The measurement of dissolved and gaseous carbon dioxide concentration
Identifieur interne : 001723 ( Istex/Corpus ); précédent : 001722; suivant : 001724The measurement of dissolved and gaseous carbon dioxide concentration
Auteurs : J. Zosel ; W. Oelner ; M. Decker ; G. Gerlach ; U. GuthSource :
- Measurement Science and Technology [ 0957-0233 ] ; 2011.
Abstract
In this review the basic principles of carbon dioxide sensors and their manifold applications in environmental control, biotechnology, biology, medicine and food industry are reported. Electrochemical CO2 sensors based on the Severinghaus principle and solid electrolyte sensors operating at high temperatures have been manufactured and widely applied already for a long time. Besides these, nowadays infrared, non-dispersive infrared and acoustic CO2 sensors, which use physical measuring methods, are being increasingly used in some fields of application. The advantages and drawbacks of the different sensor technologies are outlined. Electrochemical sensors for the CO2 measurement in aqueous media are pointed out in more detail because of their simple setup and the resulting low costs. A detailed knowledge of the basic detection principles and the windows for their applications is necessary to find an appropriate decision on the technology to be applied for measuring dissolved CO2. In particular the pH value and the composition of the analyte matrix exert important influence on the results of the measurements.
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DOI: 10.1088/0957-0233/22/7/072001
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<record><TEI wicri:istexFullTextTei="biblStruct"><teiHeader><fileDesc><titleStmt><title xml:lang="en">The measurement of dissolved and gaseous carbon dioxide concentration</title><author><name sortKey="Zosel, J" sort="Zosel, J" uniqKey="Zosel J" first="J" last="Zosel">J. Zosel</name><affiliation><mods:affiliation>Meinsberg Kurt Schwabe Research Institute, Kurt-Schwabe-Strae 4, D-04720 Ziegra-Knobelsdorf, Germany</mods:affiliation></affiliation></author><author><name sortKey="Oelner, W" sort="Oelner, W" uniqKey="Oelner W" first="W" last="Oelner">W. Oelner</name><affiliation><mods:affiliation>Meinsberg Kurt Schwabe Research Institute, Kurt-Schwabe-Strae 4, D-04720 Ziegra-Knobelsdorf, Germany</mods:affiliation></affiliation></author><author><name sortKey="Decker, M" sort="Decker, M" uniqKey="Decker M" first="M" last="Decker">M. Decker</name><affiliation><mods:affiliation>Meinsberg Kurt Schwabe Research Institute, Kurt-Schwabe-Strae 4, D-04720 Ziegra-Knobelsdorf, Germany</mods:affiliation></affiliation></author><author><name sortKey="Gerlach, G" sort="Gerlach, G" uniqKey="Gerlach G" first="G" last="Gerlach">G. Gerlach</name><affiliation><mods:affiliation>Institute of Solid State Electronics, Dresden University of Technology, D-01062 Dresden, Germany</mods:affiliation></affiliation></author><author><name sortKey="Guth, U" sort="Guth, U" uniqKey="Guth U" first="U" last="Guth">U. Guth</name><affiliation><mods:affiliation>Department of Chemistry and Food Chemistry, Dresden University of Technology, D-01062 Dresden, Germany</mods:affiliation></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">ISTEX</idno><idno type="RBID">ISTEX:1518AFDAF6F3212D5984FCD25E802E024B790A6A</idno><date when="2011" year="2011">2011</date><idno type="doi">10.1088/0957-0233/22/7/072001</idno><idno type="url">https://api.istex.fr/document/1518AFDAF6F3212D5984FCD25E802E024B790A6A/fulltext/pdf</idno><idno type="wicri:Area/Istex/Corpus">001723</idno><idno type="wicri:explorRef" wicri:stream="Istex" wicri:step="Corpus" wicri:corpus="ISTEX">001723</idno></publicationStmt><sourceDesc><biblStruct><analytic><title level="a" type="main" xml:lang="en">The measurement of dissolved and gaseous carbon dioxide concentration</title><author><name sortKey="Zosel, J" sort="Zosel, J" uniqKey="Zosel J" first="J" last="Zosel">J. Zosel</name><affiliation><mods:affiliation>Meinsberg Kurt Schwabe Research Institute, Kurt-Schwabe-Strae 4, D-04720 Ziegra-Knobelsdorf, Germany</mods:affiliation></affiliation></author><author><name sortKey="Oelner, W" sort="Oelner, W" uniqKey="Oelner W" first="W" last="Oelner">W. Oelner</name><affiliation><mods:affiliation>Meinsberg Kurt Schwabe Research Institute, Kurt-Schwabe-Strae 4, D-04720 Ziegra-Knobelsdorf, Germany</mods:affiliation></affiliation></author><author><name sortKey="Decker, M" sort="Decker, M" uniqKey="Decker M" first="M" last="Decker">M. Decker</name><affiliation><mods:affiliation>Meinsberg Kurt Schwabe Research Institute, Kurt-Schwabe-Strae 4, D-04720 Ziegra-Knobelsdorf, Germany</mods:affiliation></affiliation></author><author><name sortKey="Gerlach, G" sort="Gerlach, G" uniqKey="Gerlach G" first="G" last="Gerlach">G. Gerlach</name><affiliation><mods:affiliation>Institute of Solid State Electronics, Dresden University of Technology, D-01062 Dresden, Germany</mods:affiliation></affiliation></author><author><name sortKey="Guth, U" sort="Guth, U" uniqKey="Guth U" first="U" last="Guth">U. Guth</name><affiliation><mods:affiliation>Department of Chemistry and Food Chemistry, Dresden University of Technology, D-01062 Dresden, Germany</mods:affiliation></affiliation></author></analytic><monogr></monogr><series><title level="j">Measurement Science and Technology</title><title level="j" type="abbrev">Meas. Sci. Technol.</title><idno type="ISSN">0957-0233</idno><idno type="eISSN">1361-6501</idno><imprint><publisher>IOP Publishing</publisher><date type="published" when="2011">2011</date><biblScope unit="volume">22</biblScope><biblScope unit="issue">7</biblScope><biblScope unit="page" from="1">1</biblScope><biblScope unit="page" to="45">45</biblScope><biblScope unit="production">Printed in the UK & the USA</biblScope></imprint><idno type="ISSN">0957-0233</idno></series><idno type="istex">1518AFDAF6F3212D5984FCD25E802E024B790A6A</idno><idno type="DOI">10.1088/0957-0233/22/7/072001</idno><idno type="PII">S0957-0233(11)22926-6</idno><idno type="articleID">322926</idno><idno type="articleNumber">072001</idno></biblStruct></sourceDesc><seriesStmt><idno type="ISSN">0957-0233</idno></seriesStmt></fileDesc><profileDesc><textClass></textClass><langUsage><language ident="en">en</language></langUsage></profileDesc></teiHeader><front><div type="abstract">In this review the basic principles of carbon dioxide sensors and their manifold applications in environmental control, biotechnology, biology, medicine and food industry are reported. Electrochemical CO2 sensors based on the Severinghaus principle and solid electrolyte sensors operating at high temperatures have been manufactured and widely applied already for a long time. Besides these, nowadays infrared, non-dispersive infrared and acoustic CO2 sensors, which use physical measuring methods, are being increasingly used in some fields of application. The advantages and drawbacks of the different sensor technologies are outlined. Electrochemical sensors for the CO2 measurement in aqueous media are pointed out in more detail because of their simple setup and the resulting low costs. A detailed knowledge of the basic detection principles and the windows for their applications is necessary to find an appropriate decision on the technology to be applied for measuring dissolved CO2. In particular the pH value and the composition of the analyte matrix exert important influence on the results of the measurements.</div></front></TEI><istex><corpusName>iop</corpusName><author><json:item><name>J Zosel</name><affiliations><json:string>Meinsberg Kurt Schwabe Research Institute, Kurt-Schwabe-Strae 4, D-04720 Ziegra-Knobelsdorf, Germany</json:string></affiliations></json:item><json:item><name>W Oelner</name><affiliations><json:string>Meinsberg Kurt Schwabe Research Institute, Kurt-Schwabe-Strae 4, D-04720 Ziegra-Knobelsdorf, Germany</json:string></affiliations></json:item><json:item><name>M Decker</name><affiliations><json:string>Meinsberg Kurt Schwabe Research Institute, Kurt-Schwabe-Strae 4, D-04720 Ziegra-Knobelsdorf, Germany</json:string></affiliations></json:item><json:item><name>G Gerlach</name><affiliations><json:string>Institute of Solid State Electronics, Dresden University of Technology, D-01062 Dresden, Germany</json:string></affiliations></json:item><json:item><name>U Guth</name><affiliations><json:string>Department of Chemistry and Food Chemistry, Dresden University of Technology, D-01062 Dresden, Germany</json:string></affiliations></json:item></author><subject><json:item><lang><json:string>eng</json:string></lang><value>CO2 sensor</value></json:item><json:item><lang><json:string>eng</json:string></lang><value>Severinghaus electrode</value></json:item><json:item><lang><json:string>eng</json:string></lang><value>solid electrolyte sensor</value></json:item><json:item><lang><json:string>eng</json:string></lang><value>IR</value></json:item><json:item><lang><json:string>eng</json:string></lang><value>NDIR</value></json:item><json:item><lang><json:string>eng</json:string></lang><value>sensor application</value></json:item></subject><language><json:string>eng</json:string></language><originalGenre><json:string>review</json:string><json:string>review</json:string></originalGenre><abstract>In this review the basic principles of carbon dioxide sensors and their manifold applications in environmental control, biotechnology, biology, medicine and food industry are reported. 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Sci. Technol.</title><idno type="pISSN">0957-0233</idno><idno type="eISSN">1361-6501</idno><imprint><publisher>IOP Publishing</publisher><date type="published" when="2011"></date><biblScope unit="volume">22</biblScope><biblScope unit="issue">7</biblScope><biblScope unit="page" from="1">1</biblScope><biblScope unit="page" to="45">45</biblScope><biblScope unit="production">Printed in the UK & the USA</biblScope></imprint></monogr><idno type="istex">1518AFDAF6F3212D5984FCD25E802E024B790A6A</idno><idno type="DOI">10.1088/0957-0233/22/7/072001</idno><idno type="PII">S0957-0233(11)22926-6</idno><idno type="articleID">322926</idno><idno type="articleNumber">072001</idno></biblStruct></sourceDesc></fileDesc><profileDesc><creation><date>2011</date></creation><langUsage><language ident="en">en</language></langUsage><abstract><p>In this review the basic principles of carbon dioxide sensors and their manifold applications in environmental control, biotechnology, biology, medicine and food industry are reported. Electrochemical CO2 sensors based on the Severinghaus principle and solid electrolyte sensors operating at high temperatures have been manufactured and widely applied already for a long time. Besides these, nowadays infrared, non-dispersive infrared and acoustic CO2 sensors, which use physical measuring methods, are being increasingly used in some fields of application. The advantages and drawbacks of the different sensor technologies are outlined. Electrochemical sensors for the CO2 measurement in aqueous media are pointed out in more detail because of their simple setup and the resulting low costs. A detailed knowledge of the basic detection principles and the windows for their applications is necessary to find an appropriate decision on the technology to be applied for measuring dissolved CO2. 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Sci. Technol.</jnl-abbreviation><jnl-shortname>MST</jnl-shortname><jnl-issn>0957-0233</jnl-issn><jnl-coden>MSTCEP</jnl-coden><jnl-imprint>IOP Publishing</jnl-imprint><jnl-web-address>stacks.iop.org/MST</jnl-web-address></jnl-data><volume-data><year-publication>2011</year-publication><volume-number>22</volume-number></volume-data><issue-data><issue-number>7</issue-number><coverdate>July 2011</coverdate></issue-data><article-data><article-type type="review" sort="regular">TOPICAL REVIEW</article-type><type-number numbering="article" type="review" artnum="072001">072001</type-number><article-number>322926</article-number><first-page>1</first-page><last-page>45</last-page><length>45</length><pii>S0957-0233(11)22926-6</pii><doi>10.1088/0957-0233/22/7/072001</doi><copyright>2011 IOP Publishing Ltd</copyright><ccc>0957-0233/11/072001+45$33.00</ccc><printed>Printed in the UK & the USA</printed><features colour="global" mmedia="no" suppdata="no"></features></article-data></article-metadata><header><title-group><title>The measurement of dissolved and gaseous carbon dioxide concentration</title><short-title>Topical Review</short-title><ej-title>Topical Review</ej-title></title-group><author-group><author address="mst322926ad1"><first-names>J</first-names><second-name>Zosel</second-name></author><author address="mst322926ad1"><first-names>W</first-names><second-name>Oelßner</second-name></author><author address="mst322926ad1"><first-names>M</first-names><second-name>Decker</second-name></author><author address="mst322926ad2"><first-names>G</first-names><second-name>Gerlach</second-name></author><author address="mst322926ad3"><first-names>U</first-names><second-name>Guth</second-name></author></author-group><address-group><address id="mst322926ad1"><orgname>Meinsberg Kurt Schwabe Research Institute</orgname>, Kurt-Schwabe-Straße 4, D-04720 Ziegra-Knobelsdorf, <country>Germany</country></address><address id="mst322926ad2"><orgname>Institute of Solid State Electronics</orgname>, <orgname>Dresden University of Technology</orgname>, D-01062 Dresden, <country>Germany</country></address><address id="mst322926ad3"><orgname>Department of Chemistry and Food Chemistry, Dresden University of Technology</orgname>, D-01062 Dresden, <country>Germany</country></address></address-group><history received="5 August 2010" finalform="16 March 2011" online="20 May 2011"></history><abstract-group><abstract><heading>Abstract</heading><p indent="no">In this review the basic principles of carbon dioxide sensors and their manifold applications in environmental control, biotechnology, biology, medicine and food industry are reported. Electrochemical CO<sub>2</sub> sensors based on the Severinghaus principle and solid electrolyte sensors operating at high temperatures have been manufactured and widely applied already for a long time. Besides these, nowadays infrared, non-dispersive infrared and acoustic CO<sub>2</sub> sensors, which use physical measuring methods, are being increasingly used in some fields of application. The advantages and drawbacks of the different sensor technologies are outlined. Electrochemical sensors for the CO<sub>2</sub> measurement in aqueous media are pointed out in more detail because of their simple setup and the resulting low costs. A detailed knowledge of the basic detection principles and the windows for their applications is necessary to find an appropriate decision on the technology to be applied for measuring dissolved CO<sub>2</sub>. In particular the pH value and the composition of the analyte matrix exert important influence on the results of the measurements.</p></abstract></abstract-group><classifications><keywords print="yes"><keyword>CO<sub>2</sub> sensor</keyword><keyword>Severinghaus electrode</keyword><keyword>solid electrolyte sensor</keyword><keyword>IR</keyword><keyword>NDIR</keyword><keyword>sensor application</keyword></keywords></classifications></header><body numbering="bysection"><sec-level1 id="mst322926s1" label="1"><heading>Introduction</heading><p indent="no">Carbon dioxide is one of the key components in our life. It is important for the growth of plants and phyto cells in the biological carbon cycle and, on the other hand, it is the ‘waste’ produced by the metabolism of most of the living creatures on our Earth. In process technology CO<sub>2</sub> is an important reagent for the manufacturing of a lot of products. In contrast, in combustion of fossil fuels it is the main reaction product (besides water) and is widely understood to be one of the crucial sources of the greenhouse effect. Carbon dioxide is dissolved in natural liquids such as drinking water and mineral springs and a refreshing component in beer, sparkling wine and minerals. On the other hand, for example, the lakes Manoun and Nyos in Cameroon bear such high concentrations of CO<sub>2</sub> in their deep waters that sudden outbursts of carbon dioxide killed a lot of people in 1984 and 1986.</p><p>Observation of CO<sub>2</sub> in the atmosphere and in the oceans yields important signals for long-term predictions of the world's climate. Since carbon dioxide in higher concentration can be lethal for human beings, CO<sub>2</sub> warning devices are needed. The monitoring of CO<sub>2</sub> in chemical as well as in biotechnological processes is a valuable tool to control the efficiency of the production processes. This short listing indicates the importance of determining CO<sub>2</sub> in gases as well as in liquids over a wide range of concentrations from ppm level up to 100%.</p><p>Depending on the medium in which CO<sub>2</sub> needs to be measured and the requirements for measuring range, accuracy, long-term stability, selectivity and maintenance different methods can be applied. Standard test methods for the analytical determination of total and dissolved carbon dioxide in water require the titration of test samples. More user-friendly and therefore preferably applied are CO<sub>2</sub> sensors, which are based on various chemical or physical measuring methods.</p><p>Membrane-covered electrochemical CO<sub>2</sub> sensors according to the Severinghaus principle have been manufactured and widely applied already for a long time. With these analytical probes the CO<sub>2</sub> partial pressure is determined indirectly by measuring the pH value of a bicarbonate solution, which is separated from the surrounding gaseous or liquid medium being measured by a CO<sub>2</sub> permeable membrane. This pH value is strictly dependent on the amount of carbon dioxide reversibly permeating through the membrane into the electrolyte. An increase of the carbon dioxide concentration in the analyte acidifies the bicarbonate electrolyte in front of the pH probe and, vice versa, a decrease causes its alkalization. Typically, if the CO<sub>2</sub> partial pressure in the measuring medium changes by one order of magnitude the pH value of the bicarbonate solution changes by about 1 U. As opposed to other types of CO<sub>2</sub> sensors, Severinghaus sensors can be applied not only in gases but also for measurements in liquid media.</p><p>Nowadays infrared (IR), non-dispersive infrared (NDIR) and acoustic methods are being increasingly used. Furthermore, sensor principles based on conductometric or thermal conductivity measurements have been tested as well as measuring systems based on mass spectrometry. The current development activities in CO<sub>2</sub> sensor technology and application are characterized by
<itemized-list id="mst322926il1"><list-item id="mst322926il1.1" marker="•"><p indent="no">miniaturization of electrochemical sensors based on the Severinghaus principle, e.g. for measurement in liquid biological systems, in cell cultures, cell tissues and living organisms;</p></list-item><list-item id="mst322926il1.2" marker="•"><p indent="no">development of sterilizable and even CIP (cleaning in process)-resistant sensors for measurement of dissolved CO<sub>2</sub> in biotechnological processes and foodstuffs production;</p></list-item><list-item id="mst322926il1.3" marker="•"><p indent="no">extension of the measuring ranges to higher or lower concentrations, as required;</p></list-item><list-item id="mst322926il1.4" marker="•"><p indent="no">extension of the sensor service life and the calibration intervals;</p></list-item><list-item id="mst322926il1.5" marker="•"><p indent="no">application of thin-film and thick-film manufacturing technologies for the mass production of low-cost sensors;</p></list-item><list-item id="mst322926il1.6" marker="•"><p indent="no">development of solid electrolyte CO<sub>2</sub> sensors with short response time for <italic>in situ</italic> breath analysis;</p></list-item><list-item id="mst322926il1.7" marker="•"><p indent="no">miniaturization and improvement of selectivity and sensitivity of IR sensors and</p></list-item><list-item id="mst322926il1.8" marker="•"><p indent="no">utilization of ultrasonic sensors for breath gas analysis in medical and sportive applications.</p></list-item></itemized-list></p><p>At first this review gives an overview of the different chemical and physical measuring methods and sensors for the determination of CO<sub>2</sub> in liquids and gases with the main focus directed towards electrochemical sensors, that means devices which can be applied directly (<italic>in situ</italic>) without sampling. In particular the sampling of liquids in which CO<sub>2</sub> is dissolved is a source of errors. Subsequently, the wide variety of applications are illustrated by some typical and also somewhat original examples ranging from the atmosphere to the depth of the ocean. As compared to spectrometric (FTIR, UV–VIS), mass spectrometric (MS) and chromatographic techniques (GC, HPLC), electrochemical sensors are simple in their setup as well as in the electronic equipment necessary for operation and data acquisition. The effort for maintenance and calibration is low. Since sensor signals are obtained directly (<italic>in situ</italic>) real-time information for process control are delivered. Therefore, they are preferred tools for screenings in field application. On the other hand, electrochemical sensors cannot replace the above-mentioned standard methods in laboratories in terms of precision, detection limit, etc.</p><p>Depending on the special field of application the goals of the investigation, the measuring conditions and technical requirements on the sensors can be very different. Sensors for safety control should be mechanically robust, long-term stable and have low maintenance, whereas for measurements in boreholes or in the deep sea great demands on pressure resistance and compensation of rapid temperature changes have to be fulfilled. In biology and medicine, often small dimensions and short response times are required. In biotechnology precise, real-time data on CO<sub>2</sub> concentration increase an understanding of critical fermentation and cell culture processes and can help in gaining insight into cell metabolism, cell culture productivity and other processes within bioreactors. But in this case the sensor must be sterilizable. When being applied online in food industry, it is required that the sensor is non-breakable and even survives the rigorous CIP cleaning procedures. There is no CO<sub>2</sub> sensor available to date which meets these partly contrary requirements all at the same time. For this reason, the review is intended to be an invitation to the reader to accept the challenge to continue developing and improving CO<sub>2</sub> sensors and a motivation to open up new areas for their application. Of course, each application has its own scientific background without this the results of measurement cannot be interpreted. Therefore, we will give some details on the respective application to a certain extent without covering completely the whole phenomenon.</p></sec-level1><sec-level1 id="mst322926s2" label="2"><heading>Basic principles for CO<sub>2</sub> measurements using classical electrochemical principles</heading><sec-level2 id="mst322926s2-1" label="2.1"><heading>Analytical determination of CO<sub>2</sub> in liquids</heading><p indent="no">Unlike other gases such as oxygen, the determination of dissolved CO<sub>2</sub> is more difficult due to its chemical reactions with water. This is important for interpreting analytical results as well as for understanding the measurement principle applied e.g. in the Severinghaus electrode. Therefore, the interaction of CO<sub>2</sub> with water and the acid–base equilibrium have to be discussed in more detail. The concentration of CO<sub>2</sub> is only reasonable with the knowledge of the pH of the analysed medium.</p><p>In the ASTM standard D 513-02 [<cite linkend="mst322926bib01">1</cite>] two test methods (A and B) are proposed, providing the measurement of total or dissolved carbon dioxide present as carbon dioxide (CO<sub>2</sub>), carbonic acid (H<sub>2</sub>CO<sub>3</sub>), bicarbonate ion (HCO<sup>−</sup><sub valign="yes">3</sub>) and carbonate ion (CO<sup>2 −</sup><sub valign="yes">3</sub>) in water.</p><p>The test method A (gas sensing electrode test method) stipulates that carbon dioxide is liberated by acidification of the sample to pH = 5.0. The expelled CO<sub>2</sub> is measured by a gas-sensing electrode such as a Severinghaus CO<sub>2</sub> sensor. Volatile weak acids as well as water vapour are potential interferences and can cause errors. In our own experiments with formic acid HCOOH added as an interfering substance this could be quantitatively confirmed and, on the other hand, it could be demonstrated that by lowering the pH value to pH = 4.2 the error was considerably reduced.</p><p>By the test method B (CO<sub>2</sub> evolution, coulometric titration test method) samples containing between 5 and 800 mg L<sup>−1</sup> total CO<sub>2</sub> can be analysed. Carbon dioxide is liberated by acidification and heating the samples. The liberated CO<sub>2</sub> is swept through a scrubber by carbon dioxide-free air into an absorption cell where it is automatically coulometrically titrated. Individual concentrations of the several carbonate species are determined from the pH and total CO<sub>2</sub> values.</p><p>The actual content of aqueous solutions of free carbon dioxide depends significantly on its pH value. In figure <figref linkend="mst322926fig01">1</figref> the characteristic relationship between the CO<sub>2</sub> equilibrium concentration and the pH value in water is shown. Apart from the pH value, the curves depend slightly on temperature and on the alkalinity and salinity of the solution.
<figure id="mst322926fig01"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig01.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig01.jpg"></graphic-file></graphic><caption id="mst322926fc01" label="Figure 1"><p indent="no">pH dependence of the carbonate system.</p></caption></figure></p><p>As can be seen in the diagram, the fractional amount of the individual carbonate species changes with the pH value. At pH values below 4.3 nearly the total carbonate is existent as free dissolved CO<sub>2</sub> in the solution. With increasing pH value its percentage decreases to almost zero at pH = 8.2. In the pH range 7 to 10, hydrogen carbonate HCO<sup>−</sup><sub valign="yes">3</sub> is the predominant species. At higher pH values the carbonate equilibrium shifts increasingly to CO<sup>2 −</sup><sub valign="yes">3</sub> ions. The conventional analytical determination of carbon dioxide by titration procedures and the evaluation of the so-called <italic>p</italic>- and <italic>m</italic>-values are based on these facts.</p><p>According to the German Industrial Standard DIN 38405 (D8) [<cite linkend="mst322926bib02">2</cite>], in water dissolved carbon dioxide and the anions of the carbonic acid can be quantified indirectly only. The application of this method requires
<itemized-list id="mst322926il2"><list-item id="mst322926il2.1" marker="•"><p indent="no">measurement of pH value, temperature, conductivity and</p></list-item><list-item id="mst322926il2.2" marker="•"><p indent="no">analytical determination of acid capacity (<italic>K</italic><sub>a</sub>) and base capacity (<italic>K</italic><sub>b</sub>) or the formerly used <italic>m</italic>-value and <italic>p</italic>-value.</p></list-item></itemized-list></p><p>In the Standard DIN 38409 (H7) [<cite linkend="mst322926bib03">3</cite>], the acid capacity (<italic>K</italic><sub>a</sub>) and base capacity (<italic>K</italic><sub>b</sub>) are defined as
<itemized-list id="mst322926il3"><list-item id="mst322926il3.1" marker="•"><p indent="no"><italic>K</italic><sub>a</sub> = <italic>n</italic>(H<sub>3</sub>O<sup>+</sup>)/<italic>v</italic>(H<sub>2</sub>O) (mmol L<sup>−1</sup>)</p></list-item><list-item id="mst322926il3.2" marker="•"><p indent="no"><italic>K</italic><sub>b</sub> = <italic>n</italic>(OH<sup>−</sup>)/<italic>v</italic>(H<sub>2</sub>O) (mmol L<sup>−1</sup>)</p></list-item></itemized-list></p><p>where <italic>n</italic> is the amount of substance of the specified ions that the water (volume <italic>v</italic>) can take up until defined pH values are reached. <italic>K</italic><sub>a</sub> and <italic>K</italic><sub>b</sub> or the <italic>m</italic>-value and <italic>p</italic>-value, respectively, are determined by titration with HCl and NaOH to the characteristic pH values 8.2 and 4.3 or with indicator substances according to table <tabref linkend="mst322926tab01">1</tabref>. It is obvious that in the pH range 4.5–7.8 the +<italic>m</italic>-value (alkalinity) and the −<italic>p</italic>-value are determined.
<table id="mst322926tab01" frame="topbot"><caption id="mst322926tc01" label="Table 1"><p indent="no">Analytical determination of <italic>K</italic><sub>a</sub> and <italic>K</italic><sub>b</sub> and <italic>m</italic>-value, <italic>p</italic>-value.</p></caption><tgroup cols="4"><colspec colnum="1" colname="col1" align="left"></colspec><colspec colnum="2" colname="col2" align="left"></colspec><colspec colnum="3" colname="col3" align="left"></colspec><colspec colnum="4" colname="col4" align="left"></colspec><spanspec spanname="2to3" namest="col2" nameend="col3" align="center"></spanspec><spanspec spanname="1to4" namest="col1" nameend="col4" align="center"></spanspec><thead><row><entry>Determination</entry><entry spanname="2to3">Electrometric</entry><entry></entry></row><row><entry>Determination</entry><entry spanname="2to3"></entry><entry></entry></row><row><entry><italic>K</italic><sub>a</sub> resp.</entry><entry>Titration with</entry><entry>To</entry><entry>With</entry></row><row><entry><italic>K</italic><sub>b</sub></entry><entry>(0.1 or 0.02 mol L<sup>−1</sup>)</entry><entry>pH</entry><entry>indicator</entry></row></thead><tbody><row><entry><italic>K</italic><sub>a</sub> (8.2)</entry><entry>HCl</entry><entry>8.2</entry><entry>= +<italic>p</italic>-value</entry></row><row><entry><italic>K</italic><sub>a</sub> (4.3)</entry><entry>HCl</entry><entry>4.3</entry><entry>≈ +<italic>m</italic>-value</entry></row><row><entry><italic>K</italic><sub>b</sub> (8.2)</entry><entry>NaOH</entry><entry>8.2</entry><entry>≈ –<italic>p</italic>-value</entry></row><row><entry><italic>K</italic><sub>b</sub> (4.3)</entry><entry>NaOH</entry><entry>4.3</entry><entry>= –<italic>m</italic>-value</entry></row></tbody><tfoot><italic>p:</italic> phenolphthalein; <italic>m:</italic> methyl orange.</tfoot></tgroup></table></p><p>From the <italic>m</italic>- and −<italic>p</italic>-values the main components of the sample can be estimated:
<itemized-list id="mst322926il4"><list-item id="mst322926il4.1" marker="•"><p indent="no"><italic>m</italic> > 2<italic>p</italic>: sample contains CO<sup>2 −</sup><sub valign="yes">3</sub>, HCO<sup>−</sup><sub valign="yes">3</sub></p></list-item><list-item id="mst322926il4.2" marker="•"><p indent="no"><italic>m</italic> = 2<italic>p</italic>: sample contains CO<sup>2 −</sup><sub valign="yes">3</sub></p></list-item><list-item id="mst322926il4.3" marker="•"><p indent="no"><italic>p</italic> < <italic>m</italic> < 2<italic>p</italic>: sample contains CO<sup>2 −</sup><sub valign="yes">3</sub>, OH<sup>−</sup></p></list-item><list-item id="mst322926il4.4" marker="•"><p indent="no"><italic>m</italic> = <italic>p</italic>: sample contains OH<sup>−</sup></p></list-item><list-item id="mst322926il4.5" marker="•"><p indent="no"><italic>p</italic> = 0: sample contains CO<sub>2</sub>, HCO<sup>−</sup><sub valign="yes">3</sub>.</p></list-item></itemized-list></p><p>The total amount of inorganic carbon (TIC), referred to as <italic>Q</italic><sub>c</sub>, can be calculated from the <italic>m</italic>-value and <italic>p</italic>-value according to <italic>Q</italic><sub>c</sub> = Σ <italic>c</italic>(CO<sub>2</sub>; HCO<sup>−</sup><sub valign="yes">3</sub>; CO<sup>2 −</sup><sub valign="yes">3</sub>) = <italic>m</italic> − <italic>p</italic>.</p><p>Provided that the values of pH, temperature, conductivity and the analytical parameters <italic>m</italic>, <italic>p</italic> and <italic>Q</italic><sub>c</sub> are known, the CO<sub>2</sub> concentration can be calculated by two methods. While the range of application of the first method, which evaluates the <italic>p-</italic> and <italic>m</italic>-values, is restricted, the second method, which uses the <italic>Q</italic><sub>c</sub> value, is more generally applicable. The CO<sub>2</sub> concentration can be determined as follows.</p><p indent="no">Calculation according to method 1.
<itemized-list id="mst322926il5"><list-item id="mst322926il5.1" marker="•"><p indent="no">Calculation of <italic>m</italic><sub>relative</sub> = <italic>m</italic>/(<italic>m</italic> − <italic>p</italic>)</p></list-item><list-item id="mst322926il5.2" marker="•"><p indent="no">Determination of the corresponding pH value from the titration curve shown in figure <figref linkend="mst322926fig02">2</figref></p></list-item><list-item id="mst322926il5.3" marker="•"><p indent="no">Comparison with the measured pH value.</p><p>In the case of conformity of both pH values,</p></list-item></itemized-list></p><p><itemized-list id="mst322926il6" type="unnum"><list-item id="mst322926il6.1"><p indent="no"> if pH < 4.5: <italic>c</italic>(CO<sub>2</sub>) = −<italic>p</italic> (mmol L<sup>−1</sup>) – [H<sub>3</sub>O<sup>+</sup>]</p></list-item><list-item id="mst322926il6.2"><p indent="no"> if 4.5 < pH < 7.8: <italic>c</italic>(CO<sub>2</sub>) = −<italic>p</italic> (mmol L<sup>−1</sup>).</p></list-item></itemized-list></p><p indent="no">Calculation according to method 2:
<itemized-list id="mst322926il7"><list-item id="mst322926il7.1" marker="•"><p indent="no">calculation of <italic>Q</italic><sub>c</sub> = <italic>m</italic> − <italic>p</italic> (mmol L<sup>−1</sup>)</p></list-item><list-item id="mst322926il7.2" marker="•"><p indent="no">determination of the percent content of CO<sub>2</sub> of total carbonic acid <italic>Q</italic><sub>c</sub> from a table in [<cite linkend="mst322926bib04">4</cite>] or from equation (<eqnref linkend="mst322926eqn2-5">5</eqnref>) in [<cite linkend="mst322926bib02">2</cite>]:
<display-eqn id="mst322926eqn2-1" eqnnum="2.1"></display-eqn>In this equation <inline-eqn></inline-eqn> is the concentration of dissolved carbon dioxide in the solution in mmol L<sup>−1</sup>; <inline-eqn></inline-eqn> is the activity of the hydronium ions in mol L<sup>−1</sup>; <italic>Q</italic><sub>C</sub> is the sum of dissolved CO<sub>2</sub>, HCO<sup>−</sup><sub valign="yes">3</sub> and CO<sup>2 −</sup><sub valign="yes">3</sub> ions in mmol L<sup>−1</sup>; <italic>k</italic><sub>1</sub>, <italic>k</italic><sub>2</sub> are the temperature-dependent quantities; and <italic>f</italic><sub>1</sub>, <italic>f</italic><sub>2</sub> are the activity coefficients (also sometimes denoted by γ<sub>1</sub>, γ<sub>2</sub>) dependent on the total ion content of the water.</p></list-item></itemized-list><figure id="mst322926fig02"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig02.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig02.jpg"></graphic-file></graphic><caption id="mst322926fc02" label="Figure 2"><p indent="no">General titration curve of carbonic acid at 10 °C according to [<cite linkend="mst322926bib02">2</cite>].</p></caption></figure></p><p>Numerical values for the thermodynamic constants <italic>k</italic><sub>1</sub>(<italic>T</italic>), <italic>k</italic><sub>2</sub>(<italic>T</italic>) and <italic>f</italic><sub>1</sub>(κ), <italic>f</italic><sub>2</sub>(κ) as functions of the temperature <italic>T</italic> and the specific conductivity κ of the water may be taken from tables <tabref linkend="mst322926tab01">1</tabref> and <tabref linkend="mst322926tab02">2</tabref> in the standard. For illustration, for some selected values of temperature <italic>T</italic> and conductivity κ in figure <figref linkend="mst322926fig03">3</figref> the numerical values of the table in [<cite linkend="mst322926bib04">4</cite>] for the percentage of free carbonic acid as function of the total carbonic acid <italic>Q</italic><sub>c</sub> are plotted as curves in a diagram.
<figure id="mst322926fig03"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig03.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig03.jpg"></graphic-file></graphic><caption id="mst322926fc03" label="Figure 3"><p indent="no">Percentage of free carbonic acid from total carbonic acid <italic>Q</italic><sub>c</sub> depending on temperature and conductivity derived from the table in [<cite linkend="mst322926bib04">4</cite>].</p></caption></figure><table id="mst322926tab02" frame="topbot"><caption id="mst322926tc02" label="Table 2"><p indent="no">Conditions for the ion exchange process in layered β″-alumina.</p></caption><tgroup cols="4"><colspec colnum="1" colname="col1" align="left"></colspec><colspec colnum="2" colname="col2" align="left"></colspec><colspec colnum="3" colname="col3" align="left"></colspec><colspec colnum="4" colname="col4" align="left"></colspec><spanspec spanname="1to4" namest="col1" nameend="col4" align="center"></spanspec><spanspec spanname="1to2" namest="col1" nameend="col2" align="center"></spanspec><spanspec spanname="3to4" namest="col3" nameend="col4" align="center"></spanspec><thead><row><entry spanname="1to4">Basis material</entry></row><row><entry spanname="1to4"></entry></row><row><entry spanname="1to2">Na<sup>+</sup>-β″-alumina layer on α-alumina sheet</entry><entry spanname="3to4">K<sup>+</sup>-β″-alumina on α-alumina sheet</entry></row><row><entry spanname="1to2"></entry><entry spanname="3to4"></entry></row><row><entry>Modified β″-alumina</entry><entry>Conditions for ionic exchange</entry><entry>Modified β″-alumina</entry><entry>Conditions for ionic exchange</entry></row></thead><tbody><row><entry>Li<sup>+</sup>-β″-alumina</entry><entry>Molten LiNO<sub>3</sub></entry><entry>H<sup>+</sup>-β″-alumina</entry><entry>H<sub>2</sub>O</entry></row><row><entry></entry><entry>300 °C, 168 h</entry><entry></entry><entry>95 °C, 48 h</entry></row><row><entry>Ca<sup>2+</sup>-β″-alumina</entry><entry>Molten CaCl<sub>2</sub></entry><entry></entry><entry></entry></row><row><entry></entry><entry>800 °C, 48 h</entry><entry></entry><entry></entry></row><row><entry>Sr<sup>2+</sup>-β″-alumina</entry><entry>Molten SrCl<sub>2</sub></entry><entry></entry><entry></entry></row><row><entry></entry><entry>800 °C, 48 h</entry><entry></entry><entry></entry></row></tbody></tgroup></table></p><p>In practice till today the CO<sub>2</sub> concentration is often taken from tables. In this case it is generally necessary to determine the total alkalinity analytically by titration. Rebsdorf [<cite linkend="mst322926bib05">5</cite>] presented a set of tables for easy calculation of total carbon dioxide, the partial pressure of free CO<sub>2</sub> and other components of the carbon dioxide system in freshwater. The tables are based on measuring values of pH, temperature, total alkalinity and electrolytic conductivity, from which an approximate figure of the ionic strength can be determined. They are valid only in aqueous solutions where the overall dominating buffer capacity is caused by the carbonate system, and where the Debye–Hückel formula for the mean activity coefficients in diluted solutions can be applied without great error. Consequently, the accuracy of the table values decreases for higher ionic strengths. Ca<sup>++</sup> and Mg<sup>++</sup> ions can also influence the calculation of CO<sub>2</sub> from titration alkalinity. Neglecting this influence can cause considerable errors. General relationships among the influencing factors are given by tables too [<cite linkend="mst322926bib06">6</cite>].</p><p>Figure <figref linkend="mst322926fig04">4</figref> illustrates the dependence of the CO<sub>2</sub> concentration on the pH value and alkalinity of two solutions with different alkalinities at selected values of temperature and conductivity.
<figure id="mst322926fig04"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig04.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig04.jpg"></graphic-file></graphic><caption id="mst322926fc04" label="Figure 4"><p indent="no">Dependence of the CO<sub>2</sub> concentration on pH value and alkalinity of solution.</p></caption></figure></p><p>Of course, it is possible to store the table values in a microprocessor as was realized by the microprocessor-based handheld pH/CO<sub>2</sub> analyser (Model 503, Royce Instrument Corporation, USA). This device uses a conventional combination pH glass electrode and calculates the CO<sub>2</sub> concentration from the measured current pH value and temperature and the analytically determined alkalinity (<italic>m</italic>-value) and salinity of the water being tested. If the water contains substances that affect the ionic strength, then this may be compensated by adjusting a salinity factor.</p><p>For an experimental comparison at the Meinsberg Kurt Schwabe Research Institute (KSI) CO<sub>2</sub> measurements were performed in more or less diluted mineral waters by using the different analytical methods described above and with a conventional electrochemical CO<sub>2</sub> sensor. The results are presented in figure <figref linkend="mst322926fig05">5</figref>. Despite great experimental care noticeable differences occurred, which seem not to be systematic and increase with CO<sub>2</sub> concentration. They may have different reasons. One source of deviation is the essential difference between the kinds of CO<sub>2</sub> quantities to be measured. The analytical methods determine the CO<sub>2</sub> concentration, whereas the electrochemical CO<sub>2</sub> sensor primarily measures the CO<sub>2</sub> partial pressure in the solution. This, as a matter of fact, plays a role in many of the application examples described in section 7 of this review, where CO<sub>2</sub> is measured in natural waters containing various organic or inorganic substances, which can affect the titration.
<figure id="mst322926fig05"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig05.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig05.jpg"></graphic-file></graphic><caption id="mst322926fc05" label="Figure 5"><p indent="no">Results of determinations of CO<sub>2</sub> concentrations in various samples of diluted mineral waters by using different methods (KSI).</p></caption></figure></p></sec-level2><sec-level2 id="mst322926s2-2" label="2.2"><heading>Potentiometric sensing principle</heading><sec-level3 id="mst322926s2-2-1" label="2.2.1"><heading>Severinghaus-type electrochemical carbon dioxide sensors</heading><p indent="no">CO<sub>2</sub> cannot be reduced electrochemically in an aqueous solution; hence, a simple sensor setup such as the one used for determination of oxygen is not possible [<cite linkend="mst322926bib07">7</cite>]. However, CO<sub>2</sub> can be measured indirectly by dissolving it in an aqueous electrolyte solution and monitoring the resulting change in pH. The fundamental idea of the membrane-covered potentiometric carbon dioxide sensor consists of separating the sample being measured from the internal electrolyte solution of the sensor by a thin rubber or polymer membrane, which is permeable to CO<sub>2</sub> gas but not to ions or water. It was first suggested by Stow and Randall [<cite linkend="mst322926bib08">8</cite>, <cite linkend="mst322926bib09">9</cite>] about 50 years ago and shortly afterwards improved by Severinghaus and Bradley [<cite linkend="mst322926bib10">10</cite>], who increased the sensitivity and stability of the sensor considerably by using a hydrogen carbonate solution instead of pure water as the electrolyte contained in the sensor. Although in the meantime there was much development in this area and consequently numerous relevant publications and patent applications appeared, the basic principle has remained almost unchanged since its introduction.</p><p>Figure <figref linkend="mst322926fig06">6</figref> shows schematically the so-called Severinghaus carbon dioxide sensor (sometimes also called as electrode) and illustrates its mode of operation. Main constituents of the sensor are a thin polymer membrane, the hydrogen carbonate containing electrolyte solution, a thin hydrophilic spacer sheet soaked with the electrolyte solution and a pH probe. CO<sub>2</sub> permeates from the gaseous or liquid specimen through the membrane into the electrolyte film in the spacer until equilibrium between the CO<sub>2</sub> partial pressure on both sides of the membrane has been established. During measurement virtually no CO<sub>2</sub> is consumed.
<figure id="mst322926fig06"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig06.eps" width="14pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig06.jpg"></graphic-file></graphic><caption id="mst322926fc06" label="Figure 6"><p indent="no">Main components and mode of operation of the Severinghaus carbon dioxide sensor.</p></caption></figure></p><p>As indicated in figure <figref linkend="mst322926fig06">6</figref>, in the sensor electrolyte a series of equilibrium reactions take place resulting in the formation of H<sup>+</sup> ions and thus in a CO<sub>2</sub>-dependent shift of the pH value, which is measured by means of the integrated pH electrode. The equation for the output voltage signal <italic>E</italic> of the sensor given in figure <figref linkend="mst322926fig06">6</figref> can be derived as follows.</p><p>Since the concentration of H<sub>2</sub>CO<sub>3</sub> (carbonic acid) in the sensor electrolyte is much lower than that of CO<sub>2</sub>, the first two equations in figure <figref linkend="mst322926fig06">6</figref> may be summarized as
<display-eqn id="mst322926eqn2-2" eqnnum="2.2"></display-eqn>The equilibrium is expressed by
<display-eqn id="mst322926eqn2-3" eqnnum="2.3"></display-eqn>with <italic>K</italic><sub>1</sub> = 4.3 × 10<sup>−7</sup> mol L<sup>−1</sup> and p<italic>K</italic><sub>1</sub> = 6.36.</p><p>According to Henry's law, in the case of equilibrium between the gaseous and the liquid phases the concentration of dissolved carbon dioxide [CO<sub>2</sub>] = <italic>c</italic>(CO<sub>2</sub>) in the sensor electrolyte is
<display-eqn id="mst322926eqn2-4" eqnnum="2.4"></display-eqn>with <italic>K</italic><sub>H</sub> = 3.4 × 10<sup>−2</sup> (mol L<sup>−1</sup>) atm<sup>‐1</sup> (at 25 °C). <italic>p</italic>(CO<sub>2</sub>) is the partial pressure of CO<sub>2</sub> in the sample (and finally also in the sensor electrolyte) and <italic>K</italic><sub>H</sub> is the solubility coefficient of CO<sub>2</sub>:
<display-eqn id="mst322926eqn2-5" eqnnum="2.5"></display-eqn>Inserting equations (<eqnref linkend="mst322926eqn2-4">2.4</eqnref>) and (<eqnref linkend="mst322926eqn2-5">2.5</eqnref>) in equation (<eqnref linkend="mst322926eqn2-3">2.3</eqnref>) and taking the logarithm results in the Henderson–Hasselbalch equation (<eqnref linkend="mst322926eqn2-6">2.6</eqnref>)
<display-eqn id="mst322926eqn2-6" eqnnum="2.6"></display-eqn>Since the electrolyte of the Severinghaus carbon dioxide sensor already contains a large concentration of hydrogen carbonate ions (e.g. 10<sup>−2</sup> M), the changes in this concentration due to the dissociation of carbonic acid are negligible and thus the term [HCO<sup>−</sup><sub valign="yes">3</sub>] can be regarded as a constant and added to p<italic>K</italic><sub>2</sub> in equation (<eqnref linkend="mst322926eqn2-6">2.6</eqnref>), resulting at last in the interesting dependence of the pH value on the partial pressure of CO<sub>2</sub> in the sample:
<display-eqn id="mst322926eqn2-7" eqnnum="2.7"></display-eqn>Finally, as indicated in figure <figref linkend="mst322926fig06">6</figref>, the output voltage signal <italic>E</italic> of the pH electrode can be written as
<display-eqn id="mst322926eqn2-8" lines="multiline" eqnnum="2.8" eqnalign="left"></display-eqn>where <italic>F</italic><sub>N</sub> is the Nernst factor (59.16 mV/decade at 25 °C) and <italic>S</italic> is the relative sensitivity of the carbon dioxide sensor, which practically ranges between 0.85 and 0.98.</p><p>In more details the dependence of the CO<sub>2</sub> sensor function on the NaHCO<sub>3</sub> (sodium bicarbonate) concentration and the resulting Na<sup>+</sup> concentration in the sensor electrolyte can be derived as follows:
<eqn-group id="mst322926eqngrp01"><display-eqn id="mst322926eqn2-9" eqnnum="2.9"></display-eqn><display-eqn id="mst322926eqn2-10" eqnnum="2.10"></display-eqn></eqn-group>with <italic>K</italic><sub>2</sub> = 5.61 × 10<sup>−11</sup> mol L<sup>−1</sup> and p<italic>K</italic><sub>2</sub> = 10.25 (at 25 °C)
<display-eqn id="mst322926eqn2-11" eqnnum="2.11"></display-eqn>with <italic>K</italic><sub>W</sub> = 1.008 × 10<sup>−14</sup> (mol L<sup>−1</sup>)<sup>2</sup> (at 25 °C).</p><p>Introducing in the equation for charge neutrality
<display-eqn id="mst322926eqn2-12" eqnnum="2.12"></display-eqn>equations (<eqnref linkend="mst322926eqn2-3">2.3</eqnref>), (<eqnref linkend="mst322926eqn2-10">2.10</eqnref>) and (<eqnref linkend="mst322926eqn2-11">2.11</eqnref>) result in
<display-eqn id="mst322926eqn2-13" lines="multiline" eqnnum="2.13" eqnalign="left"></display-eqn>and in the cubic equation
<display-eqn id="mst322926eqn2-14" lines="multiline" eqnnum="2.14" eqnalign="left"></display-eqn>For solving this equation Cardano's formula for a root of a cubic equation (casus irreducibilis) can be used with the following transformations:
<eqn-group id="mst322926eqngrp02"><display-eqn id="mst322926eqn2-15" eqnnum="2.15"></display-eqn><display-eqn id="mst322926eqn2-16" eqnnum="2.16"></display-eqn><display-eqn id="mst322926eqn2-17" eqnnum="2.17"></display-eqn><display-eqn id="mst322926eqn2-18" lines="multiline" eqnnum="2.18" eqnalign="left"></display-eqn></eqn-group>The desired dependence of [H<sup>+</sup>] on the concentration [Na<sup>+</sup>] is
<display-eqn id="mst322926eqn2-19" lines="multiline" eqnnum="2.19" eqnalign="left"></display-eqn>and finally
<display-eqn id="mst322926eqn2-20" eqnnum="2.20"></display-eqn></p><p>Figure <figref linkend="mst322926fig07">7</figref> demonstrates the influence of the NaHCO<sub>3</sub> concentration of the sensor electrolyte on the sensitivity of the Severinghaus carbon dioxide sensor. The curves calculated from equations (<eqnref linkend="mst322926eqn2-19">2.19</eqnref>) and (<eqnref linkend="mst322926eqn2-20">2.20</eqnref>) are in good accordance with our own experimental results and those published by other authors [<cite linkend="mst322926bib11">11</cite>]. They were helpful to find an optimum sensor design.
<figure id="mst322926fig07"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig07.eps" width="18pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig07.jpg"></graphic-file></graphic><caption id="mst322926fc07" label="Figure 7"><p indent="no">Dependence of the sensitivity of the Severinghaus carbon dioxide sensor on the NaHCO<sub>3</sub> concentration in the sensor electrolyte, calculated from equations (<eqnref linkend="mst322926eqn2-19">2.19</eqnref>) and (<eqnref linkend="mst322926eqn2-20">2.20</eqnref>).</p></caption></figure></p><p>In principle, the sensor works even if pure water is used as a sensor electrolyte. But in this case (curve 4 in figure <figref linkend="mst322926fig07">7</figref>) the CO<sub>2</sub> sensitivity is only ΔpH = –0.5/decade CO<sub>2</sub>, and, furthermore, the sensor function would be less stable. By adding comparatively small amounts of NaHCO<sub>3</sub>, as proposed in [<cite linkend="mst322926bib10">10</cite>], the sensitivity can be considerably raised to about ΔpH = – 1/decade CO<sub>2</sub>. The comparison of curves 1 to 3 in figure <figref linkend="mst322926fig07">7</figref> shows that the sensitivity cannot be further increased by adding more NaHCO<sub>3</sub>. To achieve appropriate sensor stability, typically 10<sup>−2</sup> mol L<sup>−1</sup> NaHCO<sub>3</sub> are applied.</p><p>If the sensor is used for measurements in gases or if it is simply stored in ambient air the internal electrolyte filling tends to dry out, causing elevated sensor drifts and anticipating frequent recalibrations. To prevent this, the water vapour pressure of the sensor electrolyte can be decreased, e.g. by adding ethylene glycol. This admixture does not influence the main sensor parameters significantly.</p><p>Provided the measuring conditions are known and constant, the concentration can be adapted to the relative humidity of the ambient air. The water vapour pressure <italic>p</italic><sub>w</sub> above an ethylene glycol/water mixture can be calculated at ϑ = 25 °C from the volumetric water concentration <italic>c</italic><sub>w</sub> by the following empirical equation:
<display-eqn id="mst322926eqn2-21" eqnnum="2.21"></display-eqn>with <italic>a</italic><sub>0</sub> = 95.97, <italic>a</italic><sub>1</sub> = 80.19, <italic>a</italic><sub>2</sub> = −0.905 <italic>a</italic><sub>3</sub> = 4.4 × 10<sup>−3</sup>. An admixture of nearly 75 vol.% ethylene glycol results therefore in zero water permeation through the membrane if the measuring gas has the relative humidity r.h. = 50%. This strategy enables calibration-free monitoring of ambient air in a room with Severinghaus carbon dioxide sensors for more than 6 months. If the sensor is used for measurement in liquids, the admixture of water vapour suppressors is not necessary. Those sensors have to be stored under wet conditions.</p><p>Typical materials used as sensor membranes are polytetrafluoroethylene (PTFE), polymethylpentene (TPX), silicone and polypropylene (PP). Their thicknesses range typically between 5 and 30 µm. To improve the mechanical stability, the thin membranes are often mechanically protected or reinforced by a perforated metal foil or other embedded compounds.</p><p>Apart from the mechanical, thermal and electrical properties the permeability for CO<sub>2</sub> and H<sub>2</sub>O is one of the most important parameters of the membrane material. The sensitivity of equilibrium-type sensors is not influenced by the permeabilities, but long-term behaviour and response time depend on them as described in section 2.2.2. Sensitivities of non-equilibrium-type sensors as discussed in section <secref linkend="mst322926s2-5">2.5</secref> depend directly on the membrane CO<sub>2</sub> permeability.</p><p>The temperature dependence of the permeability <italic>P</italic> can be calculated from the activation energy <italic>E</italic><sub>a</sub> using the following equation:
<display-eqn id="mst322926eqn2-22" eqnnum="2.22"></display-eqn></p><p>The ratio of the membrane permeabilities for H<sub>2</sub>O and CO<sub>2</sub> is of special importance regarding the long-term stability of equilibrium-type carbon dioxide sensors. This parameter depends on the membrane material and amounts for silicone to <italic>p</italic>(H<sub>2</sub>O)/<italic>p</italic>(CO<sub>2</sub>) ≈ 13 and for polymethylpentene to <italic>p</italic>(H<sub>2</sub>O)/<italic>p</italic>(CO<sub>2</sub>) < 1, which is therefore especially suited as membrane material for electrochemical CO<sub>2</sub> gas sensors.</p></sec-level3><sec-level3 id="mst322926s2-2-2" label="2.2.2"><heading>Response behaviour of electrochemical carbon dioxide sensors</heading><p indent="no">The response time of the Severinghaus-type CO<sub>2</sub> electrode depends on different parameters. On one hand, the thickness and the material of the gas permeable membrane have a significant influence on the transport of carbon dioxide (and of other gaseous compounds and water) from the analysed solution to the inner electrolyte and vice versa. On the other hand, the thickness of the electrolyte film in front of the pH-sensing electrode and the concentrations of H<sub>2</sub>CO<sub>3</sub> and HCO<sup>−</sup><sub valign="yes">3</sub> are important for the time required for the adjustment of the pH equilibrium.</p><p>Ross <italic>et al</italic> [<cite linkend="mst322926bib12">12</cite>] have calculated the response time of a Severinghaus-type CO<sub>2</sub> electrode under the assumption that the transport process through the membrane is the time-depending step for the establishment of the equilibrium partial pressure of the analysed gas between the inner and outer solutions.</p><p>Figure <figref linkend="mst322926fig08">8</figref> [<cite linkend="mst322926bib12">12</cite>] shows the concentration profile of the diffusing species in membrane and electrolytes during the change of the outer concentration from <italic>c</italic><sub>1</sub> to <italic>c</italic><sub>2</sub>. Presuppositions for this model are a rapid achievement of the equilibrium at the phase boundaries membrane/aqueous solutions, a rapid diffusion of the gas within the membrane and that the thickness of the electrolyte layer <italic>l</italic> in front of the sensing electrode is small compared with the membrane thickness <italic>m</italic>.
<figure id="mst322926fig08"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig08.eps" width="18pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig08.jpg"></graphic-file></graphic><caption id="mst322926fc08" label="Figure 8"><p indent="no">Steady state model of the electrode response of the CO<sub>2</sub> electrode according to Ross [<cite linkend="mst322926bib12">12</cite>].</p></caption></figure></p><p>The equation of the response time <italic>t</italic> is given as follows:
<display-eqn id="mst322926eqn2-23" eqnnum="2.23"></display-eqn>with
<display-eqn id="mst322926eqn2-24" eqnnum="2.24"></display-eqn><italic>c</italic><sub>2</sub> is the final concentration of H<sub>2</sub>CO<sub>3</sub>, Δ<italic>c</italic> is the difference between starting and end concentration of H<sub>2</sub>CO<sub>3</sub>, <italic>m</italic> is the thickness of the membrane and <italic>l</italic> is that of the inner electrolyte between electrode surface and membrane. <italic>D</italic> expresses the diffusion coefficient of the gas in the membrane and <italic>k</italic> is the partition coefficient between solution and the bulk of the membrane. The concentration-dependent dissociation of H<sub>2</sub>CO<sub>3</sub> into HCO<sup>−</sup><sub valign="yes">3</sub> and CO<sup>2 −</sup><sub valign="yes">3</sub> is described by the expression d<italic>c</italic><sub>B</sub>/d<italic>c</italic>. In <italic>c</italic><sub>B</sub> the ionic species are summarized. The fractional approach of H<sub>2</sub>CO<sub>3</sub> to the final concentration in the inner electrolyte is given by the factor ϵ. At higher concentrations of H<sub>2</sub>CO<sub>3</sub> the relation d<italic>c</italic><sub>B</sub>/d<italic>c</italic> is approximately constant in equation (<eqnref linkend="mst322926eqn2-23">2.23</eqnref>) and the dependence of the time response can be estimated for different ratios of <italic>c</italic><sub>1</sub>/<italic>c</italic><sub>2</sub>. It is important to keep in mind that the response time is dependent on the direction of the concentration change. For example, for an increase of the analyte concentration from 10<sup>−3</sup> to 10<sup>−1</sup> mol L<sup>−1</sup> the response time (for reaching 99% of the equilibrium, ϵ = 0.01) is twofold faster than for the reverse process.</p><p>The assumptions for equation (<eqnref linkend="mst322926eqn2-23">2.23</eqnref>) are only valid for higher sample concentrations of H<sub>2</sub>CO<sub>3</sub>. At concentrations of H<sub>2</sub>CO<sub>3</sub> lower than 2 mmol L<sup>−1</sup> the response time of the Severinghaus electrode is dependent on the bicarbonate concentration in the internal electrolyte. High amounts of bicarbonate in the internal electrolyte space and carbonic acid concentrations in the analyte solution less than 2 mmol L<sup>−1</sup> evoke a substantial increase of the response time. At low concentrations the term
<display-eqn id="mst322926eqn2-25" eqnnum="2.25"></display-eqn>in equation (<eqnref linkend="mst322926eqn2-23">2.23</eqnref>) is no longer constant. At low levels of H<sub>2</sub>CO<sub>3</sub> the permeating carbon dioxide leads to an increase of HCO<sup>−</sup><sub valign="yes">3</sub> in the internal electrolyte. This results finally in a further consumption of carbonic acid which has to be replenished by CO<sub>2</sub> passing through the membrane to establish the equilibrium in the space before the glass electrode. Examples for the concentration dependence of the response time are given in the publication of Jensen <italic>et al</italic> [<cite linkend="mst322926bib13">13</cite>].</p><p>All these assumptions are made for an ‘ideally’ constructed CO<sub>2</sub> electrode with only small spaces between the pH sensor and the membrane. The electrolyte beside the membrane in the deeper bulk of the electrode is not considered in these investigations. The CO<sub>2</sub> passing through the membrane has also to equilibrate in the electrolyte volume in the rim near the sensing part of the pH electrode surface, which takes a certain time and influences the stability of the potentiometric signal. A closer view of the influence of bicarbonate electrolyte in the entire system on the response time and of drift effects has been described by Samukawa <italic>et al</italic> [<cite linkend="mst322926bib14">14</cite>].</p></sec-level3><sec-level3 id="mst322926s2-2-3" label="2.2.3"><heading>Calibration of electrochemical carbon dioxide sensors</heading><p indent="no">As is generally required for electrochemical sensors, Severinghaus-type CO<sub>2</sub> sensors have to be calibrated regularly according to given temporal or user-specific regimes to compensate for unavoidable changes of the sensor parameters and to maintain high measuring accuracy over longer periods.</p><p>Whereas CO<sub>2</sub> sensors for measurements in gases can simply be calibrated by using commercially available test gases, the calibration of electrochemical CO<sub>2</sub> sensors for measurements in liquids is not trivial [<cite linkend="mst322926bib15">15</cite>]. Contrary to standardized, long-term stable buffer solutions with defined pH values, which are generally used in pH measuring techniques, solutions with defined CO<sub>2</sub> contents are not stable over longer periods of time and therefore commercially not available. For this reason, calibration solutions with defined CO<sub>2</sub> concentrations must be prepared immediately before starting the calibration procedure. These can be made as follows.</p><p>At first, solutions with the required NaHCO<sub>3</sub> concentrations are prepared. Then, by adding an organic acid (e.g. succinic or oxalic acid) the pH value of the solution is decreased to pH ≈ 3. At this pH value the total carbonate in the solution changes completely into dissolved free CO<sub>2</sub> in the desired concentration. A special calibrating vessel and high experimental care are necessary to prevent the escape of CO<sub>2</sub> from the calibrating solution as it happens when opening a mineral water bottle. In particular the user has to avoid an air space in the vessel to prevent a change of dissolved CO<sub>2</sub> in the gas phase. Nevertheless, the calibrating solutions cannot be stored for a long time.</p><p>Since the sensitivity of electrochemical CO<sub>2</sub> sensors changes only slightly in the course of time, it is sufficient to carry out regularly one-point calibrations in order to correct the potential drift of the sensor. To correct temporal changes of the sensitivity, two-point calibrations are only necessary in longer time intervals or after the sensor had been out of use for a longer period. If the sensor is continuously in use, as a rule, one-point calibrations at least once per week and two-point calibrations at least once per month are recommended. For two-point calibrations two calibration solutions with different CO<sub>2</sub> concentrations are needed. Generally, the expected CO<sub>2</sub> concentration of the measuring liquid should be within these concentration values. To avoid larger temperature-dependent errors, the temperature of the calibration solution should deviate by no more than 5 K from that of the measuring solution. Before starting the calibration procedure, the electrode should be inserted at least for 15 min into the solution.</p><p>However, the calibration of electrochemical CO<sub>2</sub> sensors is particularly difficult, if measurements under field conditions and in solutions of unknown and changing composition must be carried out. It was found that considerable deviations might occur between the analytically determined CO<sub>2</sub> concentrations and those measured with the sensor, if the measuring and the calibrating solutions differ substantially in their composition. For this reason it was proposed to check the electrochemical CO<sub>2</sub> sensor in the actual measuring solution with a multi-parameter measuring instrument also containing sensors for the determination of pH value, conductivity and temperature [<cite linkend="mst322926bib16">16</cite>]. During measurement and calibration an integrated microcomputer compares the measured values of the CO<sub>2</sub> sensor with the values measured with the other sensors according to given functions and limit values.</p></sec-level3></sec-level2><sec-level2 id="mst322926s2-3" label="2.3"><heading>Opto-chemical CO<sub>2</sub> sensing principle</heading><p indent="no">Alternatively and somewhat time delayed to the electrochemical principles described in section <secref linkend="mst322926s2-2">2.2</secref>, different opto-chemical sensing principles for CO<sub>2</sub> have been developed and published in numerous papers [<cite linkend="mst322926bib17" range="mst322926bib17,mst322926bib18,mst322926bib19,mst322926bib20,mst322926bib21,mst322926bib22">17–22</cite>]. Mills [<cite linkend="mst322926bib23">23</cite>] reviewed and illustrated with examples the basic concepts of the different colorimetric and luminescent optical sensors for the detection and quantitative analysis of carbon dioxide. Furthermore, in this review the major applications of these sensors are discussed and their strengths and weaknesses are highlighted.</p><p>In another case a patent [<cite linkend="mst322926bib24">24</cite>] describes in detail an invention for optical chemical sensing of CO<sub>2</sub>, which ‘comprises a method for determining an analyte in which a dye solution is illuminated to induce a first and a second output light at the first and second wavelength, respectively, and the analyte concentration is determined from the measured first and second output intensities’. Based on this patent, a CO<sub>2</sub> monitor for <italic>in situ</italic> measurement of dissolved CO<sub>2</sub> has been developed and commercialized (Model 8500, YSI Inc., USA). Figure <figref linkend="mst322926fig09">9</figref> illustrates the main components of the opto-chemical sensor capsule of the YSI 8500 CO<sub>2</sub> monitor and their functions [<cite linkend="mst322926bib25">25</cite>].
<figure id="mst322926fig09"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig09.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig09.jpg"></graphic-file></graphic><caption id="mst322926fc09" label="Figure 9"><p indent="no">Main components of the sensor capsule of the YSI 8500 CO<sub>2</sub> monitor [according to <cite linkend="mst322926bib25">25</cite>].</p></caption></figure></p><p>The disposable stainless steel sensor capsule of the CO<sub>2</sub> monitor contains essentially a small reservoir of bicarbonate buffer with an admixture of HPTS (hydroxypyrene trisulfonic acid), a pH-sensitive fluorescent dye. CO<sub>2</sub> diffuses through a polymer membrane into the buffer solution, changing its pH. The thin membrane is mechanically protected by a perforated stainless steel foil.</p><p>As explained in [<cite linkend="mst322926bib25">25</cite>], CO<sub>2</sub> that permeates into the capsule's dye layer causes a reaction which changes the pH value and consequently the fluorescence of the dye according to
<eqn-group id="mst322926eqngrp03"><display-eqn id="mst322926eqn2-26" eqnnum="2.26"></display-eqn><display-eqn id="mst322926eqn2-27" eqnnum="2.27"></display-eqn></eqn-group></p><p>A fibre-optic cable transfers light into the sensor capsule with the fluorescent dye. To determine the CO<sub>2</sub> concentration of the sample medium, the instrument compares the pH-dependent fluorescence of the dye at two different wavelengths. The resulting light emission from the dye is transferred back through the cable to the monitor, which calculates the level of dissolved CO<sub>2</sub> based on ratiometric analysis of the dye's fluorescence.</p><p>The sensor capsule is easy to replace and can be autoclaved multiple times. It measures dissolved CO<sub>2</sub> over the range of 1% to 25% with an accuracy of 5% of the reading or 0.2% absolute. Over a 7 day period the sensor drift is 2% of the reading, and the 90% response time is 7 min. The sensor was found to show good shelf life for over 45 days of continuous use, depending upon the special application. Applications of this opto-chemical CO<sub>2</sub> sensor are reported particularly with regard to measurement and control of dissolved carbon dioxide in mammalian cell culture processes and microbial fermentations [<cite linkend="mst322926bib26">26</cite>].</p></sec-level2><sec-level2 id="mst322926s2-4" label="2.4"><heading>Coulometric CO<sub>2</sub> sensing principle</heading><p indent="no">Albery <italic>et al</italic> [<cite linkend="mst322926bib27">27</cite>, <cite linkend="mst322926bib28">28</cite>] have described a CO<sub>2</sub> sensor in which the titration of carbon dioxide with OH<sup>−</sup> is an essential step. The reagent hydroxide is generated by a cathodic electrolysis process. Simultaneously, the pH of the titration process is controlled by a pH electrode. The starting alkaline pH in the measured adsorbing electrolyte is precisely preset. Carbon dioxide permeating through the gas permeable membrane acidifies the solution until another defined pH has been exactly reached. Then the cathodic evolution of OH<sup>−</sup> starts, and the electric charge needed for the re-establishment of the starting pH is proportional to the amount of carbon dioxide that had infiltrated into the solution. But for the concentration measurement, the time for acidification of the analyte is taken into account—the required charge should ideally always be the same for each titration. The participating reactions can be written as follows:
<itemized-list id="mst322926il8"><list-item id="mst322926il8.1" marker="•"><p indent="no">reactions at the cathode:</p></list-item></itemized-list><display-eqn id="mst322926eqn2-28" eqnnum="2.28"></display-eqn><itemized-list id="mst322926il9"><list-item id="mst322926il9.1" marker="•"><p indent="no">titration</p></list-item></itemized-list><display-eqn id="mst322926eqn2-29" eqnnum="2.29"></display-eqn><itemized-list id="mst322926il10"><list-item id="mst322926il10.1" marker="•"><p indent="no">reaction at the anode:
<display-eqn id="mst322926eqn2-30" eqnnum="2.30"></display-eqn></p></list-item></itemized-list></p><p>At the cathode H<sub>2</sub> is evolved from water, and the remaining hydroxide neutralizes the dissolved carbon dioxide. At the anode, however, the oxidation leads to molecular oxygen and protons dissolved immediately in the solution. To avoid a rapid recombination of the ionic products of the electrode process to form water, the separation of catholyte and anolyte by a diaphragm is necessary.</p><p>Detailed investigations of the coulometric CO<sub>2</sub> sensor have been carried out by Trapp <italic>et al</italic> [<cite linkend="mst322926bib29">29</cite>] and Wiegran <italic>et al</italic> [<cite linkend="mst322926bib30">30</cite>]. A schematic drawing of their cell design is shown in figure <figref linkend="mst322926fig10">10</figref>.
<figure id="mst322926fig10"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig10.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig10.jpg"></graphic-file></graphic><caption id="mst322926fc10" label="Figure 10"><p indent="no">Schematic drawing of the coulometric cell design according to [<cite linkend="mst322926bib29">29</cite>]</p></caption></figure></p><p>The authors have constructed a measuring cell with an iridium oxide electrode as central module. This electrode serves as a cathode as well as a pH-sensing device. The iridium oxide has been realized by thermal treatment of IrCl<sub>3</sub> or by reactive sputtering of iridium in an oxidizing atmosphere. This cathode is surrounded by a screen-printed Ag/AgCl layer serving as a reference electrode. The anode is separated from the electrolyte solution by a ceramic diaphragm. A nonporous Teflon® membrane with a thickness of 12.5 µm is used as a gas permeable membrane. The electrolyte for the CO<sub>2</sub> adsorption is a mixture of 1 M KCl with 1 M BaCl<sub>2</sub> adjusted at a starting pH of 10.5, which ensures a rapid reaction between CO<sub>2</sub> and OH<sup>−</sup>. Addition of excess of Ba<sup>2+</sup> salt is chosen in order to eliminate carbonate by precipitation of hardly soluble barium carbonate. After a decrease of 0.3 pH units the cathodic regeneration of consumed hydroxide starts. Small current pulses are applied to establish the starting pH again. The electrochemical pulses are frequently interrupted to determine the potential between the reference electrode and the IrO<sub>2</sub> pH electrode for pH calculation. After reaching the alkaline starting pH again the adsorption cycle begins anew. The time for the acidification of the electrolyte is dependent on the carbon dioxide uptake or rather on the CO<sub>2</sub> concentration. In figure <figref linkend="mst322926fig11">11</figref> the principle of the pH development during adsorption and coulometric titration is pictured.
<figure id="mst322926fig11"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig11.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig11.jpg"></graphic-file></graphic><caption id="mst322926fc11" label="Figure 11"><p indent="no">Schematic diagram of the pH development during measuring cycle and coulometric titration according to [<cite linkend="mst322926bib29">29</cite>] sections I and III: measuring mode, sections II and IV: coulometric titration mode.</p></caption></figure></p><p>To increase the lifetime of the sensors, hygroscopic admixtures, e.g. glycols, are added to the sensor electrolyte to reduce its drying. These coulometric sensors allow CO<sub>2</sub> determinations in the range between 200 and 20 000 ppm in air [<cite linkend="mst322926bib29">29</cite>] and had also been successfully tested in seawater [<cite linkend="mst322926bib30">30</cite>]. They are more suitable to measure low CO<sub>2</sub> concentrations. The response time of the coulometric sensors (time for acidification versus CO<sub>2</sub> concentration) depends on the actual CO<sub>2</sub> concentration of the analyte. For a sensor with a 12.5 µm Teflon membrane it ranges from some seconds for CO<sub>2</sub> concentrations higher than 10 000 ppm to some minutes for concentrations below 1000 ppm.</p></sec-level2><sec-level2 id="mst322926s2-5" label="2.5"><heading>Conductometric CO<sub>2</sub> sensing principle</heading><p indent="no">Another principle of a carbon dioxide sensor, which also uses a membrane-covered design as discussed in section <secref linkend="mst322926s2-2">2.2</secref>, consists of conductometric measurements. One of the first publications on CO<sub>2</sub> sensors working on conductometry [<cite linkend="mst322926bib31">31</cite>] indicates that the motivation behind this effort of development was the significant decrease of the relatively long response times of equilibrium sensors to meet the requirements of medical application, i.e. breath gas monitoring. The idea is based on the fact that the relatively slow dissociation reaction of CO<sub>2</sub> in a liquid electrolyte can be excluded from the sensor response by its dislocation into a flowing electrolyte. Additionally, the flowing electrolyte causes maximum partial pressure difference on both sides of the diffusion membrane, which promotes short response times, too. Those sensors usually contain two conductometric flow-through cells upstream and downstream of the membrane diffusion zone. The sensor signal is generated by conductivity changes in the electrolyte due to CO<sub>2</sub> dissociation and measured as a difference in conductivities between the upstream and the downstream cell. The conductivity difference Δσ depends on the CO<sub>2</sub> flow across the membrane and the electrolyte flow rate and can be calculated roughly according to
<display-eqn id="mst322926eqn2-31" eqnnum="2.31"></display-eqn></p><p>During the last four decades different attempts have been made to adapt this sensor principle to the requirements of different applications [<cite linkend="mst322926bib32">32</cite>, <cite linkend="mst322926bib33">33</cite>]. One of these [<cite linkend="mst322926bib34">34</cite>] was directed to the introduction of the multilayer printed circuit board technology as used today in highly integrated electronic devices such as mobile phones and portable computers. The planar sensor design, as shown in figure <figref linkend="mst322926fig12">12</figref>, also contains two conductometric cells upstream and downstream of the diffusion zone.
<figure id="mst322926fig12"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig12.eps" width="15pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig12.jpg"></graphic-file></graphic><caption id="mst322926fc12" label="Figure 12"><p indent="no">Schematic drawing of the cross section of a planar conductivity type sensor manufactured with printed circuit board technology as a four-layer structure, all dimensions in µm.</p></caption></figure></p><p>All electrodes are introduced as Pt foils with a thickness of 20 µm during the build-up process of the multilayer structure with a distance of 100 µm between the two electrodes of every cell. The micro-channel with a cross section of 300 × 30 µm<sup>2</sup> and 10 mm length is engraved in the upper layer by milling. Silicon membranes of different thicknesses are glued onto the surface of the substrate. Due to the relatively low H<sub>2</sub>CO<sub>3</sub> concentrations in the flow at the downstream cell the sensor is fed with deionized water with conductivities below 1 µS cm<sup>−1</sup> at different flow rates.</p><p>As illustrated in figure <figref linkend="mst322926fig13">13</figref> the response time decreases slightly with increasing flow. The sensor responds faster to 5 vol.% CO<sub>2</sub> than an equilibrium-type sensor described in section <secref linkend="mst322926s2-2">2.2</secref>. The increase in the conductivity of the electrolyte during its passage through the diffusion zone on exposing the sensor to N<sub>2</sub> atmosphere is mainly caused by copper corrosion in the flow channel, which can be prevented by covering the cannel walls with a thin gold layer.
<figure id="mst322926fig13"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig13.eps" width="18pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig13.jpg"></graphic-file></graphic><caption id="mst322926fc13" label="Figure 13"><p indent="no">Response behaviour of a planar conductometric CO<sub>2</sub> sensor according to figure <figref linkend="mst322926fig12">12</figref> during sudden changes between the CO<sub>2</sub> concentration, membrane thickness 25 µm, membrane material silicon, temperature 22 °C, electrolyte deionized water, flow rates (μL min<sup>−1</sup>): a: 20; b: 40; c: 80.</p></caption></figure></p><p>To understand the sensor behaviour, the concentrations of dissolved CO<sub>2</sub> and H<sub>2</sub>CO<sub>3</sub> in the electrolyte were also investigated by numerical simulation on the basis of the software CFX solver (Ansys, Germany). The hexagonal grid, which fills the channel and the conductivity cells, contains 80 000 nodes. The calculated H<sub>2</sub>CO<sub>3</sub>-concentration profile at the CO<sub>2</sub> concentration 5 vol.% outside the membrane, as shown in figure <figref linkend="mst322926fig14">14</figref>, indicates that the regions of higher concentrations are situated at the edges of the channel. Here the flow velocity is lower and therefore more CO<sub>2</sub> can dissociate than in the channel centre.
<figure id="mst322926fig14"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig14.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig14.jpg"></graphic-file></graphic><caption id="mst322926fc14" label="Figure 14"><p indent="no">Simulated concentration profile in the diffusion channel at the position 7 mm downstream of the electrolyte inlet, H<sub>2</sub>CO<sub>3</sub> concentration in 10<sup>−4</sup> mg L<sup>−1</sup>: a: 3.6–4; b: 3.0–3.6; c: 2.8–3.0; d: 2.6–2.8.</p></caption></figure></p><p>The temporal change of the concentration profile in the downstream conductivity cell was also simulated after a sudden change in the CO<sub>2</sub> concentration outside the membrane. For a real sensor under similar conditions, measured response curves were compared with a calculated curve as illustrated in figure <figref linkend="mst322926fig15">15</figref>. The simulated response time for the membrane thickness 30 µm fits very well the two experimental results for 15 and 25 µm. Therefore, the established simulation model is suited to describe the sensor behaviour and will deliver appropriate leads for design optimization.
<figure id="mst322926fig15"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig15.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig15.jpg"></graphic-file></graphic><caption id="mst322926fc15" label="Figure 15"><p indent="no">Comparison between experimental and simulation results of the sensor response to a concentration change between 0 and 5 vol.% CO<sub>2</sub> for different membrane thicknesses: a: experimental, <italic>d</italic> = 15 µm, <italic>t</italic><sub>90</sub> = 1.9 s; b: simulation, <italic>d</italic> = 30 µm, <italic>t</italic><sub>90</sub> = 3.7 s; c: experimental, <italic>d</italic> = 25 µm, <italic>t</italic><sub>90</sub> = 4.9 s.</p></caption></figure></p></sec-level2><sec-level2 id="mst322926s2-6" label="2.6"><heading>Miniaturization of electrochemical carbon dioxide sensors with liquid electrolytes</heading><p indent="no">The ‘classical’ electrochemical CO<sub>2</sub> sensor is appropriate for many fields of application, but its use in small sample volumes is not possible. The necessary dimensions caused by the glass electrode are obstacles to extend this device in the micro scale. A simple reduction of the contact area is not sufficient enough, because the bicarbonate volume in front of the pH electrode needs a certain amount of permeating carbon dioxide from the analyte sample volume to establish the equilibrium at the whole pH-sensitive surface. To solve this problem ion-selective field effect transistors (ISFETs) have been introduced. These transducers allow the miniaturization of the sensing area down to the mm<sup>2</sup> range, are robust and shatterproof, and the low-ohmic resistance reduces the requirements for the signal measurement. The sensitivity of the pH-ISFETs is induced by pH-sensitive layers on the gate, mostly based on Si<sub>3</sub>N<sub>4</sub>, Al<sub>2</sub>O<sub>3</sub> or Ta<sub>2</sub>O<sub>5</sub>. They show near Nernstian slope and short response times.</p><p>The main focus during the development of ISFET-based CO<sub>2</sub> sensors had to be stressed on the integration of the reference electrode and on the covering of the sensing area by the inner electrolyte and by the gas permeable membrane.</p><p>Two exemplary designs will be presented more in detail. Shoji <italic>et al</italic> [<cite linkend="mst322926bib35">35</cite>] reported on a micro flow cell for blood gas analysis based on ISFET modules. A cross section of the CO<sub>2</sub> module is shown in figure <figref linkend="mst322926fig16">16</figref>.
<figure id="mst322926fig16"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig16.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig16.jpg"></graphic-file></graphic><caption id="mst322926fc16" label="Figure 16"><p indent="no">Cross section of an ISFET with an Ag/AgCl electrode placed on the back [<cite linkend="mst322926bib35">35</cite>].</p></caption></figure></p><p>The Si<sub>3</sub>N<sub>4</sub> pH-ISFET was placed on the front side of the wafer facing the measuring solution. The electrolytic contact to the Ag/AgCl reference electrode positioned on the back of the transducer was established by a channel in the wafer. The gas permeable membrane consisted of a negative photoresist. To ensure a small gap between ISFET and gas permeable membrane for the bicarbonate solution, a positive photoresist had been patterned on the ISFET and removed by washing after printing of the gas permeable resin layer. The chamber volume was about 5 nL and filled with a 0.005 M bicarbonate solution containing 0.02 M sodium chloride to ensure a stable reference potential at the Ag/AgCl electrode.</p><p>The sensors showed good results in the physiological range of CO<sub>2</sub>, but response times of 3 to 5 min were not acceptable for blood gas analysis. The authors assumed that the poor permeability of carbon dioxide through by the negative photoresist was the cause of slow response.</p><p>A slight improvement of the response time had been achieved by Arquint <italic>et al</italic> [<cite linkend="mst322926bib36">36</cite>, <cite linkend="mst322926bib37">37</cite>] describing an ISFET-based CO<sub>2</sub> sensor covered with a hydrogel and polysiloxane membrane—both directly patterned on the sensor surface. Besides the Al<sub>2</sub>O<sub>3</sub> pH-ISFET on the wafer a thin-film Ag/AgCl electrode was screen printed. A cross section of this sensor type is presented in figure <figref linkend="mst322926fig17">17</figref>.
<figure id="mst322926fig17"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig17.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig17.jpg"></graphic-file></graphic><caption id="mst322926fc17" label="Figure 17"><p indent="no">Cross section of an ISFET with a screen-printed hydrogel and a siloxane membrane [<cite linkend="mst322926bib36">36</cite>].</p></caption></figure></p><p>Between <italic>p</italic>(CO<sub>2</sub>) = 50–20 000 Pa the sensitivity was found to be −49 mV/decade <italic>p</italic>(CO<sub>2</sub>) and the <italic>t</italic><sub>95</sub> time of 2 min. The response time <italic>t</italic><sub>95</sub> increased to about 5 min and the slope to −52 mV/decade <italic>p</italic>(CO<sub>2</sub>) when used in 37 °C transfusion blood. The sensor device was stable for more than 2 weeks under continuous operation.</p><p>ISFETs covered with carbonate-selective polymer membranes on the surface of the gate, combined with a reference electrode, result in a potentiometric carbonate measuring chain. Abramova <italic>et al</italic> [<cite linkend="mst322926bib38">38</cite>] showed that the measurement of carbonate with polymer matrix-based carbonate electrodes at constant pH (pH = 8.7) is possible. An influence of penetrating CO<sub>2</sub> and water molecules on the stability of the contact surface between membrane and gate of the ISFET had not been observed in contrast to the assumption of many authors.</p><p>New investigations concerning the solid-state electrolyte-based FET-type CO<sub>2</sub> sensor have been published by Shimanoe <italic>et al</italic> [<cite linkend="mst322926bib39">39</cite>]. For the use of FETs only elevated temperatures below 180 °C are acceptable, and research is focussed on solid electrolytes, which allow lower or preferably room-temperature applications of these materials. The CO<sub>2</sub>-sensitive layers developed by Shimanoe <italic>et al</italic> are sensitive to carbon dioxide even at 30 °C. The gate insulated by a Ta<sub>2</sub>O<sub>5</sub> layer was covered with the Na<sup>+</sup> form of a cation exchange membrane as an ionic conductor. An auxiliary phase for the establishment of the CO<sub>2</sub> sensitivity consisted of Li<sub>2</sub>CO<sub>3</sub>BaCO<sub>3</sub>/ITO (indium tin oxide) and was deposited on the surface. The electrochemical reaction for the detection is described as follows:
<display-eqn id="mst322926eqn2-32" eqnnum="2.32"></display-eqn></p><p>The slope of the sensors was nearly the Nernstian value for an electrochemical reaction involving a two electron step, and the response time was determined between 1 and 2 min. In particular the remarkable fact is that the sensor was not influenced by humidity between 30% and 70% r.h.</p><p>Although the ISFET has a lot of advantages, this tool has not yet reached a breakthrough in pH with respect to CO<sub>2</sub> determination. This might be due to the fact that packaging is still an ambitious challenge and that the costs for the production of these devices are too high at the moment to meet the need of the market.</p><p>Another way to miniaturize the electrode is the use of needle-type electrodes for CO<sub>2</sub> detection. Such a Severinghaus sensor with a tip diameter less than 20 µm has been developed e.g. for the investigation of biofilms. A schematic drawing of the electrode is presented in figure <figref linkend="mst322926fig18">18</figref>.
<figure id="mst322926fig18"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig18.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig18.jpg"></graphic-file></graphic><caption id="mst322926fc18" label="Figure 18"><p indent="no">Design of a Severinghaus-type <italic>p</italic>(CO<sub>2</sub>) microelectrode.</p></caption></figure></p><p>The main element of the probe is the pH electrode, which has to be placed near the gas permeable membrane on the tip of the sensor. This requires special pH electrodes with a small diameter, too. For the establishment of the pH sensor two different ways have been examined. Zhao <italic>et al</italic> [<cite linkend="mst322926bib40">40</cite>] and de Beer <italic>et al</italic> [<cite linkend="mst322926bib41">41</cite>] use an appropriate liquid exchange membrane for the pH detection. This pH-sensitive viscous liquid can be placed in the tip of another needle and when combined together with a reference electrode allows a reliable H<sup>+</sup> detection. These ionophore-based electrodes showed Nernstian behaviour between <italic>p</italic>(CO<sub>2</sub>) = 50 and 5000 Pa.</p><p>Beyenal <italic>et al</italic> [<cite linkend="mst322926bib42">42</cite>], in contrast, decided to use pH-sensitive iridium oxide for the determination of the <italic>p</italic>(CO<sub>2</sub>)-dependent pH change. The AIROF (anodically grown iridium oxide film) tips were produced by electrochemical oxidation of an Ir wire, melted in a glass tube, in 0.5 M sulfuric acid. These pH electrodes showed a super-Nernstian sensitivity between 65 and 80 mV pH<sup>−1</sup>. For the corresponding CO<sub>2</sub> sensors the slope was found to be between 65 and 76 mV/decade <italic>p</italic>(CO<sub>2</sub>) for a linear range up to <italic>p</italic>(CO<sub>2</sub>) = 3000 Pa at the detection limit of 15 Pa. The response time was about 2 min for a change from <italic>p</italic>(CO<sub>2</sub>) = 65 Pa to 3000 Pa and 8 min for the reverse direction. This kind of sensors has been used successfully for carbon dioxide detection in <italic>Staphylococcus aureas</italic> biofilms.</p></sec-level2><sec-level2 id="mst322926s2-7" label="2.7"><heading>Quinhydrone CO<sub>2</sub> electrode</heading><p indent="no">To simplify the relative complex design of the pH sensor inside an equilibrium-type CO<sub>2</sub> sensor the introduction of an easily fabricable pH electrode on the basis of the quinhydrone system was suggested and investigated relatively early [<cite linkend="mst322926bib31">31</cite>]. The detection principle resides on the oxidation and reduction of hydroquinone (H<sub>2</sub>Q) and quinone (Q), respectively. The corresponding potential of the redox electrode is described by
<display-eqn id="mst322926eqn2-33" eqnnum="2.33"></display-eqn></p><p>At equal activities of quinone and hydroquinone (the quinhydrone system) the electrode potential depends on temperature and hydrogen ion activity only. The performance of this electrode is comparable to that of the conventional CO<sub>2</sub> electrode. It has the advantage of being an inexpensive low-impedance electrode with a possibility for miniaturization.</p><p>Additional investigations and developments, which were directed to the utilization of this electrochemical system for CO<sub>2</sub> sensors, were published in a variety of papers [<cite linkend="mst322926bib43" range="mst322926bib43,mst322926bib44,mst322926bib45">43–45</cite>].</p></sec-level2></sec-level1><sec-level1 id="mst322926s3" label="3"><heading>Solid electrolyte CO<sub>2</sub> sensors</heading><sec-level2 id="mst322926s3-1" label="3.1"><heading>Basic principles</heading><p indent="no">Electrochemical cells based on solid electrolytes operating at temperatures between 350 and 1400 °C are commonly used to measure directly gaseous and in molten metals dissolved oxygen in this temperature range. Such cells exhibit a lot of advantages. They work potentiometrically according to the Nernst equation or in coulometric mode according to Faraday's law over a long time without calibration with response times less than 1 s. The solid electrolyte that is used for such cells is an oxide ion conductor. Mostly yttria-stabilized zirconia (YSZ) is utilized for oxygen sensors [<cite linkend="mst322926bib46">46</cite>]. Such sensors are widely used to control high-temperature processes such as combustions for industrial and automotive applications. Therefore, it seemed to be successful to follow this strategy for other gases.</p><p>However, there is no solid electrolyte available in which carbonate anions are mobile. Nevertheless, potentiometric determination is possible if an electrochemical equilibrium between the ions of solid electrolytes, the CO<sub>2</sub> and electrons in the electrodes can be established. Solid alkali carbonates are solid electrolytes at temperatures >300 °C because the charge carriers, the alkaline ions, are mobile via alkali ion vacancies <italic>V</italic>′<sub valign="yes">Na</sub>. That is due to the intrinsic disorder of Frenkel type:
<display-eqn id="mst322926eqn3-1" eqnnum="3.1"></display-eqn></p><p>The mobility and therefore the electrical conductivity can be enhanced by doping with divalent or trivalent ions. In that case the doping leads to alkaline ion vacancies [<cite linkend="mst322926bib47">47</cite>]:
<display-eqn id="mst322926eqn3-2" eqnnum="3.2"></display-eqn></p><p>For the equations Kröger–Vink's relative notation is used. With such solid carbonates Nernstian gas concentration cells can be built up. These cells consist of sintered gas-tight sodium carbonate (or doped sodium carbonate) covered on both sides with gold layers that separate the two gas chambers (figure <figref linkend="mst322926fig19" override="yes">19(<italic>A</italic>)</figref>). According to the following equations, the cell emf <italic>E</italic> (or cell voltage <italic>U</italic>) depends on both concentrations of CO<sub>2</sub> and O<sub>2</sub>:
<eqn-group id="mst322926eqngrp04"><display-eqn id="mst322926eqn3-3" lines="multiline" eqnnum="3.3" eqnalign="left"></display-eqn><display-eqn id="mst322926eqn3-4" lines="multiline" eqnnum="3.4" eqnalign="left"></display-eqn><display-eqn id="mst322926eqn3-5" lines="multiline" eqnnum="3.5" eqnalign="left"></display-eqn></eqn-group><figure id="mst322926fig19"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig19.eps" width="15pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig19.jpg"></graphic-file></graphic><caption id="mst322926fc19" label="Figure 19"><p indent="no">Basic principles of solid electrolyte CO<sub>2</sub> sensors.</p></caption></figure></p><p>The charge compensation occurs by migration of sodium ions from the side having lower CO<sub>2</sub> and O<sub>2</sub> concentrations to the side with higher concentration. If a current flows sodium carbonate is formed on the side with higher gas concentration and it disappears on the other side. This behaviour is different to that of oxygen concentration cells using stabilized zirconia. This is the reason why the measurement of cell emf should be made with an instrument having high input impedance. Carbonate cells cannot be used stably in amperometric or coulometric mode. Nevertheless, the sintered solid carbonates remain gas-tight over a long time. Using such cells CO<sub>2</sub> can be measured continuously from ppm to the percentage level over more than 1 year in the cell temperature range of 450–700 °C according to Nernst's equation [<cite linkend="mst322926bib48">48</cite>, <cite linkend="mst322926bib49">49</cite>] provided that the cell is continuously heated.</p><p>There are two main problems which had to be solved for practical applications. Pressed alkali carbonates are often hygroscopic and tend to sublimate so that electrolytes become porous. Besides this effect the realization of a stable reference system is a topic of interest. To establish a gas reference CO<sub>2</sub> and O<sub>2</sub> concentrations have to be fixed. This is not possible using air (as in the case of oxygen concentration cells) because the CO<sub>2</sub> content in the air is normally not fixed (see below).</p><p>To overcome this problem of the solid electrolytes, various admixtures of alio valent carbonates (homogeneous doping) and of γ-alumina (heterogeneous doping) were proposed to enhance the mechanical strength of the carbonates and to increase the electrical conductivity [<cite linkend="mst322926bib50">50</cite>, <cite linkend="mst322926bib51">51</cite>].</p><p>The crucial idea was to combine alkali carbonates with a stable alkali ion conductor such as Nasicon, β′-alumina or β″-alumina as an electrolyte (figure <figref linkend="mst322926fig19" override="yes">19(<italic>B</italic>)</figref>) [<cite linkend="mst322926bib52">52</cite>]. Nasicon is a three-dimensional sodium ion conductor having the general chemical composition: Na<sub>1+<italic>x</italic></sub>Zr<sub>2</sub>Si<italic><sub>x</sub></italic>P<sub>3−</sub><italic><sub>x</sub></italic>O<sub>12</sub>. The stoichiometric factor for sodium can vary from 1 to 3. With Na<sup>+</sup> = 2.2 the compound exhibits the highest electrical conductivity. β-alumina is a two-dimensional sodium ion conductor of the general composition Na<sub>2</sub>O⋅11Al<sub>2</sub>O<sub>3</sub>. β″-alumina contains only 5–7 Al<sub>2</sub>O<sub>3</sub> per unit Na<sub>2</sub>O and is often stabilized by MgO. β,β″-Alumina can be formed as a thin layer on an α-alumina substrate [<cite linkend="mst322926bib53">53</cite>, <cite linkend="mst322926bib54">54</cite>]. These electrolytes are gas-tight so that two separate compartments can be realized:
<display-eqn id="mst322926eqn3-6" lines="multiline" eqnnum="3.6" eqnalign="left"></display-eqn><display-eqn id="mst322926eqn3-7" lines="multiline" eqnnum="3.7" eqnalign="left"></display-eqn><display-eqn id="mst322926eqn3-8" lines="multiline" eqnnum="3.8" eqnalign="left"></display-eqn></p><p>At the measuring electrode CO<sub>2</sub> and O<sub>2</sub> are electrochemically transformed, whereas on the reference side oxygen is reduced. At the phase boundary between sodium ion conducting carbonate and sodium ion conductor, e.g. β-alumina, only sodium ions can be exchanged. The sensitive electrode consisting of Na<sub>2</sub>CO<sub>3</sub>/Au should be porous. After Weppner [<cite linkend="mst322926bib53">53</cite>] this type of sensor is called a potentiometric sensor type III. The gradual loss in carbonate leads to an increase of porosity and can diminish the three phase contacts between gold, carbonate and gas.</p><p>The reference electrode potential can be established by fixing sodium oxide activity using Na<italic><sub>x</sub></italic>(Hg) and Na<italic><sub>x</sub></italic>CoO<sub>2</sub>, respectively [<cite linkend="mst322926bib55">55</cite>, <cite linkend="mst322926bib56">56</cite>]. It is also possible to use a gold or a platinum layer covered by a glass. Under these circumstances the oxygen surface concentration and hence the oxide ion activity is fixed if no current flows. An elegant solution is to fix the O<sub>2</sub> by a solid thermodynamic system such as SiO<sub>2</sub>: Na<sub>2</sub>Si<sub>2</sub>O<sub>5</sub> (figure <figref linkend="mst322926fig19" override="yes">19(<italic>C</italic>)</figref>) [<cite linkend="mst322926bib57">57</cite>, <cite linkend="mst322926bib58">58</cite>]. Then the reaction at the reference electrode turns out to be
<display-eqn id="mst322926eqn3-9" lines="multiline" eqnnum="3.9" eqnalign="left"></display-eqn>and hence the net cell reaction results in
<display-eqn id="mst322926eqn3-10" eqnnum="3.10" eqnalign="left"></display-eqn>If the solid components are pure then the cell emf depends only on <italic>p</italic>(CO<sub>2</sub>):
<display-eqn id="mst322926eqn3-11" eqnnum="3.11"></display-eqn></p><p>Instead of the silica, silicate system, other systems such as titania, titanate or zirconia, zirconate can also be used as reference systems [<cite linkend="mst322926bib59">59</cite>]. In such cases no gas separation is necessary because only one electrode is gas sensitive. Other reference systems based on contacts with an additional solid electrolyte (YSZ) were also proposed [<cite linkend="mst322926bib60">60</cite>].</p><p>The response time of freshly fabricated thick film sensors based on thin film β-alumina is very short (figure <figref linkend="mst322926fig20">20</figref>, about 11 ms at 650 °C).
<figure id="mst322926fig20"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig20.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig20.jpg"></graphic-file></graphic><caption id="mst322926fc20" label="Figure 20"><p indent="no">Response behaviour of a CO<sub>2</sub>-solid electrolyte sensor during a pressure leap (our own results).</p></caption></figure></p><p>After several weeks of operation this time increases tenfold (150 ms) [<cite linkend="mst322926bib61">61</cite>]. Solid electrolyte CO<sub>2</sub> sensors using the Ni/carbonate composite as a measuring electrode are suited for measurement of CO<sub>2</sub> in equilibrated water gases [<cite linkend="mst322926bib62">62</cite>]. Some authors reported that Nasicon-based CO<sub>2</sub> sensors using mixtures of semi-conducting oxides and carbonates such as SnO<sub>2</sub> or ITO (indium tin oxide) [<cite linkend="mst322926bib63">63</cite>] and In<sub>2</sub>O<sub>3</sub> [<cite linkend="mst322926bib64">64</cite>] as electrodes are able to measure at room temperature. But all solid electrolyte-based sensors which are commercially available operate at high temperature.</p></sec-level2><sec-level2 id="mst322926s3-2" label="3.2"><heading>Technology</heading><sec-level3 id="mst322926s3-2-1" label="3.2.1"><heading>General setup</heading><p indent="no">Commercially available CO<sub>2</sub> solid electrolyte sensors can be divided into two main types: the pellet-shape design and the thick film setup.</p><sec-level4 id="mst322926s3-2-1-1" label="3.2.1.1"><heading>Pellet sensor</heading><p indent="no">The setup of the electrochemical cell according to the principle is shown in figure <figref linkend="mst322926fig19">19</figref> and its cross section in figure <figref linkend="mst322926fig21">21</figref>. Three sintered pellets are arranged by a spring load mechanism in a heatable quartz tube as a measuring electrode, an auxiliary solid electrolyte and a reference electrode, respectively. The measuring electrode consists of an alkaline carbonate, mostly sodium carbonate, mixed with small gold particles. The pellets are produced by uni-axial pressing of corresponding powders and subsequently sintering them [<cite linkend="mst322926bib65">65</cite>]. All three parts of the cell were modified to overcome some drawbacks, which are described in the literature in more detail [<cite linkend="mst322926bib58">58</cite>, <cite linkend="mst322926bib59">59</cite>].
<figure id="mst322926fig21"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig21.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig21.jpg"></graphic-file></graphic><caption id="mst322926fc21" label="Figure 21"><p indent="no">Schematic cross section of a CO<sub>2</sub> sensor using pellet design (courtesy Zirox® GmbH Greifswald, Germany).</p></caption></figure></p><p>According to our own investigations the long-term stability can be influenced by the sublimation of alkaline carbonates. This phenomenon can be easily observed in the carbonate consisting dendrites formed in the colder part of the oven. Carbonates can move via a gas phase onto the reference side and influence the cell voltage. Additionally, the loss of carbonates leads to a loss of the three phase contacts on the measuring electrode, which in turn is connected with an increase of the response time. Impedance studies are useful to investigate kinetic and aging effects of sensors [<cite linkend="mst322926bib66">66</cite>].</p><p>The fabrication of a sensor with pellet-shape design needs a lot of manual work and cannot be easily automated. Nevertheless, sensors according to this design are commercially available and operate at the temperature 600 °C over a very long time (several years) in clean air without calibration (figure <figref linkend="mst322926fig21">21</figref>).</p></sec-level4><sec-level4 id="mst322926s3-2-1-2" label="3.2.1.2"><heading>Thick film sensor</heading><p indent="no">As an alternative, a thick film sensor was developed by several groups, which can be produced economically on a large scale [<cite linkend="mst322926bib59">59</cite>, <cite linkend="mst322926bib67" range="mst322926bib67,mst322926bib68,mst322926bib69,mst322926bib70">67–70</cite>]. As an electrolyte a sheet of pure β,β″-alumina or Nasicon may be used. The drawback of such conducting materials is that the metallic heater structures cannot be printed directly on the reverse side of the sheets without an additional insulating layer. An elegant method is to modify a thick film substrate, e.g. an α-alumina substrate, by a thin film of β,β″-alumina via a high-temperature topotactically growing process. By this means the mechanical strength of the α-alumina and the sodium ion conductivity are combined. Moreover, on the reverse side of the substrate (the β-alumina layer was removed) a platinum heater can be printed so that there is no influence of the cell voltage by the heater voltage. The setup of such a sensor is shown schematically in figure <figref linkend="mst322926fig22">22</figref>.
<figure id="mst322926fig22"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig22.eps" width="15pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig22.jpg"></graphic-file></graphic><caption id="mst322926fc22" label="Figure 22"><p indent="no">Cross section of a thick film sensor using a β-alumina layer on an alumina substrate. 1: ceramic substrate; 2: ß″-alumina solid electrolyte thin film; 3: heater; 4: Au/SiO<sub>2</sub>/Na<sub>2</sub>Si<sub>2</sub>O<sub>5</sub> mix (reference electrode); 5: Au/Na<sub>2</sub>CO<sub>3</sub>/BaCO<sub>3</sub> mix (measuring electrode).</p></caption></figure></p><p>The preparation of layered alumina is shown in figure <figref linkend="mst322926fig23">23</figref>. A thin layer of β-alumina (few µm thick) is formed by tempering the α-alumina substrate in a bed of β-alumina powder at 1000 °C for 12 h. The alkaline ion can be easily exchanged by other mono or divalent ions. The conditions for the exchange process and the results are given in figure <figref linkend="mst322926fig23">23</figref>. The excess of ion exchange could be observed by <italic>x-ray diffraction</italic> (XRD) and energy-dispersive x-ray (EDX) investigations. Also, the conductivity, given in figure <figref linkend="mst322926fig24">24</figref>, for the divalent-doped material is high enough for the use in potentiometric cells [<cite linkend="mst322926bib71">71</cite>].
<figure id="mst322926fig23" width="page"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig23.eps" width="26pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig23.jpg"></graphic-file></graphic><caption id="mst322926fc23" label="Figure 23"><p indent="no">SEM images of the layered alumina substrates, (<italic>A</italic>) Na<sup>+</sup>-β″-Al<sub>2</sub>O<sub>3</sub>, (<italic>B</italic>) K<sup>+</sup>-β″-Al<sub>2</sub>O<sub>3</sub>, (<italic>C</italic>) Li<sup>+</sup>-β″-Al<sub>2</sub>O<sub>3</sub>, H<sub>3</sub>O<sup>+</sup>-β″-Al<sub>2</sub>O<sub>3</sub>, conditions for the ion exchange process, see table <tabref linkend="mst322926tab02">2</tabref>.</p></caption></figure><figure id="mst322926fig24"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig24.eps" width="16.8pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig24.jpg"></graphic-file></graphic><caption id="mst322926fc24" label="Figure 24"><p indent="no">Temperature dependence of the conductivity in the Arrhenius like plot for ion exchanged β″-alumina.</p></caption></figure></p><p>For the fabrication of thick film sensors mostly the screen printing technology was used. Although the sensor design looks very easy and the fabrication process seems to be also simple, the windows for the process parameters are small. Even the firing temperature turns out to be very important for the sensor behaviour.</p><p>In order to achieve a good adhesion this temperature should be high enough. On the other hand, the higher the temperature the more reaction between the different materials of the layers can take place. In this contact region a new phase with different properties is formed. This in turn leads to both a deviation in cell emf and a longer response time as can be seen in figure <figref linkend="mst322926fig25">25</figref>.
<figure id="mst322926fig25"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig25.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig25.jpg"></graphic-file></graphic><caption id="mst322926fc25" label="Figure 25"><p indent="no">Influence of the firing temperature of Nasicon on the CO<sub>2</sub> signal.</p></caption></figure></p></sec-level4></sec-level3><sec-level3 id="mst322926s3-2-2" label="3.2.2"><heading>Technological experiences</heading><p indent="no">In this section some own experimental details obtained on thick film CO<sub>2</sub> sensors are reported, above all concerning the influence of the electrolyte.</p><p>The sensor consists of two electrolytes: the carbonate electrolyte which is necessary for the electrochemical (sensing) process and the auxiliary electrolyte in which no electrochemical reaction with CO<sub>2</sub> but a reaction with oxygen and a subsequent alkaline ion transport takes place. (Sometimes the carbonate/Au phase is called the auxiliary phase. But this phase is an essential one, whereas the alkali ion electrolyte is not obligatory.) Both electrolytes were modified mainly in order to improve the sensor stability and its selectivity and to reduce the cross sensitivity towards water vapour and combustibles [<cite linkend="mst322926bib58">58</cite>, <cite linkend="mst322926bib59">59</cite>, <cite linkend="mst322926bib64">64</cite>]. Although the conductivity of pure SrCO<sub>3</sub> is much lower than that of sodium or sodium-doped compounds, CO<sub>2</sub> measurements are possible with nearly the same time characteristics (figure <figref linkend="mst322926fig27">27</figref>, bottom). This result suggests that the conductivity of a solid electrolyte is not important for the sensor response.
<figure id="mst322926fig27"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig27.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig27.jpg"></graphic-file></graphic><caption id="mst322926fc27" label="Figure 27"><p indent="no">Cell voltage response of electrolyte-modified cells.</p></caption></figure></p><p>The auxiliary electrolyte has no influence on the cell performance. Different compositions of Nasicon and β-alumina were tested successfully [<cite linkend="mst322926bib54">54</cite>, <cite linkend="mst322926bib56">56</cite>, <cite linkend="mst322926bib61">61</cite>]. As observed in our own experiments it seems that Nasicon is less stable against alkali carbonate than β-aluminas.</p><p>In this thick film arrangement CO<sub>2</sub> can be measured with a sodium carbonate electrolyte mixed with gold particles over a temperature range of 250 to 700 °C according to Nernst's equation (figure <figref linkend="mst322926fig26">26</figref>), whereas with lithium carbonate this is possible only at temperatures >500 °C.
<figure id="mst322926fig26"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig26.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig26.jpg"></graphic-file></graphic><caption id="mst322926fc26" label="Figure 26"><p indent="no">Temperature dependence of the cell voltage of Na<sub>2</sub>CO<sub>3</sub> and Li<sub>2</sub>CO<sub>3</sub> cells.</p></caption></figure></p><p>The doping of Na<sub>2</sub>CO<sub>3</sub> with a small amount of BaCO<sub>3</sub> has only a small influence on the cell voltage (figure <figref linkend="mst322926fig27">27</figref>).</p><p>The cross sensitivity towards water vapour is displayed in figure <figref linkend="mst322926fig28">28</figref>. In general, the sensitivity is low. The influence on the signal can be explained by a different heat conductivity of gas with and without water vapour. Taking into account the results of impedance measurements shown as Nyquist plots (figure <figref linkend="mst322926fig28">28</figref>) we can conclude that water reduces the total cell resistance in fresh cells. This result suggests that water plays an important role in electrode kinetics.
<figure id="mst322926fig28"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig28.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig28.jpg"></graphic-file></graphic><caption id="mst322926fc28" label="Figure 28"><p indent="no">Influence of water vapour on the cell voltage and on the impedance behaviour.</p></caption></figure></p><p>Higher concentrations of combustibles in air disturb the signal. If the temperature and the residence time are high enough, the organic components are burnt off completely so that a higher CO<sub>2</sub> value (a lower cell voltage) is displayed. At temperatures lower than 500 °C the gold electrode is also sensitive to combustibles and a mixed potential, similar to that observed in YSZ cells at low temperatures, with lower values is established (figure <figref linkend="mst322926fig29">29</figref>). This influence can be avoided by removing the combustible components from the measuring gas by an activated charcoal filter.
<figure id="mst322926fig29"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig29.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig29.jpg"></graphic-file></graphic><caption id="mst322926fc29" label="Figure 29"><p indent="no">Influence of combustibles on the signal 2-propanol (<italic>A</italic>) and acetone (<italic>B</italic>); the dotted lines are calculated assuming a complete combustion.</p></caption></figure></p></sec-level3></sec-level2></sec-level1><sec-level1 id="mst322926s4" label="4"><heading>Non-dispersive infrared CO<sub>2</sub> sensors</heading><sec-level2 id="mst322926s4-1" label="4.1"><heading>Basic principle and general setup</heading><p indent="no">Non-dispersive infrared (NDIR) sensors use the concentration-dependent absorption of electromagnetic radiation in the IR range. Unlike spectrometers, often used to identify materials, NDIR sensors do not comprise any dispersive optical component but colour filters, hence avoiding the most cost-driving elements in spectrometers. Therefore, NDIR analysis can be considered a special operating case of infrared absorption spectrometers where not the entire spectrum but only the transmission for single or several selected wavelengths is recorded. Figure <figref linkend="mst322926fig30">30</figref> compares the functional principal of NDIR gas sensors with that of IR spectrometers.
<figure id="mst322926fig30"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig30.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig30.jpg"></graphic-file></graphic><caption id="mst322926fc30" label="Figure 30"><p indent="no">Schematic setup of (<italic>A</italic>) an absorption spectrometer and (<italic>B</italic>) a multi-spectral NDIR gas sensor for two gases.</p></caption></figure><figure id="mst322926fig31"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig31.eps" width="8pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig31.jpg"></graphic-file></graphic><caption id="mst322926fc31" label="Figure 31"><p indent="no">Oscillation modes of the CO<sub>2</sub> molecule, further explanation in table <tabref linkend="mst322926tab05">5</tabref>.</p></caption></figure></p><p>The key components of NDIR sensors are an IR source (lamp), a sample chamber or light tube (cuvette), a wavelength filter, and an infrared detector (table <tabref linkend="mst322926tab03">3</tabref>).
<table id="mst322926tab03" frame="topbot"><caption id="mst322926tc03" label="Table 3"><p indent="no">Structural components of NDIR gas sensors.</p></caption><tgroup cols="2"><colspec colnum="1" colname="col1" align="left" colwidth="6pc"></colspec><colspec colnum="2" colname="col2" align="left" colwidth="35pc"></colspec><thead><row><entry>Component</entry><entry>Functions</entry></row></thead><tbody><row><entry>IR radiation source</entry><entry>• IR lamp, i.e. spectral broadband thermal emitter, mainly IR incandescent lamps
• Light-emitting diodes (LEDs) or laser diodes as quantum emitters with an emission wavelength that is specifically selected according to the gas to be detected (see table <tabref linkend="mst322926tab06">6</tabref>); instead of the combination of broadband thermal emitter, beam splitter and IR colour filter</entry></row><row><entry>Probe gas or liquid</entry><entry>• Fluids: confined between IR-transparent mineral salt plates, layer thicknesses of about 10 µm, for diluted solutions 0.1–10 mm, in order to achieve a sufficiently large absorption length
• Gases: measuring cuvettes (glass cells) of a length of 5–10 cm, at the ends closed by alkali halide windows. At low pressures, large absorption lengths for gases are achieved by multiple reflection in the cuvette
• Transmitting atmosphere</entry></row><row><entry>Optics</entry><entry>• Entrance aperture for a sensor
• Limits the spectral range</entry></row><row><entry>Beam splitter</entry><entry>• Splits the radiation as evenly as possible between reference and measuring channels (mainly mirror systems, e.g. mirrored pyramids for four-channel sensors)</entry></row><row><entry>IR filter</entry><entry>• Usually a colour filter
• Eliminates all light except the wavelength that the selected gas molecules can absorb
• Mainly interference filters due to its narrow-band nature and high edge steepness, most recently also integrated Fabry–Perot interferometers</entry></row><row><entry>IR detector</entry><entry>• Converts the gas-specific incident radiation according to elements into an electric signal
• Mainly thermopiles or pyroelectric sensors</entry></row><row><entry>Sensor signal processing</entry><entry>• Amplifies with the lowest noise possible
• Compensates interferences from ambient temperature fluctuation
• Where required compensates the effects of the atmosphere in the propagation path by reference measurements in the reference channel</entry></row></tbody></tgroup></table></p><p>The gas is pumped or diffuses into the sample chamber. The IR light is directed through the sample chamber towards the detector. It is absorbed at a specific wavelength in the IR. According to the Lambert–Beer law, absorption depends on both the path length-dependent attenuation of the radiant intensity <italic>I</italic> during passage through the absorbing gas (Bouguer–Lambert law) and the gas concentration <italic>c</italic> (Beer's law):
<display-eqn id="mst322926eqn4-1" eqnnum="4.1"></display-eqn>where <italic>E</italic><sub>λ</sub> is the extinction (absorbance of light with wavelength λ in matter), <italic>I</italic><sub>0</sub> and <italic>I</italic><sub>1</sub> are the radiant intensities of incident and transmitted light, <italic>c</italic> is the concentration of the absorbing substance (gas; in mol dm<sup>−3</sup> or mol L<sup>−1</sup>), ϵ<sub>λ</sub> is the decade extinction coefficient (in mol<sup>−1</sup> dm<sup>2</sup>), and <italic>d</italic> is the thickness of the matter. Actually, the Lambert–Beer law is valid just for monochromatic radiation. Since the relationship for high gas concentrations is nonlinear, equation (<eqnref linkend="mst322926eqn4-1">4.1</eqnref>) can be applied only for dilute solutions and gases.</p><p>The decay of radiation intensity due to transition of the gas in the cuvette results from equation (<eqnref linkend="mst322926eqn4-1">4.1</eqnref>) and can be described by an exponential function:
<display-eqn id="mst322926eqn4-2" eqnnum="4.2"></display-eqn>with ϵ<sub>λ</sub> = log (e)ϵ*<sub valign="yes">λ</sub> ≈ 0.434 ϵ*<sub valign="yes">λ</sub> as an extinction coefficient. Similar to the refractive index <italic>n</italic>, the extinction <italic>ϵ</italic><sub>λ</sub> is also dependent on the wavelength λ of the incident radiation. The Lambert–Beer law can also be used for gas concentrations in the atmosphere. Then, equation (<eqnref linkend="mst322926eqn4-2">4.2</eqnref>) results in
<display-eqn id="mst322926eqn4-3" eqnnum="4.3"></display-eqn>where <italic>m</italic> is the atmospheric mass and τ<sub>gas</sub> is the optical thickness (refractive index multiplied by geometrical thickness) of the different absorbing (CO<sub>2</sub>, O<sub>2</sub>, O<sub>3</sub>, water vapour, etc) and scattering gases (O<sub>2</sub>, N<sub>2</sub>). τ<sub>gas</sub> has to be determined for evaluating satellite images.</p></sec-level2><sec-level2 id="mst322926s4-2" label="4.2"><heading>Molecular absorption in the IR</heading><p indent="no">Atoms and molecules show a large variety of interactions with electromagnetic radiation. This refers to all frequency and wavelength ranges (table <tabref linkend="mst322926tab04">4</tabref>).
<table id="mst322926tab04" frame="topbot"><caption id="mst322926tc04" label="Table 4"><p indent="no">Interaction of atoms and molecules with electromagnetic radiation.</p></caption><tgroup cols="2"><colspec colnum="1" colname="col1" align="left" colwidth="6.5pc"></colspec><colspec colnum="2" colname="col2" align="left" colwidth="34.5pc"></colspec><thead><row><entry>Wavelength range</entry><entry>Effect</entry></row></thead><tbody><row><entry>Radio frequency range 100 m–1 cm</entry><entry>Electron spin causes a very small magnetic dipole; inverting the spin causes a change of the dipole's spatial direction that interacts with the magnetic part of the radiation and leads to absorption or emission</entry></row><row><entry>Microwave range 1 cm–10 µm</entry><entry>Molecules with permanent polar dipole moment align according to the electric component of the radiation (rotation); this leads to a wavelength-dependent absorption or emission</entry></row><row><entry>Infrared (IR) range 100–0.78 µm</entry><entry>Oscillations of the atoms in the molecules that cause a change of the dipole moment interact with the electric component of the radiation (IR-active). If the dipole moment in the molecule remains constant for specific oscillation modes, it is IR-inactive</entry></row><row><entry>Visible (VIS)/ultraviolet (UV) range 0.78 µm–10 nm</entry><entry>Alternating electric field of the radiation induces a periodic deflection of the electrons in the molecules and thus a change in the dipole moment; it causes the absorption of the radiation at resonance frequency of the deflection</entry></row></tbody></tgroup></table></p><p>The usage of IR radiation is of particular importance because the interaction is based on the spontaneous polarization in an electric field and it does not necessarily need permanent dipoles. This makes it advantageous for gas or liquid analysis. Due to the large degrees of freedom of oscillations of atoms or groups of atoms within a molecule, the infrared spectrum of large molecules shows numerous oscillations. Fundamental oscillations of linear molecules that do not generate a dipole moment, e.g. symmetric valence oscillations of CO<sub>2</sub>, are IR inactive; the others absorb at the respective resonance frequency or resonance wave number energy and are IR active. The different oscillations occur simultaneously—not only the oscillations listed in table <tabref linkend="mst322926tab05">5</tabref>, but also their harmonics, i.e. the multiples of these frequencies with intensity decreasing with increasing frequency. In addition, the oscillations affect each other causing combination oscillations.
<table id="mst322926tab05" frame="topbot"><caption id="mst322926tc05" label="Table 5"><p indent="no">Fundamental oscillations of CO<sub>2</sub> (according to [<cite linkend="mst322926bib72">72</cite>]).</p></caption><tgroup cols="4"><colspec colnum="1" colname="col1" align="left"></colspec><colspec colnum="2" colname="col2" align="left"></colspec><colspec colnum="3" colname="col3" align="left"></colspec><colspec colnum="4" colname="col4" align="left"></colspec><thead><row><entry></entry><entry>Scheme in</entry><entry>Wave number</entry><entry>IR</entry></row><row><entry>Oscillation</entry><entry>figure <figref linkend="mst322926fig31">31</figref></entry><entry>ν (cm<sup>−1</sup>)</entry><entry>active</entry></row></thead><tbody><row><entry>Symmetric valence</entry><entry>A</entry><entry>1330</entry><entry>No</entry></row><row><entry>oscillation</entry><entry></entry><entry></entry><entry></entry></row><row><entry>Anti-symmetric valence</entry><entry>B</entry><entry>2349</entry><entry>Yes</entry></row><row><entry>oscillation</entry><entry></entry><entry></entry><entry></entry></row><row><entry>Deformation</entry><entry>C</entry><entry>667.3</entry><entry>Yes</entry></row><row><entry>oscillation</entry><entry></entry><entry></entry><entry></entry></row><row><entry>Deformation oscillation</entry><entry>D</entry><entry>667.3</entry><entry>Yes</entry></row><row><entry>perpendicular to</entry><entry></entry><entry></entry><entry></entry></row><row><entry>the blade plane</entry><entry></entry><entry></entry><entry></entry></row></tbody></tgroup></table></p><p>To determine the CO<sub>2</sub> concentration, a particular wavelength has to be chosen that corresponds to the characteristic absorption bands of the gas components and that is sufficiently far away from the characteristic wavelengths (wave numbers) of the other gases or components in aqueous solutions. The molecular transmission of the different gases can be taken from HITRAN (high resolution transmission), the worldwide standard for calculating or simulating atmospheric molecular transmission and radiance from the microwave through the ultraviolet region of the spectrum including also the IR range [<cite linkend="mst322926bib73">73</cite>]. Spectra are given for molecular species along with their most significant isotopologues. Due to its abundance of 98.42%, the CO<sub>2</sub> isotopologue 626 is the most decisive [<cite linkend="mst322926bib74">74</cite>]. Usually, CO<sub>2</sub> is usefully detected at a wavelength of 4.24 µm corresponding to a wave number of 2358.5 cm<sup>−1</sup> (table <tabref linkend="mst322926tab06">6</tabref>).
<table id="mst322926tab06" frame="topbot"><caption id="mst322926tc06" label="Table 6"><p indent="no">Preferably used wavelengths for gas detection.</p></caption><tgroup cols="3"><colspec colnum="1" colname="col1" align="left"></colspec><colspec colnum="2" colname="col2" align="left"></colspec><colspec colnum="3" colname="col3" align="left"></colspec><thead><row><entry>Gas</entry><entry>Wavelength (µm)</entry><entry>Wave number (cm<sup>−1</sup>)</entry></row></thead><tbody><row><entry>CH<sub>4</sub></entry><entry>3.33</entry><entry>3003.0</entry></row><row><entry>HC</entry><entry>3.40</entry><entry>2941.2</entry></row><row><entry><bold>CO<sub>2</sub></bold></entry><entry><bold>4.24</bold></entry><entry><bold>2358.5</bold></entry></row><row><entry>CO</entry><entry>4.66</entry><entry>2145.9</entry></row><row><entry>NO<italic><sub>x</sub></italic></entry><entry>5.30</entry><entry>1886.8</entry></row><row><entry>SO<sub>2</sub></entry><entry>7.30</entry><entry>1369.8</entry></row></tbody></tgroup></table></p></sec-level2><sec-level2 id="mst322926s4-3" label="4.3"><heading>Recent developments</heading><p indent="no">Recent developments of non-dispersive infrared detection concern all components of NDIR sensors—IR radiation sources, IR filters, IR detectors, signal processing—as well as the fields of applications.</p><sec-level3 id="mst322926s4-3-1" label="4.3.1"><heading>IR detectors</heading><p indent="no">Most common detectors in NDIR sensors are pyroelectric detectors and thermopiles [<cite linkend="mst322926bib75">75</cite>]. Both detector types are thermal sensors where the incident radiation ‘heats’ up the sensor (temperature rise in the range of mK) and which do not need cooling like photon detectors.</p><p>Pyroelectric sensors exclusively detect signals that change over time. Because pulsed radiation sources are used in spectroscopy as well as in gas analysis this is not a drawback. The advantages of pyroelectric sensors are their simple and inexpensive structures together with high detectivity and excellent long-term stability.</p><p>The signal-to-noise ratio of IR detectors is determined by their specific detectivity <italic>D</italic>*:
<display-eqn id="mst322926eqn4-4" eqnnum="4.4"></display-eqn>where NEP is the noise-equivalent power, <italic>A</italic><sub>S</sub> is the sensitive sensor area and <italic>B</italic> is the frequency bandwidth. NEP of pyroelectric sensors depends on chopper frequency, the operation mode (current or voltage output mode) and is influenced by many effects [<cite linkend="mst322926bib75">75</cite>]. The temperature fluctuation noise marks the ultimate physical limit of <italic>D</italic>*. Noise of the loss resistance due to the dielectric loss of the pyroelectric sensor material is the dominating and hence limiting noise contribution for chopper frequencies larger than 100 Hz. The Johnson noise of the feedback resistor of the transimpedance amplifier in current mode and that of the series resistance of the impedance amplifier in voltage mode, respectively, are the limiting components for lower chopper frequencies.</p><p>Recent sensors based on crystalline lead zirconate titanate ((PbZr<italic><sub>x</sub></italic>Ti<sub>1−</sub><italic><sub>x</sub></italic>)O<sub>3</sub> with 0 < <italic>x</italic> < 1; also called PZT) with a drastically reduced element thickness of less than 1 µm and ultra-thin absorption layers show the values of specific detectivity <italic>D*</italic>(3 Hz, 500 K) of 5.35 × 10<sup>9</sup> cm Hz<sup>1/2</sup> W<sup>−1</sup> which are quite close to the fundamental noise limit [<cite linkend="mst322926bib76">76</cite>, <cite linkend="mst322926bib77">77</cite>]. PZT thin-film detectors with its high potential for miniaturization reach <italic>D</italic>* values of currently not more than 5 × 10<sup>8</sup> cm Hz<sup>1/2</sup> W<sup>−1</sup> [<cite linkend="mst322926bib78">78</cite>].</p><p>Due to the large progress of micromechanical manufacturing procedures, thermopiles became inexpensive thermal sensors that are available in large numbers [<cite linkend="mst322926bib79">79</cite>]. They are based on the Seebeck effect and do not necessarily require a chopper. An important advantage of thermopiles is the fact that they can be easily integrated using standard technologies of semiconductor manufacturing—which leads to low-cost mass production. A good temperature resolution always requires many thermocouples connected in series (thermopile). The space required for this sets limits to the miniaturization of thermoelectric sensors.</p><p>The most important noise source of thermopiles is the thermal resistance noise of the thermocouples. This means that the signal-to-noise ratio and hence specific detectivity <italic>D</italic>* improves with the square root of number <italic>N</italic> of the series thermocouples [<cite linkend="mst322926bib75">75</cite>]. Advanced thermopile detectors with high <italic>N</italic> numbers show <italic>D</italic>* values of about 10<sup>9</sup> cm Hz<sup>1/2</sup> W<sup>−1</sup> [<cite linkend="mst322926bib80">80</cite>].</p><p>As mentioned above, NDIR sensors often use pulsed radiation. This modulates both the radiation as required for pyroelectric sensors and reduces power consumption by an order of magnitude (pulse duration of several 100 ms, duty cycle time of several seconds). The CO<sub>2</sub> sensitivity at 2000 ppm in pulse mode and in chopper mode amounted to 95 µV ppm<sup>−1</sup> at 0.5 s pulse duration and 75 µV ppm<sup>−1</sup> at 1.5 Hz modulation frequency, respectively [<cite linkend="mst322926bib81">81</cite>]. Nevertheless, the pulse mode does not achieve the same signal-to-noise-ratio or accuracy as the common chopping mode. Therefore, a smart measurement algorithm alternates between the two modes and recalibrates the pulse mode from time to time [<cite linkend="mst322926bib82">82</cite>]. By this, the daily power consumption was reduced to 8.2 mW (23.5 h in pulse mode and 0.5 h chopping mode) compared to300 mW in chopping mode.</p></sec-level3><sec-level3 id="mst322926s4-3-2" label="4.3.2"><heading>IR radiation sources</heading><p indent="no">NDIR gas sensors need an infrared source for the excitation of the gas molecules in the wavelength range for the particular gas (cf table <tabref linkend="mst322926tab06">6</tabref>; 4.24 µm for CO<sub>2</sub>). The most desirable lamp characteristics are
<itemized-list id="mst322926il11"><list-item id="mst322926il11.1" marker="•"><p indent="no">rugged construction</p></list-item><list-item id="mst322926il11.2" marker="•"><p indent="no">high emissivity</p></list-item><list-item id="mst322926il11.3" marker="•"><p indent="no">long lifetime</p></list-item><list-item id="mst322926il11.4" marker="•"><p indent="no">low cost</p></list-item><list-item id="mst322926il11.5" marker="•"><p indent="no">small size</p></list-item><list-item id="mst322926il11.6" marker="•"><p indent="no">low power consumption</p></list-item><list-item id="mst322926il11.7" marker="•"><p indent="no">high pulse rates and, hence, a low thermal time constant for light modulation to offset thermal background signals.</p></list-item></itemized-list></p><p>Mostly, incandescent lamps are employed as thermal radiators for this task because they are very cost-effective. As given by Planck's law, their operating temperature should be as high as possible to obtain the large output intensity and detector signal.</p><p>To reduce oxidation of the filament, bulbs are filled with noble gases or evacuated. Filaments of Kanthal, a FeCrAl alloy with a maximum operation temperature of 1350 °C, are used instead of tungsten [<cite linkend="mst322926bib83">83</cite>]. However, the transmission of the glass bulb limits the useful spectral range and constrains the types of gas molecules that can be measured by NDIR. As an alternative, TO (transitor single outline) transistor standard housing with an appropriate IR window, e.g. of sapphire, ZnSe or CaF<sub>2</sub>, can be used as an envelope to avoid transmission losses of glass or quartz (transmission of only 50% at its peak value at 4.3 µm) bulbs [<cite linkend="mst322926bib83">83</cite>]. The most used types of filaments of thermal emitters and photon radiation sources are summarized in table <tabref linkend="mst322926tab07">7</tabref>.
<table id="mst322926tab07" frame="topbot"><caption id="mst322926tc07" label="Table 7"><p indent="no">IR radiation sources [<cite linkend="mst322926bib84" range="mst322926bib84,mst322926bib85,mst322926bib86,mst322926bib87">84–87</cite>].</p></caption><tgroup cols="3"><colspec colnum="1" colname="col1" align="left"></colspec><colspec colnum="2" colname="col2" align="left" colwidth="5pc"></colspec><colspec colnum="3" colname="col3" align="left" colwidth="10pc"></colspec><thead><row><entry>Emission</entry><entry>Type</entry><entry>Properties</entry></row></thead><tbody><row><entry>Thermal</entry><entry>Wound filament</entry><entry>High power output with high reliability</entry></row><row><entry></entry><entry>Ribbon filament</entry><entry>Higher pulse rates of up to 200 Hz, modulation depth of 50%, often used with reflectors to direct entire radiation out of package</entry></row><row><entry></entry><entry>Thin-film filament</entry><entry>High-volume production, low output power due to small filament size</entry></row><row><entry></entry><entry>MEMS system (hot plate)</entry><entry>Thin-film filaments on thin membranes (of silicon), 1200 °C with 10.7 mW radiation power from 1 mm<sup>2</sup> emission area</entry></row><row><entry>Photon</entry><entry>IR-LED</entry><entry>Optically pumped LEDs based on III–V semiconductors for CO<sub>2</sub> detection, emitting power of 10–30 µW</entry></row><row><entry></entry><entry>Laser diode</entry><entry>Narrower bandwidth than LEDs</entry></row></tbody></tgroup></table></p><p>The choice of emitter to be used in the NDIR sensor is driven by the requirements of the particular application. In the publication [<cite linkend="mst322926bib85">85</cite>] a decision tree is provided to make an appropriate choice. The emitter must also match the detector to give an optimal ‘optopair’ configuration (table <tabref linkend="mst322926tab08">8</tabref>). It can be seen that pyroelectric detectors and thermopiles in combination with incandescent IR lamps show the best performance, even though LEDs are needed for fast pulsing.
<table id="mst322926tab08" frame="topbot"><caption id="mst322926tc08" label="Table 8"><p indent="no">Optimal optopairs of radiation source and detector for CO<sub>2</sub> detection (4.2 µm) [<cite linkend="mst322926bib87">87</cite>].</p></caption><tgroup cols="3"><colspec colnum="1" colname="col1" align="left"></colspec><colspec colnum="2" colname="col2" align="left"></colspec><colspec colnum="3" colname="col3" align="left"></colspec><thead><row><entry></entry><entry>Time</entry><entry>Relative limit</entry></row><row><entry>Optopair</entry><entry>constant (s)</entry><entry>of detection</entry></row></thead><tbody><row><entry>Thermal + thermopile</entry><entry>0.1</entry><entry>160</entry></row><row><entry>Thermal + pyrodetector</entry><entry>0.1</entry><entry>50–160</entry></row><row><entry>Thermal + photodiode</entry><entry>0.1</entry><entry>1</entry></row><row><entry>LED + thermopile</entry><entry>0.01</entry><entry>100</entry></row><row><entry>LED + pyrodetector</entry><entry>0.1</entry><entry>100</entry></row><row><entry>LED + photodiode</entry><entry>10<sup>−8</sup></entry><entry>0.5 × 10<sup>−3</sup></entry></row></tbody></tgroup></table></p></sec-level3><sec-level3 id="mst322926s4-3-3" label="4.3.3"><heading>IR filter</heading><p indent="no">Interference band-pass filters are relatively inexpensive wavelength selectors that allow transmission of a predetermined wavelength while rejecting or blocking other wavelengths. The relative low cost and simple installation make them the preferred wavelength selector for applications such as CO<sub>2</sub> measurement, where the required wavelength is well known. The entire aperture of an interference filter can be illuminated, resulting in high throughput and an excellent signal-to-noise ratio. The central wavelength of an interference filter can shift with temperature due to the thermal expansion of the spacer layers and the change in their refractive indices. This wavelength shift is extremely small at room temperature (0.01 nm K<sup>−1</sup>).</p><p>Typical NDIR sensors use a dual-beam dual-wavelength structure where the second beam at the second wavelength serves as reference (see figure <figref linkend="mst322926fig30" override="yes">30(<italic>B</italic>)</figref>). Other applications such as in combustion control require the measurement of more than one gas and, hence, two different wavelengths. For such purposes the basic NDIR sensor setup, consisting of an IR lamp, the cuvette and the IR detector, can be used if the IR lamp is a broadband thermal emitter and if the filter wavelength can be changed in a controlled manner depending on the gases to be measured (cf table <tabref linkend="mst322926tab06">6</tabref>). This can be achieved by replacing the conventional interference filter by a tunable Fabry–Perot interferometer filter (FPF).</p><p>Grasdepot <italic>et al</italic> used for this a pressure-sensor-based FPF [<cite linkend="mst322926bib88">88</cite>]. Blomberg <italic>et al</italic> introduced an electrostatically tunable FPF for CO<sub>2</sub> detection [<cite linkend="mst322926bib89">89</cite>]. The wavelength shift is caused by an electrostatically excited displacement of one reflector of a Fabry–Perot double-reflector structure hence changing the gap width. In this particular case the wavelength was shifted to either side of the CO<sub>2</sub> wavelength. The ratio of these signals then indicates the degree of light absorption and thus the gas concentration. The ratio of the measurement to the reference channel changed only 0.6% when the power of the IR source was reduced to 50%. Noro <italic>et al</italic> described a FPF with a wide wavelength range from 2.5 to 4.5 µm that has been developed for a combined NDIR CO<sub>2</sub>/H<sub>2</sub>O gas sensor. The resolutions defined by standard deviation are about 13 ppm for CO<sub>2</sub> of 2000 ppm and 0.35 g m<sup>−3</sup> for H<sub>2</sub>O of 22.5 g m<sup>−3</sup> [<cite linkend="mst322926bib90">90</cite>]. Recent developments focus on the integration of FPFs with IR detectors and the improvement of the optical characteristics of FPFs, in particular on the increase of spectral resolution, improvement of the signal-to-noise ratio and implementation of a second separate wavelength range [<cite linkend="mst322926bib91">91</cite>, <cite linkend="mst322926bib92">92</cite>].</p></sec-level3><sec-level3 id="mst322926s4-3-4" label="4.3.4"><heading>Gas sensors for measuring gas mixtures</heading><p indent="no">As already mentioned for Fabry–Perot filters, many applications require the measurement of concentrations of a limited number of other gases in addition to CO<sub>2</sub>. For example, exhaust gases of automobiles contain CO<sub>2</sub>, CO, NO<italic><sub>x</sub></italic> and water vapour; air quality sensors measure CO<sub>2</sub> but should also detect C<sub>2</sub>H<sub>6</sub> as gas for heating and cooking for safety reasons.</p><p>Usually, multi-colour NDIR sensors with multiple channels are used as shown in figure <figref linkend="mst322926fig30" override="yes">30(<italic>B</italic>)</figref>. A beam splitter distributes the incident IR radiation equally to the corresponding detector elements [<cite linkend="mst322926bib93">93</cite>]. To provide high accuracy, the field of view for each detector element should be the same. Norkus <italic>et al</italic> proposed a solution that uses a pyramid with reflecting sidewalls as a beam splitter [<cite linkend="mst322926bib94">94</cite>]. Such an element can be manufactured as a discrete component with small dimensions and has a high measuring accuracy, whilst obtaining a high output signal. Such pyramid-shaped beam-splitters can also be fabricated by anisotropic etching
of (1 0 0)-oriented silicon wafers. A quadratic pattern with ⟨1 1 0⟩-oriented edges in the etch mask leads to inverse pyramid arrays [<cite linkend="mst322926bib95">95</cite>].</p><p>Another approach for measuring gas mixtures was presented by Rubio <italic>et al</italic> [<cite linkend="mst322926bib96">96</cite>]. The detector is a single-module thermopile array that is equipped with an array of broadband IR filters. Here, the filter elements are not designed for a specific absorption band. This increases the overall system flexibility but also the complexity of signal processing.</p><p>Andrews and King proposed a gas sensor that works without IR filters [<cite linkend="mst322926bib97">97</cite>]. Their approach uses the correlation of the detector signal and the nonlinear temperature-dependent emission spectrum of thermal radiation sources. Using the output signals at different emission temperatures, simultaneous monitoring of CO<sub>2</sub> and water vapour in the atmosphere could be demonstrated. However, the sensor's performance crucially depends on the stability of the IR source which limits the uncertainty.</p><p>Graf <italic>et al</italic> investigated another principle lying in between the both approaches of Rubio <italic>et al</italic> and Andrews <italic>et al</italic> [<cite linkend="mst322926bib98">98</cite>]. The so-called ANDIR concept presents a highly adaptable NDIR sensor using broadband filters instead of interference filters with a specifically designed narrow bandwidth. These broadband filters do not have to be adapted to specific chemical species but can overlap in their spectral transmission range. Using <italic>n</italic> emitter temperatures and <italic>m</italic> standard broadband filters enables differentiation of up to <italic>m</italic>⋅<italic>n</italic> different gases. Such a concept can be flexibly applied to many different detection and measuring tasks because no specific transmission filters are needed [<cite linkend="mst322926bib99">99</cite>]. Since the detector signals are highly correlated, particular algorithms have to be applied for signal processing. Support vector machines (SVM) have been used as the favourable method for data analysis [<cite linkend="mst322926bib100">100</cite>].</p></sec-level3></sec-level2><sec-level2 id="mst322926s4-4" label="4.4"><heading>State of the art of NDIR sensors</heading><p indent="no">Currently, there is an abundance of NDIR CO<sub>2</sub> sensors available in the market. Most sensors are intended for measuring air quality, ventilation control in incubators and greenhouse, environmental [<cite linkend="mst322926bib101">101</cite>] and combustion control, automotive applications [<cite linkend="mst322926bib102">102</cite>, <cite linkend="mst322926bib103">103</cite>] as well as for measuring dissolved CO<sub>2</sub> in freshwater [<cite linkend="mst322926bib104">104</cite>]. Table <tabref linkend="mst322926tab09">9</tabref> gives an overview of the characteristic properties of the actual NDIR CO<sub>2</sub> sensors. The values were taken from data sheets in which the lower detection limits of the various sensors were not detailed specified.
<table id="mst322926tab09" frame="topbot"><caption id="mst322926tc09" label="Table 9"><p indent="no">Characteristic properties of actual NDIR CO<sub>2</sub> sensors (values as given by the datasheets).</p></caption><tgroup cols="5"><colspec colnum="1" colname="col1" align="left" colwidth="12pc"></colspec><colspec colnum="2" colname="col2" align="left"></colspec><colspec colnum="3" colname="col3" align="left" colwidth="12pc"></colspec><colspec colnum="4" colname="col4" align="left" colwidth="6pc"></colspec><colspec colnum="5" colname="col5" align="left"></colspec><thead><row><entry>Manufacturer</entry><entry>CO<sub>2</sub> range (ppm)</entry><entry>Accuracy (ppm)</entry><entry> Operating temperature range (°C)</entry><entry>Reference</entry></row></thead><tbody><row><entry>Honeywell C7232A1008</entry><entry>0–2000</entry><entry>±100</entry><entry> 0–50</entry><entry>[<cite linkend="mst322926bib105">105</cite>]</entry></row><row><entry>Alphasense IRC-A1</entry><entry>0–5000</entry><entry>±50</entry><entry>−20–50</entry><entry>[<cite linkend="mst322926bib106">106</cite>]</entry></row><row><entry>SenseAir</entry><entry>0–6000</entry><entry>±3% of reading or ±20, whichever is greater</entry><entry> 0–50</entry><entry>[<cite linkend="mst322926bib107">107</cite>]</entry></row><row><entry>mb-systemtechnik SEN CO<sub>2</sub> S 100 2</entry><entry>0–2000 to 0–10 000</entry><entry>±4% FS ± 3% of reading</entry><entry>−40–70</entry><entry>[<cite linkend="mst322926bib108">108</cite>]</entry></row><row><entry>Micro-Hybrid</entry><entry>0–5000</entry><entry>±100</entry><entry>−20–200</entry><entry>[<cite linkend="mst322926bib109">109</cite>]</entry></row><row><entry>ZILA ZMF-100_IR</entry><entry>0–50 000</entry><entry>±2% of reading</entry><entry></entry><entry>[<cite linkend="mst322926bib110">110</cite>]</entry></row><row><entry>VTI Valtronics 2008SDH 15%</entry><entry>0–15 000</entry><entry>±5% of reading</entry><entry> 0–50</entry><entry>[<cite linkend="mst322926bib111">111</cite>]</entry></row><row><entry>Digital control systems 305e</entry><entry>0–2500</entry><entry>±5% of reading or ±100, whichever is greater</entry><entry> 0–50</entry><entry>[<cite linkend="mst322926bib112">112</cite>]</entry></row><row><entry>ELT environment leading technology H-550EV</entry><entry>0–2000, 0–5000</entry><entry>±8% of reading ±30</entry><entry> 0–50</entry><entry>[<cite linkend="mst322926bib113">113</cite>]</entry></row></tbody></tgroup></table></p><p>Most products are single-channel sensors due to high costs. They are smaller and use less energy, which is most important in battery operation. Two-channel sensors exhibit better long-term stability. The second channel (comparison channel) enables the sensor to compensate the following effects: drift of the IR lamp, temperature drift of the IR detector, changes to the measuring stretch, e.g. due to dirt.</p><p>Diffusion of sample gas into the cuvette takes between 30 s and 2 min. Response time is in the same range. A sensor life expectancy can be 15 years and more. Very small gas concentrations can be measured with a sufficient accuracy by means of pre-concentrator modules [<cite linkend="mst322926bib114">114</cite>].</p></sec-level2><sec-level2 id="mst322926s4-5" label="4.5"><heading>CO<sub>2</sub> permeation method</heading><p indent="no">Dissolved CO<sub>2</sub> in liquids can also be measured continuously by means of IR or solid electrolyte gas sensors if the dissolved gas is transferred to a defined gas atmosphere. This is mostly performed by the introduction of a gas permeable membrane into the solution to be measured and flushing the backside of the membrane with a carrier gas. According to figure <figref linkend="mst322926fig32">32</figref> the gas sensor is often positioned at the downstream side of the permeation cell.
<figure id="mst322926fig32"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig32.eps" width="3pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig32.jpg"></graphic-file></graphic><caption id="mst322926fc32" label="Figure 32"><p indent="no">Schematic drawing of a membrane covered gas transfer probe for the measurement of dissolved carbon dioxide. 1: permeation membrane; 2: carrier gas inlet; 3: gas outlet; 4: CO<sub>2</sub> sensor.</p></caption></figure></p><p>Carbon dioxide permeates through the membrane and is then carried to the CO<sub>2</sub> sensor. At constant carrier gas flow a linear relationship exists between the partial pressure of carbon dioxide inside the measuring solution and the carbon dioxide concentration downstream of the diffusion cell.</p><p>An example for this permeation method, which is already available in the market, is the steam-sterilizable silicone-tubing-probe Carboline developed by Biotechnologie Kempe GmbH [<cite linkend="mst322926bib115">115</cite>]. This probe is equipped with a modular CO<sub>2</sub> sensor operating on the NDIR single-beam dual-wavelength principle. Two of the main advantages of this system are the high long-term stability and the broad measuring range. The system is conceived for the monitoring of biotechnological processes and for quality control in the production of beverages or at breweries.</p><p>Another commercially available system working on the same principle is the Embra CarboCheck of ECM group, Bratislava. In contrast to the Carboline probe described above this system uses a constantly evacuated chamber behind the membrane [<cite linkend="mst322926bib116">116</cite>].</p></sec-level2></sec-level1><sec-level1 id="mst322926s5" label="5"><heading>Survey and comparison of methods</heading><p indent="no">CO<sub>2</sub> solid electrolyte sensors can be applied successfully in all cases of long-term measurements in air, in breath analysis and in the process measuring especially at higher temperature. As compared to sensors with aqueous electrolytes the main advantages consist of a short response time and maintenance-free operation without calibration. Water vapour and traces of combustibles should not disturb the signal. This kind of sensors can be easily miniaturized. On the other hand, the sensor has to be heated electrically up to 550–700 °C. For this reason a small electric heating power (2–3 W) is necessary.</p><p>As compared to IR sensors, Severinghaus and solid electrolyte-based sensors are simpler in setup and in most cases cheaper. The main advantage of the Severinghaus sensor consists in energy-free operation. This kind of sensor is the only one for direct application in liquids. But for its application a basic knowledge of electrochemistry is necessary.</p><p>Methods using membranes for gas separation from liquids with the subsequent CO<sub>2</sub> analysis by NDIR are frequently excessive in maintenance. The described methods are summarized in tables <tabref linkend="mst322926tab10">10</tabref> and <tabref linkend="mst322926tab11">11</tabref> with respect to their performances.
<table id="mst322926tab10" frame="topbot"><caption id="mst322926tc10" label="Table 10"><p indent="no">Gas application.</p></caption><tgroup cols="5"><colspec colnum="1" colname="col1" align="left"></colspec><colspec colnum="2" colname="col2" align="left"></colspec><colspec colnum="3" colname="col3" align="left"></colspec><colspec colnum="4" colname="col4" align="left"></colspec><colspec colnum="5" colname="col5" align="left"></colspec><thead><row><entry>Method</entry><entry>Severinghaus electrode</entry><entry>Conductivity sensor</entry><entry>Solid electrolyte sensor</entry><entry>NDIR sensor</entry></row></thead><tbody><row><entry>Application</entry><entry>Direct</entry><entry>Direct</entry><entry>Direct</entry><entry>Direct gas flow</entry></row><row><entry>Sensor function</entry><entry>Nernst logarithmic</entry><entry>Linear</entry><entry>Nernst logarithmic</entry><entry>Beer–Lambert linear</entry></row><row><entry>Concentration range</entry><entry>ppm–vol.%</entry><entry>0–5 vol.%</entry><entry>ppm–vol.%</entry><entry>0.1 ppm–vol.%</entry></row><row><entry>Response time</entry><entry>30 s</entry><entry>1–2 s</entry><entry><1 s</entry><entry>Few seconds</entry></row><row><entry>Cross sensitivity</entry><entry>Water, SO<sub>2</sub>, NO<italic><sub>x</sub></italic>, H<sub>2</sub>S</entry><entry>SO<sub>2</sub>, NO<italic><sub>x</sub></italic>, H<sub>2</sub>S</entry><entry>Combustibles SO<sub>2</sub>, NO<italic><sub>x</sub></italic>, H<sub>2</sub>S</entry><entry>Selective, dust aerosol</entry></row><row><entry>Measuring temperature</entry><entry><50 °C</entry><entry>20 °C</entry><entry>350–750 °C</entry><entry><50 °C</entry></row><row><entry>Calibration</entry><entry>Yes</entry><entry>Yes</entry><entry>No</entry><entry>Yes</entry></row><row><entry>Energy consumption</entry><entry>No</entry><entry>Yes</entry><entry>Yes</entry><entry>Yes</entry></row><row><entry>Maintenance</entry><entry>High</entry><entry>High</entry><entry>No</entry><entry>Low</entry></row></tbody></tgroup></table><table id="mst322926tab11" frame="topbot"><caption id="mst322926tc11" label="Table 11"><p indent="no">Dissolved CO<sub>2</sub> measurement.</p></caption><tgroup cols="5"><colspec colnum="1" colname="col1" align="left"></colspec><colspec colnum="2" colname="col2" align="left"></colspec><colspec colnum="3" colname="col3" align="left"></colspec><colspec colnum="4" colname="col4" align="left"></colspec><colspec colnum="5" colname="col5" align="left"></colspec><thead><row><entry>Method</entry><entry>Severinghaus electrode</entry><entry>Conductivity sensor</entry><entry>Solid electrolyte sensor</entry><entry>NDIR sensor</entry></row></thead><tbody><row><entry>Application</entry><entry>Direct</entry><entry>Membrane</entry><entry>Indirect, behind membrane</entry><entry>Indirect, behind membrane</entry></row><row><entry>Sensor function</entry><entry>Nernst logarithmic</entry><entry>Linear</entry><entry>Nernst logarithmic</entry><entry>Beer–Lambert linear</entry></row><row><entry>Concentration range</entry><entry>ppm–vol.%</entry><entry>0–5 vol.%</entry><entry>ppm–vol.%</entry><entry>0.1 ppm–vol.%</entry></row><row><entry>Response time</entry><entry>30 s</entry><entry>1–2 s</entry><entry><1s</entry><entry>Few seconds</entry></row><row><entry>Cross sensitivity</entry><entry>Water, SO<sub>2</sub>, NO<italic><sub>x</sub></italic>, H<sub>2</sub>S</entry><entry>SO<sub>2</sub>, NO<italic><sub>x</sub></italic>, H<sub>2</sub>S</entry><entry>Combustibles SO<sub>2</sub>, NO<italic><sub>x</sub></italic>, H<sub>2</sub>S</entry><entry>Selective dust</entry></row><row><entry>Measuring temperature</entry><entry><50 °C</entry><entry>20 °C</entry><entry>350–750 °C</entry><entry><50 °C</entry></row><row><entry>Calibration</entry><entry>Yes</entry><entry>Yes</entry><entry>No</entry><entry>Yes</entry></row><row><entry>Energy consumption</entry><entry>No</entry><entry>Yes</entry><entry>Yes</entry><entry>Yes</entry></row><row><entry>Maintenance</entry><entry>High</entry><entry>High</entry><entry>No</entry><entry>Low</entry></row></tbody></tgroup></table></p></sec-level1><sec-level1 id="mst322926s6" label="6"><heading>Selected examples of application for CO<sub>2</sub> measurements in liquids and gases</heading><sec-level2 id="mst322926s6-1" label="6.1"><heading>Applications in environment and for safety control</heading><sec-level3 id="mst322926s6-1-1" label="6.1.1"><heading>CO<sub>2</sub> in the air</heading><p indent="no">The atmospheric CO<sub>2</sub> concentration, currently about 0.038 vol.% (380 vol. ppm) on average, is harmless for humans and animals and indispensable for many biological processes. However, higher levels of this odourless and colourless gas affect the human body, are dangerous to health and can be even life threatening.</p><p>Medical insights into the effect of CO<sub>2</sub> in the breathing air have been gained already since a long time. As early as 1858 the hygienist Max von Pettenkofer attached great importance to CO<sub>2</sub> as an assessment criterion for the quality of ambient air in buildings [<cite linkend="mst322926bib117">117</cite>] and defined a limit value of 0.1 vol.% (1000 vol. ppm) that is still accepted. In office buildings CO<sub>2</sub> concentrations typically vary from the outdoor value up to 2500 vol. ppm. The primary source of the increased CO<sub>2</sub> is respiration by the building occupants. Even at CO<sub>2</sub> concentrations <1000 vol. ppm, workers in office buildings often suffer from the so-called sick building syndrome (SBS), which becomes apparent with various unspecific symptoms such as headache, fatigue, eye problems, nasal and respiratory tract symptoms such as cough or wheeze. Although these symptoms are caused by various gases coming from human respiration and perspiration and not only by CO<sub>2</sub> itself, results of extensive statistical studies indicate a correlation between elevated indoor CO<sub>2</sub> levels and increases in certain SBS symptoms [<cite linkend="mst322926bib118">118</cite>, <cite linkend="mst322926bib119">119</cite>]. In this context it is worth mentioning that at modestly elevated levels of CO<sub>2</sub> in breathing air the corresponding relatively low reduction of oxygen in that air in most cases is not the cause of SBS or other dangers to health. The well-being of the humans is not only influenced by the water vapour concentration but also in a higher extent by the CO<sub>2</sub> concentration. It is believed that concentrations above 1000 ppm CO<sub>2</sub> are felt as bothering. This is widely below the limits defined by law (5000 ppm).</p><p>Solid electrolyte CO<sub>2</sub> sensors are successfully applied in the measurement and control of the indoor atmosphere. For example, the concentration in a chemical laboratory in which several people work was measured over 1 week (figure <figref linkend="mst322926fig33" override="yes">33(<italic>A</italic>)</figref>). The concentration of CO<sub>2</sub> outside (figure <figref linkend="mst322926fig33" override="yes">33(<italic>B</italic>)</figref>) is strongly influenced by the day night cycle of the plant activity and also by the distribution of CO<sub>2</sub> due to the weather conditions (calm or windy) [<cite linkend="mst322926bib67">67</cite>]. To measure the CO<sub>2</sub> concentration independent of the plant activity, the measurements must be carried out in regions without such activities. In middle Europe regions at heights above 3500 m above sea level are suited for that purpose.
<figure id="mst322926fig33"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig33.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig33.jpg"></graphic-file></graphic><caption id="mst322926fc33" label="Figure 33"><p indent="no">CO<sub>2</sub> concentration in the lab atmosphere (<italic>A</italic>) and in the atmosphere of urban areas near the ground (<italic>B</italic>).</p></caption></figure></p><p>Table <tabref linkend="mst322926tab12">12</tabref> shows critical CO<sub>2</sub> values in the commonly used units and how elevated levels of carbon dioxide impair the human physical and mental well-being. The formula for converting ppm to mg m<sup>−3</sup> for CO<sub>2</sub> is mg m<sup>−3</sup> = ppm⋅<italic>M/V</italic> with <italic>M</italic> = 44 g mol<sup>−1</sup> (molecular weight) and <italic>V</italic> = 24.5 L mol<sup>−1</sup> (molar volume) at 25 °C.
<table id="mst322926tab12" frame="topbot"><caption id="mst322926tc12" label="Table 12"><p indent="no">Critical CO<sub>2</sub> concentration values in different units, effects and symptoms of CO<sub>2</sub> exposure.</p></caption><tgroup cols="4"><colspec colnum="1" colname="col1" align="left"></colspec><colspec colnum="2" colname="col2" align="left"></colspec><colspec colnum="3" colname="col3" align="left"></colspec><colspec colnum="4" colname="col4" align="left" colwidth="30pc"></colspec><spanspec spanname="1to3" namest="col1" nameend="col3" align="center"></spanspec><thead><row><entry spanname="1to3">CO<sub>2</sub> concentration</entry><entry>Critical values and symptoms</entry></row><row><entry spanname="1to3"></entry><entry></entry></row><row><entry>Vol.%</entry><entry>ppm</entry><entry>About mg m<sup>−3</sup></entry><entry></entry></row></thead><tbody><row><entry> 0.038</entry><entry> 380</entry><entry> 684</entry><entry>Actual CO<sub>2</sub> concentration in air</entry></row><row><entry> 0.1</entry><entry> 1 000</entry><entry> 1 800</entry><entry>Pettenkofer value for maximum tolerable CO<sub>2</sub> indoor air concentration</entry></row><row><entry> 0.5</entry><entry> 5 000</entry><entry> 9 000</entry><entry>Threshold limit value (TLV-TWA): long-term exposure limit; in Germany: Maximale Arbeitsplatz-Konzentration (MAK-Wert)</entry></row><row><entry> 0.7</entry><entry> 7 000</entry><entry> 12 600</entry><entry>Maximum value in cinemas after the performance</entry></row><row><entry> 1.5</entry><entry> 15 000</entry><entry> 27 000</entry><entry>Threshold limit value (TLV-STEL): short-term exposure limit, can cause drowsiness with prolonged exposure</entry></row><row><entry> 3.0</entry><entry> 30 000</entry><entry> 54 000</entry><entry>Threshold limit value (TLV-C): ceiling exposure limit, faster breathing, headaches, dizzy feeling, increased pulse rate, impaired hearing</entry></row><row><entry> >4.0</entry><entry> 40 000</entry><entry> 72 000</entry><entry>Rapid breathing, headaches, impaired hearing and vision, increased dizzy feeling, unconsciousness</entry></row><row><entry> >8.0</entry><entry> 80 000</entry><entry>144 000</entry><entry>Laboured breathing, headaches, visual and hearing dysfunctions, rapid loss of consciousness with risk of death within minutes of exposure, burning candle goes out</entry></row><row><entry>>20.0</entry><entry>200 000</entry><entry>360 000</entry><entry>Lethal within a short time</entry></row></tbody></tgroup></table></p><p>The threshold limit value–time-weighted average (TLV-TWA) or the German MAK value is the time-weighted average concentration to which it is believed a worker can be repeatedly exposed over a normal 8 h workday and a 40 h work per week without adverse health effects. The short-term exposure limit (TLV-STEL) allows exposure for the duration of 15 min, that cannot be repeated more than four times per day (the requirements of TWA must still be met). The ceiling exposure limit (TLV-C) or maximum exposure concentration is the highest threshold limit value, which should not to be exceeded under any circumstances.</p><p>Dangerous CO<sub>2</sub> concentrations e.g. can occur in wine cellars, silos, fruit and vegetable storage depots, boilers, well pits, liquid manure pits, sewage systems and caves. Since the density of CO<sub>2</sub> is higher than that of air, in these cases its concentration is particularly high at the bottom. Thus, e.g. in the famous so-called Grotta del cane (Dog's grotto) near Naples, Italy, the CO<sub>2</sub> concentration amounts to approximately 70 vol.%. In the potash salt mining quarries sudden CO<sub>2</sub> eruptions caused even deadly accidents several times [<cite linkend="mst322926bib120">120</cite>]. Also in coal mines occasionally high carbon dioxide concentrations can be present. In former times miners tried to detect dangerous levels of carbon dioxide by bringing a caged canary with them when working in a mine shaft. Positioned at ground level the bird would die already before the CO<sub>2</sub> concentration reached levels toxic to human beings. Of course, using CO<sub>2</sub> warning devices is a more acceptable method. While colorimetric detector tubes are less appropriate, electrochemical CO<sub>2</sub> sensors as well as IR CO<sub>2</sub> sensors can be used for this purpose. Figure <figref linkend="mst322926fig34">34</figref> shows a portable CO<sub>2</sub> warning device with an easily exchangeable electrochemical CO<sub>2</sub> sensor that had been developed for application in underground potash salt mining industry. Similar to traffic light signals, the normal state and the threshold limit values TLV-TWA (0.5 vol.% CO<sub>2</sub>) and TLV-STEL (1.5 vol.% CO<sub>2</sub>) were indicated by green, yellow and red light-emitting diodes (LEDs), respectively.
<figure id="mst322926fig34"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig34.eps" width="8pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig34.jpg"></graphic-file></graphic><caption id="mst322926fc34" label="Figure 34"><p indent="no">Portable CO<sub>2</sub> warning device with an electrochemical CO<sub>2</sub> sensor (KSI, Germany).</p></caption></figure></p><p>The stationary CO<sub>2</sub> warning device with an electrochemical CO<sub>2</sub> sensor shown in figure <figref linkend="mst322926fig35">35</figref> was likewise purpose-built for application in underground potash salt mining. Critical CO<sub>2</sub> concentrations are intensively signalled acoustically, optically by the rotating flashing beacon and also electrically transferred to the control centre [<cite linkend="mst322926bib121" range="mst322926bib121,mst322926bib122,mst322926bib123">121–123</cite>].
<figure id="mst322926fig35"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig35.eps" width="13pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig35.jpg"></graphic-file></graphic><caption id="mst322926fc35" label="Figure 35"><p indent="no">Stationary CO<sub>2</sub> warning device with an electrochemical CO<sub>2</sub> sensor for application in underground potash salt mining (KSI).</p></caption></figure></p><p>As a consequence of the extremely low air moisture in underground salt mines, electrochemical sensors tend to dry out rapidly in this environment. This can be prevented by reducing the water vapour pressure of the sensor electrolyte, e.g. by adding ethylene glycol.</p><p>At present, for increasing personal safety easily portable CO<sub>2</sub> warning devices are favourably equipped with CO<sub>2</sub>-sensitive infrared sensors (NDIR). Unlike electrochemical sensors, this type of sensor is not subject to cross sensitivity from toxic hydrogen sulfide and provides long-term stable detection values. As an example, in figure <figref linkend="mst322926fig36">36</figref> a multi-gas detector is shown, which, among other gases, detects carbon dioxide in ambient air by means of a NDIR CO<sub>2</sub> sensor. The CO<sub>2</sub> measuring range is 0–5 vol.% with the resolution 0.01%, and the typical lifetime of the sensor is >6 years [<cite linkend="mst322926bib124">124</cite>].
<figure id="mst322926fig36"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig36.eps" width="8pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig36.jpg"></graphic-file></graphic><caption id="mst322926fc36" label="Figure 36"><p indent="no">Microtector II G460 (GfG Gesellschaft für Gerätebau mbH, Dortmund, Germany).</p></caption></figure></p></sec-level3><sec-level3 id="mst322926s6-1-2" label="6.1.2"><heading>CO<sub>2</sub> in rivers and freshwater lakes</heading><p indent="no">Apart from pH value, oxygen concentration and conductivity, the concentration of dissolved carbon dioxide is one of the most important parameters for the assessment of the condition of natural waters. It affects substantially the natural equilibrium between the limy substratum and the water, plays a role in the solubility of different inorganic components and is involved in various biological reactions occurring in the water.</p><p>According to Henry's law the CO<sub>2</sub> concentration in pure water or in sufficiently diluted solutions being in equilibrium with the atmospheric air can be estimated by the equation
<display-eqn id="mst322926eqn6-1" eqnnum="6.1"></display-eqn>where <italic>c</italic> is the CO<sub>2</sub> concentration in the solution (mostly specified in mg L<sup>−1</sup> or mol L<sup>−1</sup>), <italic>p</italic> is the partial pressure of CO<sub>2</sub> in the atmosphere and <italic>K</italic><sub>H</sub>(<italic>T</italic>) is the Henry's law coefficient, which depends exponentially on temperature. Figure <figref linkend="mst322926fig37">37</figref> illustrates the considerable temperature dependence of the CO<sub>2</sub> concentration in water that is at its surface in equilibrium with the atmospheric CO<sub>2</sub> partial pressure of approximately 38 Pa [<cite linkend="mst322926bib125">125</cite>]. At 25 °C it amounts to approximately 0.57 mg L<sup>−1</sup>.
<figure id="mst322926fig37"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig37.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig37.jpg"></graphic-file></graphic><caption id="mst322926fc37" label="Figure 37"><p indent="no">Temperature dependence of the CO<sub>2</sub> concentration in water at the total atmospheric pressure 101 kPa and the CO<sub>2</sub> partial pressure 38 Pa, calculated from [<cite linkend="mst322926bib125">125</cite>].</p></caption></figure></p><p>In most cases under natural environmental conditions equilibrium between freshwater and the atmosphere, which is preconditioned in Henry's law, does not exist yet. Depending on local conditions and on temperature, the CO<sub>2</sub> content of natural waters can differ considerably and is also subject to seasonal fluctuations. Surface waters contain usually less than 10 mg L<sup>−1</sup> CO<sub>2</sub> originating from the atmosphere and biological processes. As a result of the dissolution of minerals and the relatively high carbon dioxide content of the pore air of soils, ground waters can exhibit up to 100 mg L<sup>−1</sup>, mineral waters even more than 1000 mg L<sup>−1</sup> CO<sub>2</sub>.</p><p>The CO<sub>2</sub> concentration in freshwater lakes is mainly determined by the organic production and decomposition of organic substances such as phytoplankton and algae. Hence, the CO<sub>2</sub> concentration represents a biological production indicator and is an important parameter for characterizing the trophic level of freshwater lakes [<cite linkend="mst322926bib126">126</cite>]. For long-term measurements of CO<sub>2</sub> concentrations at different depths of a lake (Willersinnweiher near Ludwigshafen, Germany), an electrochemical CO<sub>2</sub> sensor, together with sensors for measuring other parameters, has been used in a special flow-through system [<cite linkend="mst322926bib127">127</cite>, <cite linkend="mst322926bib128">128</cite>]. Furthermore, the CO<sub>2</sub> concentrations were calculated with the general geochemical computer program PHREEQC [<cite linkend="mst322926bib129">129</cite>], which is applicable to many hydrogeochemical environments for simulating chemical reactions and transport processes in natural or polluted water. The program is based on equilibrium chemistry of aqueous solutions interacting with minerals, gases, solid solutions, exchangers, and sorption surfaces, but also includes the capability to model kinetic reactions. In figures <figref linkend="mst322926fig38">38</figref> and <figref linkend="mst322926fig39">39</figref> results of measurements with the CO<sub>2</sub> sensor are compared with the calculated values. Generally, the comparison of measured and calculated CO<sub>2</sub> values shows good correlation. In figure <figref linkend="mst322926fig38">38</figref> some deviations are obvious only in the metalimnion between 9 and 12 m of depth, which can be attributed to the dissolution and precipitation processes of calcite and the oxidation of CH<sub>4</sub>.
<figure id="mst322926fig38"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig38.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig38.jpg"></graphic-file></graphic><caption id="mst322926fc38" label="Figure 38"><p indent="no">Comparison of calculated and measured CO<sub>2</sub> concentrations during the stagnation period in October [<cite linkend="mst322926bib127">127</cite>, <cite linkend="mst322926bib128">128</cite>].</p></caption></figure><figure id="mst322926fig39"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig39.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig39.jpg"></graphic-file></graphic><caption id="mst322926fc39" label="Figure 39"><p indent="no">Comparison of calculated and measured CO<sub>2</sub> concentrations during spring mixing in February [<cite linkend="mst322926bib127">127</cite>, <cite linkend="mst322926bib128">128</cite>].</p></caption></figure></p><p>During the mixing period in springtime the generally low CO<sub>2</sub> values show a good correlation below 10 m of water depth. Above that depth, the measured values of the sensor show a different development than the calculated ones. This deviation can be explained by the fact that the aqueous carbonate system is not in balance with the changing CO<sub>2</sub> concentrations. The data of the sensor reflect the dissolution of atmospheric CO<sub>2</sub> due to the assimilation by phytoplankton. This represents a general error source for calculated CO<sub>2</sub> concentrations in lake water, since these calculations assume a balanced aqueous carbonate system, which does not establish in those dynamic lake water systems.</p><p>The measurement results show that the CO<sub>2</sub> dynamic in freshwater lakes is strongly determined by organic processes and the interactions between numerous physical, biological and chemical influencing parameters. Furthermore, the carbon dioxide fluxes across the air–water interface have an impact on carbon availability in aquatic systems [<cite linkend="mst322926bib130">130</cite>, <cite linkend="mst322926bib131">131</cite>]. By directly measuring CO<sub>2</sub> concentrations with the CO<sub>2</sub> sensor immediately below and above the water surface the gas exchange between the water and the surrounding atmosphere can be studied. The comparison of the measured CO<sub>2</sub> concentrations with calculated values on one hand showed a strong correlation, but also gave evidence that the <italic>in situ</italic> measurement of CO<sub>2</sub> with a sensor is preferable or even necessary in many cases. Deviations from the CO<sub>2</sub> calculation can be found especially at measurements in disturbed water samples.</p><p>For the same reasons in many cases the results of analytical CO<sub>2</sub> determination according to section <secref linkend="mst322926s2-1">2.1</secref> and the measurement with an electrochemical CO<sub>2</sub> sensor in natural waters can be quite different. Table <tabref linkend="mst322926tab13">13</tabref> shows some typical results of comparative CO<sub>2</sub> determinations with an electrochemical CO<sub>2</sub> sensor and with the analytical method according to section <secref linkend="mst322926s2-1">2.1</secref> in regular tap water, in a medium-sized river (Zschopau, Germany) and in a fire protection pond.
<table id="mst322926tab13" frame="topbot"><caption id="mst322926tc13" label="Table 13"><p indent="no">Results of comparative CO<sub>2</sub> determinations on regular tap water, water from a medium-sized river (Zschopau, Germany) and from a fire protection pond, both taken in September (Meinsberg Kurt Schwabe Research Institute).</p></caption><tgroup cols="6"><colspec colnum="1" colname="col1" align="left"></colspec><colspec colnum="2" colname="col2" align="left"></colspec><colspec colnum="3" colname="col3" align="left"></colspec><colspec colnum="4" colname="col4" align="left"></colspec><colspec colnum="5" colname="col5" align="left"></colspec><colspec colnum="6" colname="col6" align="left"></colspec><thead><row><entry></entry><entry></entry><entry></entry><entry></entry><entry>CO<sub>2</sub></entry><entry>Analytical</entry></row><row><entry></entry><entry>Temperature</entry><entry>pH</entry><entry>Conductivity</entry><entry>sensor</entry><entry>method</entry></row><row><entry></entry><entry>(°C)</entry><entry>value</entry><entry>(μS cm<sup>−1</sup>)</entry><entry>(mg L<sup>−1</sup>)</entry><entry>(mg L<sup>−1</sup>)</entry></row></thead><tbody><row><entry>Tap</entry><entry>22</entry><entry>7.60</entry><entry>690</entry><entry> 4.0</entry><entry> 4.0</entry></row><row><entry>water</entry><entry></entry><entry></entry><entry></entry><entry></entry><entry></entry></row><row><entry>River in</entry><entry>20</entry><entry>7.34</entry><entry>363</entry><entry> 2.7</entry><entry> 2.6</entry></row><row><entry>September</entry><entry></entry><entry></entry><entry></entry><entry></entry><entry></entry></row><row><entry>Pond in</entry><entry>20</entry><entry>7.26</entry><entry>742</entry><entry>14.3</entry><entry>18.3</entry></row><row><entry>September</entry><entry></entry><entry></entry><entry></entry><entry></entry><entry></entry></row></tbody></tgroup></table></p><p>While the results of measurements in tap water and in the river agree very well, the measurements in the pond show noticeable deviations. Such differences have also been observed in other territorial stagnant waters, whereby the relative deviations depend on the special site and the season. This phenomenon may be explained by the fact that the CO<sub>2</sub> sensor measures the partial pressure of CO<sub>2</sub> in the liquid while the analytical method determines the concentration of carbonate and possibly also some other ingredients in the solution.</p></sec-level3><sec-level3 id="mst322926s6-1-3" label="6.1.3"><heading>CO<sub>2</sub> in boreholes</heading><p indent="no">In view of the prediction of earthquakes internationally extensive hydrogeological and hydrochemical investigations are performed with the goal to find reliable indicators that can be used for earthquake prediction with more reliable and quantitatively evaluable methods than for e.g. the observation and interpretation of strange behaviour patterns of animals (e.g. maybe of ants [<cite linkend="mst322926bib132">132</cite>]) before an eruption. But despite considerable research efforts undertaken during the last century, this problem has not been solved satisfactory so far.</p><p>Among other possibilities, monitoring of CO<sub>2</sub> could be useful for earthquake prediction. It has been found that CO<sub>2</sub>-rich springs occur worldwide along the major zones of seismicity. Consequently, the presence of such springs may indicate a potentially hazardous region [<cite linkend="mst322926bib133">133</cite>].</p><p>For already more than two decades systematic seismo-hydrological studies have been performed in the Vogtland and the neighbouring North-West Bohemian region at the German–Czech border [<cite linkend="mst322926bib134">134</cite>]. In this geodynamically active region CO<sub>2</sub> degassing takes place in numerous mineral springs and the so-called mofettes. The seismicity is characterized by numerous micro earthquakes, which on some occasions occur at high frequencies, then considered as ‘swarmquakes’. First attempts to take water samples from an exploration borehole at a depth of 135 m failed and resulted in a huge water eruption due to the devolatilization of the oversaturated with CO<sub>2</sub> fluid as a consequence of the movement of the scoop [<cite linkend="mst322926bib135">135</cite>].</p><p>For this reason, the CO<sub>2</sub> content of the water at the bottom of a borehole was determined by means of an electrochemical CO<sub>2</sub> sensor, which was modified especially for this purpose [<cite linkend="mst322926bib136">136</cite>]. Figure <figref linkend="mst322926fig40">40</figref> shows the head of the CO<sub>2</sub> probe with an unscrewed protective cap. The diameter of the CO<sub>2</sub> sensor is 10.5 mm. By means of a watertight connecting cable the CO<sub>2</sub> probe can be submerged up to a depth of 100 m. To transfer the very high ohmic sensor signal over this distance, an impedance converter is integrated into the head of the probe.
<figure id="mst322926fig40"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig40.eps" width="15pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig40.jpg"></graphic-file></graphic><caption id="mst322926fc40" label="Figure 40"><p indent="no">Detailed view of the CO<sub>2</sub> probe with an unscrewed protective cap, developed by KSI.</p></caption></figure></p><p>Figure <figref linkend="mst322926fig41">41</figref> illustrates the temporal course of the CO<sub>2</sub> concentration in a CO<sub>2</sub>-rich mineral water spring. During the study period of 16 days significant changes of the CO<sub>2</sub> concentration have been measured, even though the main influencing parameters such as water temperature and atmospheric pressure remained nearly constant during this time. When heaving the sensor after longer immersion time from a water depth of 100 m, not only the hydrostatic water pressure of 10 atm changes to the normal air pressure, but also the CO<sub>2</sub> concentration from more than 10 mg L<sup>−1</sup> in the sensor electrolyte decreases to less than 1 mg L<sup>−1</sup>. The excessive CO<sub>2</sub> tends to escape from the sensor rapidly, which can result in the destruction of the thin polymeric sensor membrane. To prevent this, the sensor must be heaved very slowly from the water depth thus allowing a gradual adaptation of CO<sub>2</sub> pressure and concentration.
<figure id="mst322926fig41"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig41.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig41.jpg"></graphic-file></graphic><caption id="mst322926fc41" label="Figure 41"><p indent="no">Temporal course of the CO<sub>2</sub> concentration in a mineral water spring (Bad Reiboldsgrün, Germany), as shown in [<cite linkend="mst322926bib136">136</cite>].</p></caption></figure></p></sec-level3><sec-level3 id="mst322926s6-1-4" label="6.1.4"><heading>CO<sub>2</sub> in the ocean</heading><p indent="no">Oceans represent an enormous carbon dioxide reservoir and are a sink for CO<sub>2</sub>. They contain much more CO<sub>2</sub> than the atmosphere and have a considerable influence on the content of CO<sub>2</sub> in the atmosphere and consequently on the world climate. For this reason, over the past few decades numerous oceanic research programmes have been carried out by a number of agencies, commissions, and organizations to obtain data on the complicated carbonate chemistry of the oceans.</p><p>Dissolved carbon dioxide in seawater exists in the forms of bicarbonate HCO<sup>−</sup><sub valign="yes">3</sub> ions, carbonate CO<sup>2 −</sup><sub valign="yes">3</sub> ions and a comparatively small portion of dissolved carbon dioxide CO<sub>2</sub>. Guideline values are 91% HCO<sup>−</sup><sub valign="yes">3</sub>, 8% CO<sup>2 −</sup><sub valign="yes">3</sub> and just 1% CO<sub>2</sub>. The relative concentrations of these components depend on pH value, temperature and many other influencing parameters. According to Henry's law in thermodynamic equilibrium the concentration of a gas in a liquid is proportional to its partial pressure in the gaseous phase above the liquid. However, the solubility coefficient of CO<sub>2</sub> depends on temperature, and the precondition of equilibrium is actually not fulfilled at any time under the prevailing circumstances. The velocity of the CO<sub>2</sub> transfer between the air and the ocean and vice versa is comparatively low and depends e.g. on the surface roughness of the ocean and the wind speed. Numerous complicated chemical, physical and biological processes, often referred to as solubility pump and biological pump, result in large seasonal as well as horizontal and vertical changes of the CO<sub>2</sub> concentration in the ocean [<cite linkend="mst322926bib137">137</cite>, <cite linkend="mst322926bib138">138</cite>].</p><p>Usually the <italic>p</italic>(CO<sub>2</sub>) of seawater is measured by equilibrating a small volume of gas with a large volume of seawater at a given temperature. Then the mixing ratio of CO<sub>2</sub> in the gas phase is determined either using a gas chromatograph or an infrared CO<sub>2</sub> analyser [<cite linkend="mst322926bib139">139</cite>]. If it is intended to apply an electrochemical carbon dioxide sensor in oceanology, this probe must meet difficult requirements concerning pressure resistance, rapid and correct compensation of pressure and temperature changes and high speed of response. When heaving or sending down the probe with a speed of 2.5 m s<sup>−1</sup>, pressure changes of up to 25 kPa s<sup>−1</sup> and temperature changes of up to 5 K s<sup>−1</sup> can occur. A technically rather complex construction for this purpose with a special electrode holder for pressure compensation is disclosed in the patent [<cite linkend="mst322926bib140">140</cite>]. As illustrated in figure <figref linkend="mst322926fig42">42</figref>, the electrochemical CO<sub>2</sub> sensor with its mechanically very sensitive thin polymer membrane is arranged freely sliding a tubular sleeve. The space volumes above the hydrogen carbonate sensor electrolyte and the inner buffer solution of the pH glass electrode are filled with highly insulating silicone oil by which the external pressure is transmitted through an opening immediately to the inner electrode system of the CO<sub>2</sub> sensor, thus preventing its damage due to enormous and rapid pressure increase when the CO<sub>2</sub> probe in the deep sea is moved down into great water depths.
<figure id="mst322926fig42"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig42.eps" width="15pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig42.jpg"></graphic-file></graphic><caption id="mst322926fc42" label="Figure 42"><p indent="no">Schematic drawing of an electrochemical carbon dioxide sensor with pressure compensation (KSI).</p></caption></figure></p><p>A prototype of such a pressure-compensated CO<sub>2</sub> probe was tested in a so-called hydro bottom station (HBS) as shown in figure <figref linkend="mst322926fig43">43</figref>. This device was constructed for sampling and monitoring hydrothermal fluids in the deep sea down to 3500 m water depth [<cite linkend="mst322926bib141">141</cite>]. Besides the CO<sub>2</sub> probe, the sensor unit of the HBS also contained sensors for pH value, redox potential, O<sub>2</sub> concentration and temperature.
<figure id="mst322926fig43"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig43.eps" width="15pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig43.jpg"></graphic-file></graphic><caption id="mst322926fc43" label="Figure 43"><p indent="no">Hydro bottom station for deep-sea research [<cite linkend="mst322926bib141">141</cite>].</p></caption></figure></p></sec-level3></sec-level2><sec-level2 id="mst322926s6-2" label="6.2"><heading>Applications in biology</heading><sec-level3 id="mst322926s6-2-1" label="6.2.1"><heading>CO<sub>2</sub> measurements on aquatic animals</heading><p indent="no">For the assessment of the physiological condition of aquatic animals the measurement of respiration is essential, which is influenced considerably by the water quality. For this reason aquaculture operations demand precise control of water quality, and the monitoring of water parameters becomes increasingly important especially for effective and high quality rearing and production of freshwater nutritious fish.</p><p>Apart from pH value, oxygen concentration, biochemical oxygen demand, salinity, etc, carbon dioxide concentration is an important parameter in fish farming [<cite linkend="mst322926bib142">142</cite>]. Not only sufficient supply of dissolved oxygen O<sub>2</sub>, but also the elimination of the breathing product CO<sub>2</sub> over the gills of the fish must be assured, which is only possible if an adequate CO<sub>2</sub> concentration gradient exists between the blood of fish and the surrounding water. High CO<sub>2</sub> concentrations in the feed water, occurring especially in lime-rich environments, reduce the capacity of the blood to transport oxygen in the brain and in other organs of fish. Consequences are increased liability to infectious diseases and reduced effectiveness of food intake and utilization despite the presence of sufficient oxygen [<cite linkend="mst322926bib143" range="mst322926bib143,mst322926bib144,mst322926bib145">143–145</cite>].</p><p>The maximum admissible CO<sub>2</sub> concentration in trout breeding ponds depends among other parameters to a great extent on the acid binding capacity or the water hardness. The guideline CO<sub>2</sub> concentrations values for rainbow trout are <10–30 mg L<sup>−1</sup> CO<sub>2</sub> for fish spawn and <15–35 mg L<sup>−1</sup> CO<sub>2</sub> for adult fish (>5 cm). The lethal concentration is >50–150 mg L<sup>−1</sup> CO<sub>2</sub> [<cite linkend="mst322926bib04">4</cite>]. The lower values are valid for water with low water hardness (acid binding capacity <0.5 mval L<sup>−1</sup>), and the indicated upper limits are admissible at high water hardness (acid binding capacity >3.5 mval L<sup>−1</sup>), respectively.</p><p>Figure <figref linkend="mst322926fig44">44</figref> shows a special aquarium that was used in a fish farm (Trout Farm Trostadt, Germany) to test experimentally whether fish instinctively avoid areas with high CO<sub>2</sub> concentration [<cite linkend="mst322926bib146">146</cite>]. This aquarium is divided by a panel into three sections. Through the right section flows water with a high CO<sub>2</sub> concentration of 32 mg L<sup>−1</sup> (pH = 7.44) and through the left one degassed water from the same spring with only 13 mg L<sup>−1</sup> CO<sub>2</sub> (pH = 8.11). In the common section the two different waters admix and then flow out.
<figure id="mst322926fig44"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig44.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig44.jpg"></graphic-file></graphic><caption id="mst322926fc44" label="Figure 44"><p indent="no">Photograph of the special aquarium with sections of different CO<sub>2</sub> concentrations and the final distribution of fish in it (taken by KSI).</p></caption></figure></p><p>The CO<sub>2</sub> concentrations in the waters were measured by means of a hand-held CO<sub>2</sub> meter with an electrochemical carbon dioxide sensor. pH value and temperature of the water were determined, too.</p><p>A shoal of 58 rainbow trout (<italic>Oncorhynchus mykiss</italic>) was exposed in the common section of the aquarium, where the fish had free choice to remain in this region or to move to one of the regions with either higher or lower CO<sub>2</sub> concentrations. As can be seen in figure <figref linkend="mst322926fig44">44</figref>, already after less than 1 min the shoal had left the common section to gather in the section with the lower CO<sub>2</sub> concentration. From this observation, the conclusion can be drawn that, contrary to human beings, fish are able to perceive carbon dioxide gradients and react appropriately [<cite linkend="mst322926bib147">147</cite>]. The experiment demonstrated that fish escape from regions with higher levels of CO<sub>2</sub> even at concentrations that are not yet life threatening.</p><p>A miniaturized electrochemical carbon dioxide sensor, which was developed at the Meinsberg Kurt Schwabe Research Institute, was also used for <italic>in vitro</italic> determination of respiratory quotients on zebra mussels (<italic>Dreissena polymorpha</italic>) with high accuracy. The respiratory quotient (RQ) describes the number of moles of carbon dioxide produced per mole of oxygen consumed. Due to its strong dependence on the composition of the metabolized food, the RQ is used to characterize the animals’ nutritional condition in the habitat and the quantity and quality of food. If carbohydrates are burnt the respiratory quotient is around 1.0, whereas a RQ of 0.7 indicates that fats (lipids) are metabolized. In the study (performed at the Dresden University of Technology in cooperation with the Meinsberg Kurt Schwabe Research Institute, Germany) oxygen consumption and carbon dioxide excretion of Zebra Mussels were tested in Winkler vials [<cite linkend="mst322926bib148">148</cite>]. The test species originated from various sewage polishing ponds and from a harbour of the river Elbe near Dresden where they had been exposed for periods of at least 1 month prior to the experiments. Statistical analysis showed significant differences between the mussels from the different sites. Furthermore, there was a significant difference between measured RQ = 0.70 in summer during the spawning period and RQ = 0.83 in winter, indicating metabolizing fat in summer and protein in winter. Due to the fact that the measurements of oxygen and carbon dioxide concentrations were carried out in the same vessel on the same animals, it was possible to compare the original data of oxygen consumption and carbon dioxide excretion. The miniaturized electrochemical CO<sub>2</sub> sensor proved to be very suitable for this application.</p></sec-level3><sec-level3 id="mst322926s6-2-2" label="6.2.2"><heading>CO<sub>2</sub> measurements on insects</heading><p indent="no">Insects, like most other animals, inhale oxygen to live, but they have a unique respiratory system. CO<sub>2</sub> measurements on insects have been performed with quite different methods, instrumentation and goals. As two characteristic examples in this section the results of scientific fundamental studies on butterfly pupae and more phenomenological investigations on honey bees are presented.</p><p>Butterfly pupae breathe through small openings in the cuticle, called spiracles. These are connected to the inner organs by a system of highly branched tubes, called tracheae. Oxygen enters through the spiracles and diffuses into the blood. The cells release carbon dioxide, which is carried back to the spiracles. Insect spiracles can open and close. They behave like valves, opening and closing to allow or restrict the insect's gas exchange [<cite linkend="mst322926bib149">149</cite>].</p><p>A flow-through measurement setup was constructed for simultaneous and continuous measurement of oxygen partial pressure, carbon dioxide output and intratracheal hydrostatic pressure on lepidopterous pupae [<cite linkend="mst322926bib149">149</cite>, <cite linkend="mst322926bib150">150</cite>]. Figure <figref linkend="mst322926fig45">45</figref> shows the transparent measuring chamber with an Atlas moth (<italic>Attacus atlas</italic>) pupa in it. The chamber was permanently rinsed with carbon-dioxide-free moisturized air at a constant flow rate. For intratracheal measurement of oxygen and carbon dioxide miniaturized electrochemical sensors (from Meinsberg Kurt Schwabe Research Institute, Germany) were used. Two spiracles of each pupa were intubated for the experiments and connected to the sensors via a short piece of polyethylene tubing. The amount of carbon dioxide released by the pupae was measured using a flow-through IR CO<sub>2</sub> gas analyser URAS 3G, Hartmann & Braun, Frankfurt, Germany).
<figure id="mst322926fig45"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig45.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig45.jpg"></graphic-file></graphic><caption id="mst322926fc45" label="Figure 45"><p indent="no">Measuring chamber with Atlas moth pupa, length 4.5 cm (photo taken by KSI).</p></caption></figure></p><p>Figure <figref linkend="mst322926fig46">46</figref> shows typical results of <italic>in vivo</italic> measured respiratory cycles of an Atlas moth pupa. This unusual respiratory behaviour, which has been observed in many adult insects as well as in resting butterfly and moth pupae, is referred to as discontinuous gas-exchange cycle (DGC). In insects exhibiting DGC, the spiracles close for long periods up to several hours or even days and open occasionally for only a few minutes. Initiated by a critically high amount of CO<sub>2</sub> in blood, a burst of CO<sub>2</sub> release is observed. During the longer closed phase CO<sub>2</sub> release is very low. Initiated by critically low levels of oxygen, the closed phase is followed by a flutter phase during which CO<sub>2</sub> is released in short intervals.
<figure id="mst322926fig46"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig46.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig46.jpg"></graphic-file></graphic><caption id="mst322926fc46" label="Figure 46"><p indent="no"><italic>In vivo</italic> measured respiratory cycles of an Atlas moth pupa: (<italic>A</italic>) carbon dioxide output, (<italic>B</italic>) intratracheal oxygen partial pressure [<cite linkend="mst322926bib149">149</cite>].</p></caption></figure></p><p>For a long time scientists had suggested that this opening and closing of the spiracles helps to prevent water loss, but recently, based on experimental facts, it is assumed that insects do it to protect their tissues from getting too much oxygen [<cite linkend="mst322926bib151">151</cite>]. The cyclical pattern of open and closed spiracles observed in resting insects is supposed to be a necessary consequence of the need to rid the respiratory system of accumulated CO<sub>2</sub>, followed by the need to reduce oxygen toxicity.</p><p>Since it is very difficult to investigate flying insects, most respiration measurements have been performed on resting insects and developing instars. A highly sophisticated method and experimental setup for CO<sub>2</sub> measurements on insects also during flight activities have been published by Wasserthal [<cite linkend="mst322926bib152">152</cite>]. For measuring tracheal pressure and CO<sub>2</sub> emission from specified spiracles of hawkmoths he used a special split-specimen chamber with a controlled constant-speed airflow and adjustable pressure and temperature. In the chamber the healthy moths were suspended at the descaled mesoscutellum. The tubes leading from the anterior spiracles were connected to a small anterior compartment of the split-specimen chamber, whereas the posterior thoracic and abdominal spiracles opened into a larger posterior specimen compartment. CO<sub>2</sub> emission from the anterior spiracles and the posterior thoracic and abdominal spiracles was measured separately with the split-specimen chamber. For CO<sub>2</sub> measurement, the airflow from either the anterior or the posterior compartment was conveyed directly to an infrared gas analyser, while the air from the other compartment was conveyed to a CO<sub>2</sub>-absorbing vessel containing NaOH. Flight was initiated some hours after the moths had been mounted in the chamber. All moths were used at first for tracheal pressure measurements and then for CO<sub>2</sub> emission measurements. Furthermore, the wingbeat was recorded by projecting the shadow of the moving wings onto photocells installed on the bottom of the Perspex specimen chamber. As an interesting result of the studies it was stated that during steady flight air is inspired through the anterior spiracles and expired through the posterior thoracic spiracles. CO<sub>2</sub> is emitted only at the posterior spiracles. No CO<sub>2</sub> emission was recorded from the anterior spiracles, which opened into the anterior chamber.</p><p>Many insects are capable of detecting elevated levels of carbon dioxide [<cite linkend="mst322926bib153">153</cite>, <cite linkend="mst322926bib154">154</cite>] and react reasonably to this sensory perception. Some use elevated CO<sub>2</sub> concentrations or CO<sub>2</sub> gradients, respectively, to locate their vertebrate hosts [<cite linkend="mst322926bib154" range="mst322926bib154,mst322926bib155,mst322926bib156">154–156</cite>] or to evaluate floral quality [<cite linkend="mst322926bib157">157</cite>, <cite linkend="mst322926bib158">158</cite>]. Others, like honeybees and ants, regulate potentially lethal CO<sub>2</sub> concentrations in their social colonies [<cite linkend="mst322926bib159">159</cite>, <cite linkend="mst322926bib160">160</cite>]. Recently, it has been found that some insects possess and utilize special carbon dioxide chemoreceptors [<cite linkend="mst322926bib161" range="mst322926bib161,mst322926bib162,mst322926bib163">161–163</cite>], but honeybees do not. It is supposed that they must have evolved other senses to detect carbon dioxide.</p><p>CO<sub>2</sub> measurements on honeybees showed that the carbon dioxide levels in a beehive can rise to excessive levels. Figure <figref linkend="mst322926fig47">47</figref> presents results of CO<sub>2</sub> measurements in two beehives, which had been carried out by Meinsberg Kurt Schwabe Research Institute in cooperation with Freie Universität Berlin, Faculty of Biology, Animal Physiology. The measured carbon dioxide concentration in the open beehive was permanently 0.5 vol.% or even higher. This is just the TLV-TWA value, the maximum allowable concentration for human beings. After the entrance of the beehive had been closed, the CO<sub>2</sub> concentration in the beehive rose within 90 min to more than 7 vol.%. While such a high CO<sub>2</sub> concentration would have been lethal for human beings within a short time, the bees got over this concentration without recognizable injury.
<figure id="mst322926fig47"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig47.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig47.jpg"></graphic-file></graphic><caption id="mst322926fc47" label="Figure 47"><p indent="no">Measurement of carbon dioxide concentrations in two beehives.</p></caption></figure></p><p>Nevertheless, it is assumed that high carbon dioxide concentrations in the hive could be as detrimental for the bees as high temperature or humidity and could cause reductions in honey production and pollination. To reduce these stress factors it is recommended to install a beehive ventilator at the hive entrance to exhaust the stale air [<cite linkend="mst322926bib163">163</cite>, <cite linkend="mst322926bib164">164</cite>]. Furthermore, the exhausted air should help bees to orient to their domestic hive.</p><p>For more detailed measurements of carbon dioxide concentration in beehives an electrochemical carbon dioxide sensor (EMCO2, Meinsberg Kurt Schwabe Research Institute) was embedded in a customary honeycomb as shown in figure <figref linkend="mst322926fig48">48</figref>. Thereby precaution was taken to prevent bees from wrapping up the sensor in honey. Apart from CO<sub>2</sub> the temperature inside and outside the hive, the humidity and the time of sunrise and sundown were recorded, and a bee counter was installed at the entrance of the hive.
<figure id="mst322926fig48"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig48.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig48.jpg"></graphic-file></graphic><caption id="mst322926fc48" label="Figure 48"><p indent="no">Electrochemical carbon dioxide sensor (EMCO2, KSI) in a honeycomb.</p></caption></figure></p><p>Figure <figref linkend="mst322926fig49">49</figref> shows typical results of CO<sub>2</sub> concentration measurements in the beehive over a period of 5 days. From the diagram no direct relationship between the CO<sub>2</sub> concentration and the temperature may be derived. Obviously the bees attend the hive exactly in accordance with sunrise and sundown.
<figure id="mst322926fig49"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig49.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig49.jpg"></graphic-file></graphic><caption id="mst322926fc49" label="Figure 49"><p indent="no">CO<sub>2</sub> concentration in a beehive over a period of 5 days. SR: sunrise; SD: sundown; <italic>T</italic>(in): temperature inside the hive; <italic>T</italic>(out): outside temperature.</p></caption></figure></p></sec-level3><sec-level3 id="mst322926s6-2-3" label="6.2.3"><heading>CO<sub>2</sub> measurements on plants</heading><p indent="no">Respiration of plants and photosynthesis are of utmost importance for environment and living organisms. Using solar energy, plants transform CO<sub>2</sub> absorbed from the atmosphere via photosynthesis into chemical energy stored in organic carbon compounds such as glucose and, furthermore, supply the atmosphere with oxygen, while aquatic plants such as algae absorb CO<sub>2</sub> dissolved in the water, and the terrestrial green plants take up CO<sub>2</sub> from the air through tiny openings on their leaves. Through these stomata, which are able to rapidly change their aperture, carbon dioxide penetrates leaves and in the presence of water it is dissolved and enters into cells. Once inside the leaf, CO<sub>2</sub> has to diffuse from the intercellular air spaces to the sites of carboxylation, where photosynthesis occurs. The CO<sub>2</sub> gradient within the leaf and the internal resistance to carbon dioxide diffusion affect the efficiency of the leaf, which depends on the partial pressure of CO<sub>2</sub> at the sites of carboxylation. For this reason, the internal diffusion paths of CO<sub>2</sub> in the leaf are not only of scientific but also of practical interest and the topic of numerous intensive studies and related publications [<cite linkend="mst322926bib165">165</cite>].</p><p>Apparently it is not possible to measure directly the gradient of CO<sub>2</sub> partial pressure to the sites of carboxylation in leaves with the electrochemical CO<sub>2</sub> sensors and methods described in this review. In [<cite linkend="mst322926bib166">166</cite>] two other possibilities are discussed and applied. The first method is based on carbon isotope discrimination and measures the change in carbon isotopic composition of CO<sub>2</sub> passing over the leaf, while the second one is a non-intrusive optical method and measures fluorescence during photosynthesis.</p><p>According to the equation
<display-eqn id="mst322926eqn6-2" eqnnum="6.2"></display-eqn>during the process of photosynthesis leaves convert carbon dioxide and water with the help of light energy to glucose and free oxygen.</p><p>The fundamental significance of the light for this process has been illustrated in a simple demonstration experiment as shown in figure <figref linkend="mst322926fig50">50</figref>. A miniaturized electrochemical CO<sub>2</sub> sensor with a diameter of only 10 mm was used for carbon dioxide measurement on indoor plants. The room fern was arranged near a window in a transparent thermostated vessel that provided for constant temperature during the measurements.
<figure id="mst322926fig50"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig50.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig50.jpg"></graphic-file></graphic><caption id="mst322926fc50" label="Figure 50"><p indent="no">Carbon dioxide measurement on plants using a miniaturized electrochemical CO<sub>2</sub> sensor (KSI).</p></caption></figure></p><p>Figure <figref linkend="mst322926fig51">51</figref> shows a typical result of such carbon dioxide measurements on indoor plants. As expected, due to the photosynthetic assimilation of CO<sub>2</sub> under the influence of day and room light it was observed that the brighter the daylight the lower was the carbon dioxide concentration in the test vessel.
<figure id="mst322926fig51"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig51.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig51.jpg"></graphic-file></graphic><caption id="mst322926fc51" label="Figure 51"><p indent="no">Result of the CO<sub>2</sub> measurement on a room fern.</p></caption></figure></p><p>To permit CO<sub>2</sub> measurements behind stomatal pores of plant leaves, a miniaturized potentiometric CO<sub>2</sub> biosensor was built with a tip diameter of 2 µm [<cite linkend="mst322926bib167">167</cite>]. It consists of a H<sup>+</sup> carrier-based pH microelectrode concentrically arranged within a sheathing micropipette, the tip of which is filled with carbonic anhydrase solution. Due to incorporated carbonic anhydrase the pH of the solution responds quickly to CO<sub>2</sub> concentration changes. The CO<sub>2</sub> microsensor shows a logarithmic response to CO<sub>2</sub> in the physiological relevant concentration range of 50 to 800 ppm. Its 90% response time varied between 18 and 63 s. Sensor calibration and leaf experiments were performed in an open-flow tube-like minicuvette, allowing tangential air flow along the leaf surface with controlled gas mixtures and flow rates of choice. At an external CO<sub>2</sub> concentration of 800 ppm, CO<sub>2</sub> concentration within the leaf was close to this value when stomatal pores were wide open. During stomatal closure the concentration dropped to 350 ppm due to CO<sub>2</sub> consumption by photosynthesis, thus demonstrating distinct sensing of internal leaf CO<sub>2</sub>. Following ‘light-off’ internal CO<sub>2</sub> rapidly rose close to 700 ppm, which was completely reversed by ‘light-on’. It was concluded that this sensor is a suitable tool for CO<sub>2</sub> monitoring in places too small to be accessible by conventional tools.</p><p>The measurement of CO<sub>2</sub> as a product of metabolism of plants and animals is also possible with high-temperature sensors as described in section <secref linkend="mst322926s3">3</secref>, because these sensors are very small and the entrance of heat into the analysing system is very low (approximately 3 W).</p><p>By the way, a plant leaf itself can be used as CO<sub>2</sub> gas sensing probe [<cite linkend="mst322926bib168">168</cite>]. In an experiment concerning this matter a leaf of spiderwort (<italic>Commelina communis</italic>) was attached on a glass slide under a microscope. The stem of the leaf was dipped in a bathing solution. Then the intracellular potential (ICP) was measured with a glass capillary electrode referred to another electrode dipped in the bathing solution. When CO<sub>2</sub> gas was supplied to the leaf under illumination, a considerable shift of the ICP was observed. This process was reversible, when CO<sub>2</sub> supply was stopped, the ICP returned to the initial level immediately. The authors concluded that these results demonstrate the promising property of a plant leaf as a CO<sub>2</sub> gas-sensing element.</p></sec-level3></sec-level2><sec-level2 id="mst322926s6-3" label="6.3"><heading>Applications in biotechnology</heading><sec-level3 id="mst322926s6-3-1" label="6.3.1"><heading>CO<sub>2</sub> measurements in bioreactors</heading><p indent="no">Carbon dioxide is a product of cellular metabolism of microorganisms used in biotechnology. During fermentation the carbon dioxide content is the result of carbon dioxide formation by microorganisms and its transport by aeration. Apart from pH and dissolved oxygen measurements, reliable monitoring and control of the carbon dioxide partial pressure is important for successful fermentation and attracts more and more attention for the large-scale production of monoclonal antibodies and pharmaceutical products. Precise, real-time data on CO<sub>2</sub> concentration increase an understanding of critical fermentation and cell culture processes and can help in gaining insight into cell metabolism, cell culture productivity, and other changes within bioreactors. By means of online CO<sub>2</sub> measurements in the cell suspension culture with an electrochemical CO<sub>2</sub> probe it is possible to gain additional data for process characterization and to detect such carbon dioxide concentrations which inhibit metabolism and growth of micro organisms. The extension of observed process states to dissolved CO<sub>2</sub> concentration, HCO<sup>−</sup><sub valign="yes">3</sub> concentration, carbon dioxide production rate and respiratory quotient offers the possibility of closed mass balances for the bioreactor and its liquid and gas phase. So the user is able to start compensating steps, e.g. aeration, to get information about the growth and activity of microorganisms during fermentation and to realize the control of anaerobic fermentations [<cite linkend="mst322926bib169" range="mst322926bib169,mst322926bib170,mst322926bib171,mst322926bib172">169–172</cite>].</p><p>For this reason fed-batch cultures have been intensively studied and optimized with the aid of kinetic models [<cite linkend="mst322926bib169">169</cite>]. Dissolved CO<sub>2</sub> concentration, hydrogen carbonate (HCO<sup>−</sup><sub valign="yes">3</sub>) concentration as well as carbon dioxide production rate of mammalian cell suspension culture have great impact on growth conditions. But the carbon dioxide transfer rate (CTR), which can be directly calculated from off-gas-measurement, is not necessarily equal to the interesting carbon dioxide production rate (CPR). Hence various mathematical methods have been developed for the estimation of the carbon dioxide production rate and related parameters such as the concentrations of dissolved CO<sub>2</sub> and HCO<sup>−</sup><sub valign="yes">3</sub> and total dissolved carbonate directly from the off-gas data [<cite linkend="mst322926bib170" range="mst322926bib170,mst322926bib171,mst322926bib172">170–172</cite>]. In [<cite linkend="mst322926bib171">171</cite>, <cite linkend="mst322926bib172">172</cite>] a mathematical model for the determination of dissolved CO<sub>2</sub> concentration, HCO<sup>−</sup><sub valign="yes">3</sub> concentration, and respiratory activity of mammalian cell suspension culture based on off-gas-measurement is presented. Additionally, the calculated dissolved CO<sub>2</sub> concentrations are compared with results of online CO<sub>2</sub> measurements by using an electrochemical CO<sub>2</sub> probe. In order to test the developed simulation, batch experiments were carried out in a 2 L stirred bioreactor as shown in figure <figref linkend="mst322926fig52">52</figref>.
<figure id="mst322926fig52"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig52.eps" width="12pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig52.jpg"></graphic-file></graphic><caption id="mst322926fc52" label="Figure 52"><p indent="no">2 L stirred bioreactor (VSF 2000, Bioengineering AG, CH) with an electrochemical carbon dioxide sensor (KSI).</p></caption></figure></p><p>Off-gas CO<sub>2</sub> measurement was provided by a dual-beam IR CO<sub>2</sub> sensor (0–5%). With the accuracy 0.05 vol.% this CO<sub>2</sub> sensor was especially suitable for the high demands of measurements of mammalian cell culture. Online dissolved CO<sub>2</sub> concentration measurements were carried out by an electrochemical CO<sub>2</sub> probe (Meinsberg Kurt Schwabe Research Institute). Although this sensor cannot be steam sterilized or autoclaved, it could be used successfully for determination of dissolved CO<sub>2</sub> concentration in mammalian cell suspension in the bioreactor. For this purpose, the sensor was sterilized chemically with ethyl alcohol and connected to the reactor by a sterile adapter. In figure <figref linkend="mst322926fig53">53</figref> results of measured and calculated dissolved CO<sub>2</sub> concentrations during cultivation time of a reactor batch experiment are compared.
<figure id="mst322926fig53"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig53.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig53.jpg"></graphic-file></graphic><caption id="mst322926fc53" label="Figure 53"><p indent="no">Comparison of simulated and measured dissolved CO<sub>2</sub> concentrations in a reactor batch experiment [<cite linkend="mst322926bib171">171</cite>].</p></caption></figure></p><p>Figure <figref linkend="mst322926fig53">53</figref> shows generally satisfactory conformity between simulated and measured curves. The online dissolved CO<sub>2</sub> concentration measurements approved the simulations, which provide valuable information for process modelling and model improvement of cell culture growth. Only during the first 25 h of the experiment the measured CO<sub>2</sub> concentration was higher. Not yet formed CO<sub>2</sub> mass transfer and CO<sub>2</sub> oversaturation exceeding simulated conditions can cause the slightly underestimated simulations of dissolved CO<sub>2</sub>. Furthermore, a spatial CO<sub>2</sub> profile in the liquid phase can evolve with preceding cultivation time.</p></sec-level3><sec-level3 id="mst322926s6-3-2" label="6.3.2"><heading>CO<sub>2</sub> measurements in biogas</heading><p indent="no">The sensory control of biogas production plants is essential for effectiveness and safety of this important source of renewable energy. Online monitoring by means of electrochemical sensors enables the early detection of critical feeding situations and helps to prevent cost-intensive biogas production breakdowns. Today a variety of methods are used to characterize and control the biogas process in liquid-filled reactors by measuring a number of critical parameters in the gas phase as well as in the liquid phase [<cite linkend="mst322926bib173">173</cite>, <cite linkend="mst322926bib174">174</cite>]. Figure <figref linkend="mst322926fig54">54</figref> shows the schematic design of a temperature-controlled biogas laboratory plant for the investigation of biogas production kinetics with sensors for the liquid phase (pH, temperature, dissolved hydrogen) and the gas phase (CH<sub>4</sub>, CO<sub>2</sub>, H<sub>2</sub>, flow rate) [<cite linkend="mst322926bib174">174</cite>, <cite linkend="mst322926bib175">175</cite>]. For measuring the CO<sub>2</sub> concentration in the biogas an IR sensor (AGM 32, Sensors Europe GmbH) was used.
<figure id="mst322926fig54"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig54.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig54.jpg"></graphic-file></graphic><caption id="mst322926fc54" label="Figure 54"><p indent="no">Schematic construction of the biogas laboratory plant, developed and manufactured by KSI. 1: two glass reactors with—2: stirrers; 3: thermal insulation; 4: sensors for the liquid phase (pH, temperature, dissolved hydrogen); 5: reactor cover; 6: stirring motor; 7: multi parameter measuring system with sensors for the gas phase (CH<sub>4</sub>, CO<sub>2</sub>, H<sub>2</sub> and flow rate) and transducers; 8: PC with measuring cards.</p></caption></figure></p><p>Apart from measuring CH<sub>4</sub> and H<sub>2</sub> as the most interesting components, it is very helpful to determine additionally the concentration of CO<sub>2</sub> in the biogas. On one hand by summarization of these components it can be determined whether the biogas contains further components. On the other hand the correct function of the other sensors can be controlled. Figure <figref linkend="mst322926fig55">55</figref> shows the course of the biogas production during the start-up period of a mesophilic biogas process in the biogas laboratory plant. After about 36 h the air had been completely expelled from the reactor vessels by the developed biogas consisting of about 2% H<sub>2</sub>, 42% CO<sub>2</sub> and 56% CH<sub>4</sub>. The sum of almost exactly 100% indicates that the biogas process runs as expected and that the hydrogen and methane sensors work perfectly.
<figure id="mst322926fig55"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig55.eps" width="20.5pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig55.jpg"></graphic-file></graphic><caption id="mst322926fc55" label="Figure 55"><p indent="no">Course of the development of the gases H<sub>2</sub>, CO<sub>2</sub> and CH<sub>4</sub> during the start-up period of a mesophilic biogas process in the biogas laboratory plant.</p></caption></figure></p></sec-level3></sec-level2><sec-level2 id="mst322926s6-4" label="6.4"><heading>CO<sub>2</sub> applications and measurement in food production and control</heading><p indent="no">The use of advanced instrumentation and sensors in the food industry has led to continuing improvement in food quality control, safety and process optimization [<cite linkend="mst322926bib176">176</cite>]. Chemical sensors as well as biosensors are increasingly used for process monitoring and for quality and freshness control of meat, dairy products, beverages and a variety of other foodstuffs. Apart from pH value, oxygen concentration and conductivity, the CO<sub>2</sub> concentration is an interesting parameter, which among others is measured or controlled e.g. in
<itemized-list id="mst322926il12"><list-item id="mst322926il12.1" marker="•"><p indent="no">the beverage and brewery industry,</p></list-item><list-item id="mst322926il12.2" marker="•"><p indent="no">fruit and vegetable ripening and storage,</p></list-item><list-item id="mst322926il12.3" marker="•"><p indent="no">greenhouses,</p></list-item><list-item id="mst322926il12.4" marker="•"><p indent="no">the production of yeast,</p></list-item><list-item id="mst322926il12.5" marker="•"><p indent="no">CO<sub>2</sub> monitoring in egg setters,</p></list-item><list-item id="mst322926il12.6" marker="•"><p indent="no">food packaging as well as</p></list-item><list-item id="mst322926il12.7" marker="•"><p indent="no">quality control and characterization of food quality and freshness.</p></list-item></itemized-list></p><p>In the beverage and brewery industry carbon dioxide is used to carbonate the beverages in the production of soft drinks. Carbonated mineral water, beer or Coca-Cola contain more than 2000 mg L<sup>−1</sup> CO<sub>2</sub>. The CO<sub>2</sub> concentration of the beverages is an important quality parameter. Conventional electrochemical CO<sub>2</sub> sensors do not fulfil the stringent demands of mechanical robustness, pressure and temperature stability and CIP cleaning required for online process control in the beverage industry. Therefore, to control the efficiency and quality of the processes CO<sub>2</sub> is measured in the process exhaust air. This method, which is based on Henry's law, was successfully applied e.g. for inline CO<sub>2</sub> measurements on Coca-Cola [<cite linkend="mst322926bib177">177</cite>]. Furthermore, CO<sub>2</sub> sensors working according to the gas permeation principle as described in section <secref linkend="mst322926s4-5">4.5</secref> are applied for process control in the beverage industry. But there is also another need for carbon dioxide measurement in the brewery industry. The fermentation process produces large amounts of carbon dioxide. During the bottling process CO<sub>2</sub> can be emitted from the fillers into the ambient air of the production hall. As a high concentration of CO<sub>2</sub> is hazardous, safety monitoring is essential in beverage filling facilities. For this purpose infrared CO<sub>2</sub> transmitters, e.g. Vaisala CARBOCAP® Carbon Dioxide Meters, can be used.</p><p>Fruits and vegetables are stored and ripened in specially controlled atmosphere (CA) rooms that have systems for controlling humidity, temperature as well as CO<sub>2</sub> concentration. CO<sub>2</sub> must be monitored to determine the level of ventilation, since high CO<sub>2</sub> levels can preserve the fruits in storage, but also retard the ripening. Depending upon the product being stored, CA conditions typically include decreased oxygen and increased carbon dioxide concentrations plus high relative humidity levels that have to be precisely maintained throughout the storage period. Research has shown that specific fruits require optimized storage conditions. As an example, the objective of a study [<cite linkend="mst322926bib178">178</cite>] was to determine what storage conditions and pretreatments would permit long-term storage of New Zealand limes with minimal loss of quality. Limes are an attractive fruit crop but generally suffer a loss in value as their colour changes from green to yellow. Various approaches were taken to slow degreening including low-temperature storage, use of CA environments, and treatment of fruit with physiologically active agents. However, the cold storage life of lime fruit can also be restricted by a number of factors including chilling injury. CA storage (10% O<sub>2</sub> with 0 or 3% CO<sub>2</sub>) was compared to regular air storage and treatments with varying durations across a range of temperatures. Although some CA storage regimes could assist in delaying degreening, none of the treatments provided protection against frost damage. CA storage at 3% CO<sub>2</sub> delayed yellowing and gave better fruit quality than the low CO<sub>2</sub> treatment. High CO<sub>2</sub> CA treatments at 5 or 7 °C decreased the rate of colour change compared to other constant temperature treatments but did not protect against chilling injury.</p><p>In greenhouses and mushroom farms controlling the concentration of carbon dioxide can enhance plant growth and ripening. Depending on plant type, humidity and light conditions, the optimum CO<sub>2</sub> concentration must be carefully adjusted. If the carbon dioxide level rises too high, the growth of plants can be stunted or they can even be damaged.</p><p>Yeast is an important microorganism delivering an essential contribution for our nutrition. It is used e.g. for bread dough processing and making alcoholic drinks. During the fermentation processes CO<sub>2</sub> is formed, especially in the aerobic phase. High as well as very low carbon dioxide concentrations influence the fermentation of yeast.</p><p>Proper control of humidity, temperature and carbon dioxide levels in egg setters during egg incubation can ensure an improved hatching rate and increased profitability. While slightly elevated levels of CO<sub>2</sub> during the first few days of incubation stimulate the growth of embryos, too high CO<sub>2</sub> concentration will inhibit growth.</p><p>In the food industry carbon dioxide is the most important protective gas in the packaging of food under modified atmospheres (MAP). Adding CO<sub>2</sub> to food packaging extends considerably the storage and shelf life of meat, cheese as well as fruits and vegetables. In meat packaging, for example, a high concentration of CO<sub>2</sub> in the packaging inhibits bacterial growth and retains the natural colouring of the meat. The leak detection system for packages based on CO<sub>2</sub> such as LEAK-MASTER® [<cite linkend="mst322926bib179">179</cite>] feature non-destructive detection of leaks in the packages directly after the packing process. If the test sample is leaking, the pressure difference will result in a gas flow from the package into a test chamber, and the CO<sub>2</sub> concentration within the chamber rises. By means of a highly sensitive infrared CO<sub>2</sub> sensor (measuring range 0 to 5000 ppm, resolution 1 ppm) even smallest leaks can easily be detected.</p><p>The freshness of milk and dairy products can be tested by various electrochemical and enzymatic methods and sensors [<cite linkend="mst322926bib180">180</cite>]. In comparative studies the pH value, the chloride, oxygen and lactate concentrations, the temperature, conductivity as well as carbon dioxide were measured simultaneously during the souring process of the milk [<cite linkend="mst322926bib181">181</cite>]. Figure <figref linkend="mst322926fig56">56</figref> shows results of measurement of the souring process of fresh, untreated cow's milk. The souring process begins gradually after approximately 15 h. The biochemical reactions taking place in the milk during the souring process result in a considerable increase of the lactate and decrease in the oxygen concentration. As expected, the pH value decreases with increasing milk souring, while the CO<sub>2</sub> concentration rises.
<figure id="mst322926fig56"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig56.eps" width="18pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig56.jpg"></graphic-file></graphic><caption id="mst322926fc56" label="Figure 56"><p indent="no">Results of measurement (performed at KSI) of the souring process of fresh, untreated cow's milk.</p></caption></figure></p><p>As generally in the electrochemical measuring technique, in food industry the pH value is the most measured variable. But in some cases, as shown in table <tabref linkend="mst322926tab14">14</tabref>, the CO<sub>2</sub> concentration indicates the state of spoilage much more significantly than the pH value, which remains nearly constant in some cases.
<table id="mst322926tab14" frame="topbot"><caption id="mst322926tc14" label="Table 14"><p indent="no">Changes in CO<sub>2</sub> concentration and pH value of some liquid and paste-like perishable foods in the course of the spoiling process.</p></caption><tgroup cols="3"><colspec colnum="1" colname="col1" align="left"></colspec><colspec colnum="2" colname="col2" align="left"></colspec><colspec colnum="3" colname="col3" align="left"></colspec><thead><row><entry>Food</entry><entry>CO<sub>2</sub> concentration (mg L<sup>−1</sup>)</entry><entry>pH value</entry></row></thead><tbody><row><entry>Milk fresh, untreated</entry><entry> 30</entry><entry>6.8</entry></row><row><entry>After 2 days</entry><entry> 277</entry><entry>4.4</entry></row><row><entry>Buttermilk, fresh</entry><entry> 260</entry><entry>4.4</entry></row><row><entry>After 7 days</entry><entry> 16</entry><entry>4.4</entry></row><row><entry>Apple puree, fresh</entry><entry> 17</entry><entry>3.3</entry></row><row><entry>After 2 days</entry><entry>1700</entry><entry>3.3</entry></row><row><entry>Tomato puree, fresh</entry><entry> 16</entry><entry>4.5</entry></row><row><entry>After 2 days</entry><entry>1600</entry><entry>4.4</entry></row></tbody></tgroup></table></p></sec-level2><sec-level2 id="mst322926s6-5" label="6.5"><heading>Applications in medicine</heading><sec-level3 id="mst322926s6-5-1" label="6.5.1"><heading>CO<sub>2</sub> measurements for blood gas analysis</heading><p indent="no">Although modern blood gas analysers offer the ability to measure with small blood volumes of about 200 µL a lot of important analytes for the clinical diagnosis, the detection of pH, <italic>p</italic>(CO<sub>2</sub>) and <italic>p</italic>(O<sub>2</sub>) still remains the most important measurands of these devices. The control of the interplay of these three parameters enables the physician to react immediately on serious changes of the acid base balance and/or the oxygenation of the blood.</p><p>The Henderson–Hasselbalch equation of the bicarbonate/carbon dioxide system for body temperature (37 °C) can be written as
<display-eqn id="mst322926eqn6-3" eqnnum="6.3"></display-eqn></p><p>The mean values for the bicarbonate concentration [HCO<sup>−</sup><sub valign="yes">3</sub>] and <italic>p</italic>(CO<sub>2</sub>) in human whole blood amount to 24 mmol L<sup>−1</sup> and 5.3 kPa, respectively. Taking into consideration the solubility coefficient α (0.225 mmol (L kPa)<sup>−1</sup>) the concentration of dissolved carbon dioxide α<italic>p</italic>(CO<sub>2</sub>) is 1.2 mmol L<sup>−1</sup>. This results in a pH value of 7.4 for healthy persons. The knowledge of the pH value and the <italic>p</italic>(CO<sub>2</sub>) allows the calculation of the bicarbonate concentration in the plasma [<cite linkend="mst322926bib182">182</cite>].</p><p>The total amount of CO<sub>2</sub> in blood consists of physically dissolved carbon dioxide (5–8%) and CO<sub>2</sub> bound as carbaminohaemoglobin (5–10%) or as HCO<sup>−</sup><sub valign="yes">3</sub> (65% in plasma and 20% in erythrocytes). The blood gas analyser calculates the actual bicarbonate concentration by the Henderson–Hasselbalch equation and with a base excess [<cite linkend="mst322926bib182">182</cite>] considering the buffer capacity of the extracellular fluid and of the erythrocytes.</p><p>Of course, the amount of carbon dioxide in blood is mainly dependent on the CO<sub>2</sub> production of the metabolism and on the CO<sub>2</sub> elimination in the lungs. A healthy person produces about 200 mL CO<sub>2</sub> every minute which has to be removed by expiration, e.g. an <italic>acute respiratory acidosis</italic> is caused by reduced alveolar ventilation and results in an increase of dissolved carbon dioxide in blood, a slight rise in bicarbonate and a remarkable decrease of the pH. Meanwhile, a long lasting <italic>chronic respiratory acidosis</italic> of patients, e.g. with chronic obstructive pulmonary disease with a still established metabolic compensation, leads to only a small diminishment of the pH value, an increase of <italic>p</italic>(CO<sub>2</sub>) and a higher rise in bicarbonate. On the other hand, a <italic>metabolic acidosis with a respiratory compensation</italic>, e.g. caused by renal malfunction, can be described by a reduced pH, a decrease of <italic>p</italic>(CO<sub>2</sub>) and a high loss of bicarbonate. Tendency and dimensions of deviations from the standard values and their development yield a lot of valuable hints for the diagnosis and treatment of several diseases. For a possible classification of the blood electrolyte disturbances different kinds of acid–base nomograms are available, which have also been described by Severinghaus, one of the developers of the CO<sub>2</sub> sensor.</p><p>Blood gas analysers use the Severinghaus principle for the measurement of <italic>p</italic>(CO<sub>2</sub>). A detailed description for the Radiometer ABL300 Blood Gas Analyzer can be found at Holbek [<cite linkend="mst322926bib183">183</cite>].</p><p>The pH glass electrode combined with Ag/AgCl, Cl<sup>−</sup> reference electrode is covered by a 12 µm Teflon® membrane. To ensure a reproducible and stable electrolyte film in front of the sensing pH surface, a nylon spacer is mounted on the sensitive facing side of the pH sensor. The bicarbonate/sodium chloride system contains glycerol in addition in order to avoid the evolution of bubbles in the electrolyte, which may cause instabilities of the measurement. The producer Radiometer describes the electrode function of the CO<sub>2</sub> sensor by the following equation (the sign convention of the ABL device results in an increase in the potential with a rising pH and, hence, a decrease of <italic>E</italic> with increasing <italic>p</italic>(CO<sub>2</sub>):
<display-eqn id="mst322926eqn6-4" eqnnum="6.4"></display-eqn></p><p>For the standard pressure of carbon dioxide in blood of 5.3 kPa (40 mm Hg) the <italic>E</italic>′<sub valign="yes">0</sub> value is 53.0 mV. The unit mmHg (no SI unit) is commonly used in medicine. The temporal response of the potentiometric gas electrode represents an exponential function with a time constant of 18 s, which is slightly influenced by the direction of the CO<sub>2</sub> concentration change (see also section <secref linkend="mst322926s2-2-2">2.2.2</secref>). The actual carbon dioxide pressure <italic>p<sub>x</sub></italic>(<italic>t</italic>) during the measurement in the bicarbonate electrolyte at time <italic>t</italic> is given by the following equation:
<display-eqn id="mst322926eqn6-5" eqnnum="6.5"></display-eqn>where <italic>p<sub>S</sub></italic> is the starting carbon dioxide pressure in the measuring electrolyte, <italic>p<sub>B</sub></italic> is the blood sample and τ is the time constant. The difference between the calculated potential for an elapsed time of 88 s and the measured value is used to control the correct function of the electrode. After approximately 1 month of use the electrodes show drift phenomena caused by changes in the inner electrolyte or by damages of the membrane. In this case a replacement of the solution and of the protecting membrane is advised.</p><sec-level4 id="mst322926s6-5-1-1" label="6.5.1.1"><heading><italic>In vivo</italic> measurement of CO<sub>2</sub></heading><p indent="no">In medicine there is still a great demand in introducing analyser systems for an online <italic>in vivo</italic> measurement of several blood ingredients such as <italic>p</italic>(O<sub>2</sub>), pH, glucose and also <italic>p</italic>(CO<sub>2</sub>). This enables on one hand a higher mobility of the patient by using portable analysers and, on the other hand, the physician can interpret a long-time monitoring of the required data or react immediately on sudden serious changes of the analyte. In critically ill patients these alterations in gas levels may occur within few minutes. Last but not least, time-consuming transports to the lab as well as failures during the painful sampling can be avoided. These persuasive arguments are the driving power to carry out the risky and laborious developments for <italic>in vivo</italic> measurements of CO<sub>2</sub> [<cite linkend="mst322926bib184">184</cite>].</p><p>Besides the use of optodes, the electrochemical way of <italic>in vivo</italic> detection is still based on the well-established method of Severinghaus. But for a needle-type sensor it is not possible to introduce pH glasses. Thus, alternative pH-sensitive materials have to be taken into account such as iridium oxide or polymer matrix-based pH membranes.</p><p>Wang <italic>et al</italic> [<cite linkend="mst322926bib185">185</cite>] have published results of a microfabricated needle-type sensor for the measurement of <italic>p</italic>(O<sub>2</sub>), pH and <italic>p</italic>(CO<sub>2</sub>) with the sensor structures placed on a flexible polyimide substrate. The pH sensors were based on electrochemically oxidized iridium (0.4 mm × 0.2 mm) combined with an Ag/AgCl reference electrode. All electrodes and conducting paths have been covered with intervening polyimide layers with pin-holes for the establishment of contact areas to modify metal surfaces or to enable the contact of the sensors to the analyte solution. For the <italic>p</italic>(CO<sub>2</sub>) sensor Ag/AgCl and IrO<sub>2</sub> sensors have been covered with an electrolyte including paste based on polyvinyl pyrrolidone containing 3 M KCl and 20 mM NaHCO<sub>3</sub>. A silicone rubber material has been used for covering which was carried out by dip-coating technology. The authors determined a 90% response time of 160 s for the <italic>p</italic>(CO<sub>2</sub>) sensor system with a linear response within the physiologically important range. They report a slope of 93 mV/decade <italic>p</italic>(CO<sub>2</sub>) explaining this high value with a pH shift in the PBS buffer of the measured solution. For use in the human body a successful sterilization is essential. This has been carried out by the authors with ethylene oxide at 50 °C for 3 h. After this procedure the <italic>p</italic>(CO<sub>2</sub>) sensors showed a high variation of the slope after sterilization—presumably caused by a change of the reference electrode during sterilization.</p><p>Telting-Diaz <italic>et al</italic> [<cite linkend="mst322926bib186">186</cite>], Meruva <italic>et al</italic> [<cite linkend="mst322926bib187">187</cite>] and Meyerhoff [<cite linkend="mst322926bib188">188</cite>] have published results for a dual-lumen catheter measuring <italic>p</italic>(O<sub>2</sub>), pH and dissolved carbon dioxide in arterial blood simultaneously. The main part of this catheter is a double-lumen tube based either on silicone or polyurethane. These materials can be soaked with appropriate solutions containing the pH ionophores and some additives to achieve the desired pH-sensitive membranes. After 24 h in the impregnating solutions sufficient amounts of ionophore such as tridodecylamine, plasticizer and additional organic salts are dissolved in the polymer material and result in a pH-sensitive tube tip. After sealing of the tubes one lumen is filled with a buffer solution for the assembly of a pH sensor and in the other lumen bicarbonate solution for the <italic>p</italic>(CO<sub>2</sub>) sensor is introduced. Together with a stable Ag/AgCl reference electrode two independent pH sensors are established. By permeation of carbon dioxide through the silicone tubing in the bicarbonate solution a <italic>p</italic>(CO<sub>2</sub>) measurement can be achieved. The buffered inner solution of the second tubing remains stable for the pH measurement. The characteristics of this system can be summarized as follows. Within a range of 1700–9800 Pa the potentiometric response of the carbon dioxide sensor is about 58.5 to 61.3 mV/decade <italic>p</italic>(CO<sub>2</sub>). The drift of the CO<sub>2</sub> electrode in a 5.3% containing tonometered carbon dioxide solution is about 1.6 ± 0.9 mV, while the response time <italic>t</italic><sub>90</sub> between <italic>p</italic>(CO<sub>2</sub>) = 5000 and 9800 Pa amounts to 81 ± 6 s.</p><p><italic>In vivo</italic> studies have been executed in adult mongrel dogs after implantation of the catheter in femoral or carotid arteries [<cite linkend="mst322926bib186">186</cite>]. The dogs were heparinized to avoid the coagulation of blood. Before their introduction, the sensors have been preconditioned and calibrated in a fresh blood sample of the animals to exclude matrix effects during the change of the measuring media. The analysed CO<sub>2</sub> values have been followed by measurements with a commercial blood gas analyser. A comparison of the results during a 13 h measuring cycle showed very good agreement and the mean difference between the <italic>p</italic>(CO<sub>2</sub>) pressures was 350 Pa.</p><p>In order to avoid clotting of blood on the sensor surfaces, the sensors have been treated with a tri-dodecyl-methyl-ammonium heparinate complex. But the readings of these electrodes showed a significant drift and only poor correlations to the commercial blood gas analyser, and the surface was covered with coagulated blood. A solution for the avoidance of the clotting on the membrane surfaces is still not available for the CO<sub>2</sub> electrode. Heparinization of blood is still necessary for the sample in the classical blood gas analyser as well as for the use of sophisticated <italic>in vivo</italic> probes.</p></sec-level4><sec-level4 id="mst322926s6-5-1-2" label="6.5.1.2"><heading>Non-invasive pCO<sub>2</sub> sensors</heading><p indent="no">Physicians and patients are eagerly awaiting the bloodless (non-invasive) measurement of blood gas parameters. One possible way is the transcutaneous monitoring of carbon dioxide and oxygen. The CO<sub>2</sub> sensor also uses the Severinghaus principle like the blood gas analysers. Johns <italic>et al</italic> [<cite linkend="mst322926bib189">189</cite>] showed the correlation between skin surface <italic>p</italic>(CO<sub>2</sub>) and the dissolved arterial carbon dioxide. In contrast to the transcutaneous O<sub>2</sub> measurement the difference between the <italic>p</italic>(CO<sub>2</sub>) in the capillaries during normal blood flow and carbon dioxide transmitted through the epidermis is very small. This is due to the fact that the solubility of CO<sub>2</sub> is 20 times higher in skin tissues than that of oxygen, and principally it is not necessary to heat the measuring sensor to establish a convenient capillary blood flow [<cite linkend="mst322926bib190">190</cite>]. But anyhow, the electrode sensors are heated (about 42 °C) to accelerate the diffusion process through the skin and to shorten the response time.</p><p>For the elevated temperature correction factors have to be introduced. On one hand a higher temperature results in an increase of <italic>p</italic>(CO<sub>2</sub>) in the tissue of about 4.5% K<sup>−1</sup> while on the other hand the rise of the temperature activates a higher carbon dioxide production. This metabolism of the skin tissue adds about 650 Pa to the transcutaneous carbon dioxide [<cite linkend="mst322926bib191">191</cite>].</p></sec-level4></sec-level3><sec-level3 id="mst322926s6-5-2" label="6.5.2"><heading>CO<sub>2</sub> measurements in breathing air (capnography)</heading><p indent="no">The detailed monitoring of expired carbon dioxide, called capnography, is an important tool in critical care medicine, pneumology and anaesthesiology. In the capnogram the development of the CO<sub>2</sub> concentration during one breath cycle is monitored with high time resolution and allows a distinct analysis of ventilation of the patient.</p><p>Figure <figref linkend="mst322926fig57">57</figref> shows schematically a capnogram of a single breathing cycle with the different phases of expiration.
<figure id="mst322926fig57" pageposition="bottom"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig57.eps" width="18pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig57.jpg"></graphic-file></graphic><caption id="mst322926fc57" label="Figure 57"><p indent="no">Capnogram of a single breathing cycle.</p></caption></figure></p><p>The capnogram is divided into four characteristic phases:</p><p><itemized-list id="mst322926il13" type="unnum"><list-item id="mst322926il13.1"><p indent="no"><italic>Phase I</italic>: ‘dead volume’ from the trachea and the bronchia with nearly CO<sub>2</sub>-free air</p></list-item><list-item id="mst322926il13.2"><p indent="no"><italic>Phase II</italic>: mixed air phase from the alveolar as well as the bronchial space</p></list-item><list-item id="mst322926il13.3"><p indent="no"><italic>Phase III</italic>: alveolar plateau with only a small increase—under optimal conditions the alveolar <italic>p</italic>(CO<sub>2</sub>) is comparable with the arterial <italic>p</italic>(CO<sub>2</sub>)</p></list-item><list-item id="mst322926il13.4"><p indent="no"><italic>Phase IV</italic>: ‘closing volume’—the residual volume during maximum expiration when the small airways have closed</p></list-item></itemized-list></p><p>The breath cycle is finished with the intake of fresh air.</p><p>There are two different possibilities for connecting the analysers to the patients’ airways. The measuring cell can be placed into the mainstream between tracheal tube and breathing circuit. This design allows a fast response of the analyser, but the dead space of the measuring cell is unfavourable especially for infant control, and the placement of the device near the patients is prone to movements of their body. In contrast side-stream analyser uses a pump to remove a small volume (about 100 mL min<sup>−1</sup>) of the gas sample to the measuring chamber. The necessary arrangement is less complicated. But the way from the breathing apparatus to the analyser causes a certain delay of the measurement. For pneumological purposes such as single breath analysis the path from respiration to analysis has to be very short to produce highly time resolved concentration profiles of every gasp. To achieve this time resolution, at present IR detection is the method of choice [<cite linkend="mst322926bib192" range="mst322926bib192,mst322926bib193,mst322926bib194">192–194</cite>].</p><p>IR measurement uses the strong absorption band of CO<sub>2</sub> at a wave length of 4.26 µm. In the capnometer the IR beam rapidly passes one after the other measuring chambers with the exhaled breath, a CO<sub>2</sub>-free sample and a volume with a defined carbon dioxide concentration. The resulting intensity is monitored e.g. by a lead selenide detector. For compensation of the electronic drift, the beam is periodically interrupted by an optical chopper. Interferences for the quantification of CO<sub>2</sub> can be caused by nitrous oxide N<sub>2</sub>O, an important anaesthetic gas in intensive care, which has two absorption bands at a wavelength near 4.5 µm. To enable the discrete excitation of the CO<sub>2</sub> vibration, a narrow band pass optical filter has to be placed behind the IR source. Influences of vibrational broadening of the CO<sub>2</sub> absorption band by collision with N<sub>2</sub>O can be compensated by special algorithms of the capnometers. Humidity has to be excluded from the measuring cell by a water trap to avoid the condensation of water. But for the exact determination of <italic>p</italic>(CO<sub>2</sub>) the partial pressure of the removed H<sub>2</sub>O has to be considered.</p><p>For the complex breath analysis fast sensors are needed. Perspectively, an interesting solution will be a combination of an amperometric oxygen sensor based on solid electrolyte and a sensor in which the sound velocity is used to determine a gas mixture mainly consisting of O<sub>2</sub> and CO<sub>2</sub> [<cite linkend="mst322926bib195">195</cite>].</p><p>CO<sub>2</sub> can be measured with these sensors breath by breath in real time as shown in figure <figref linkend="mst322926fig58">58</figref>. The shape of the curve depends on probing and gas transport to the sensor. But there are a lot of combustible components such as acetone, ethanol found even in healthy people, which can disturb the measurement. This can be avoided by using a catalytic converter or a sensor with high operation temperature (about 700 °C). Up to now electrochemical devices based on liquid electrolytes have not been introduced into capnography because of their slow response time.
<figure id="mst322926fig58"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig58.eps" width="17pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig58.jpg"></graphic-file></graphic><caption id="mst322926fc58" label="Figure 58"><p indent="no">CO<sub>2</sub> concentration in the breath gas (our own results).</p></caption></figure></p></sec-level3><sec-level3 id="mst322926s6-5-3" label="6.5.3"><heading>CO<sub>2</sub> measurements on baby mattresses</heading><p indent="no">Rebreathing of exhaled air is one of numerous possible (already discussed) reasons for the sudden infant death syndrome (SIDS) among prone sleeping infants. Under unfavourable conditions, while face down, babies again and again inhale parts of their exhaled breath from a pool forming around them in which the oxygen content drops and the carbon dioxide concentration continuously increases with each breath. If normal innately warning mechanisms are reduced or damaged by various influencing parameters babies do not react by moving away from the pooled CO<sub>2</sub>, and thus suffocate. Some observations and theories indicate that the rebreathing of carbon dioxide actually plays a role in the occurrence of SIDS. For this reason different kinds of specially structured baby mattresses came on the market, which are equipped with ‘climatic channels’ to improve the CO<sub>2</sub> permeability and thus to reduce the risk of SIDS. Furthermore, in several scientific investigations [<cite linkend="mst322926bib196" range="mst322926bib196,mst322926bib197,mst322926bib198">196–198</cite>] the permeation of air and CO<sub>2</sub> through different baby mattresses and bedding materials has been studied.</p><p>In [<cite linkend="mst322926bib196">196</cite>] it was evaluated how CO<sub>2</sub> dispersal was affected by a conventional crib mattress and some commercial products marketed to prevent prone rebreathing. In the tests an infant dummy with its nares connected via tubing to a reservoir filled with 5% CO<sub>2</sub> was positioned face-down or near-face-down on the different sleep surfaces. The fall in percentage end-tidal CO<sub>2</sub> was measured as the reservoir was ventilated with a piston pump. The half-time for CO<sub>2</sub> dispersal (<italic>t</italic><sub>1/2</sub>) was regarded as an index of the ability to cause or prevent rebreathing. Not only the firm mattress but also nearly all of the surfaces designed to prevent rebreathing consistently showed <italic>t</italic><sub>1/2</sub> above thresholds for the onset of CO<sub>2</sub> retention. Thus, for infants placed prone or rolling to the prone position, significant rebreathing of exhaled air would be likely on all surfaces studied, except one.</p><p>The US Consumer Product Safety Commission, Washington, recommended already in 1998 a standardized test procedure for the examination of the CO<sub>2</sub> permeability of baby mattresses by using a mechanical baby model to simulate infant breathing. Using this standardized model, in [<cite linkend="mst322926bib197">197</cite>] extensive inspection results of different baby mattresses, materials and effects of bedding on exhaled air retention are published. Under simulated rebreathing conditions, the model allows the monitoring of raised carbon dioxide inside an artificial lung–trachea system. Resulting levels of CO<sub>2</sub> suggest that common bedding materials vary widely in inherent rebreathing potential. In face-down tests, maximum airway CO<sub>2</sub> ranged from less than 5% on sheets and waterproof mattresses to over 25% on sheepskins and some pillows and comforters. Concentrations of CO<sub>2</sub> decreased with increasing head angle of the doll, away from the face-down position.</p><p>At the Dresden University of Technology, Institute of Occupational Medicine, a photoacoustic multi-gas monitor (Innova AirTech Instruments A/S) was used for investigations on the CO<sub>2</sub> permeability of three differently structured baby mattresses [<cite linkend="mst322926bib198">198</cite>]. As proposed in [<cite linkend="mst322926bib197">197</cite>], baby dummies with the body mass 4.0, 6.0 and 8.0 kg, respectively, were positioned on the mattresses in different positions, and the CO<sub>2</sub> concentrations were measured above the mattress near the nose of the baby and immediately below the mattress for 30 min in each case. Surprisingly it was found that the CO<sub>2</sub> permeabilities of the three baby mattresses were 92%, 46% and 20% and thus differed considerably.</p><p>Figure <figref linkend="mst322926fig59">59</figref> shows schematically the alternative measuring equipment that had been developed for this purpose at the Meinsberg Kurt Schwabe Research Institute. A cylindrical measuring chamber (volume 1 L), containing a solid electrolyte CO<sub>2</sub> sensor on the top, is placed with defined mechanical pressure closely on the centre of the mattress. At the beginning of the measurement the slider at the bottom of the chamber is closed, and the chamber is filled with a gas containing a defined concentration of CO<sub>2</sub>. When the chamber is opened by removing the slider, the CO<sub>2</sub> diffuses through the mattress into the surrounding air.
<figure id="mst322926fig59"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig59.eps" width="13pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig59.jpg"></graphic-file></graphic><caption id="mst322926fc59" label="Figure 59"><p indent="no">Schematic drawing of the experimental setup for investigation of CO<sub>2</sub> diffusion through mattresses and other textile materials (KSI). 1: cylindrical measuring chamber; 2: CO<sub>2</sub> solid electrolyte sensor (Zirox GmbH, Greifswald, Germany); 3: gas micropump; 4: intake and outlet pipes to the sensor; 5: gas inlet and outlet valves; 6: slider; 7: test mattress; F: defined pressing force.</p></caption></figure></p><p>Consequently, the CO<sub>2</sub> concentration in the measurement chamber decreases depending on the permeability of the mattress material as shown in figure <figref linkend="mst322926fig60">60</figref>. For comparison the unhindered diffusion of CO<sub>2</sub> from the measurement chamber through the opening into the surrounding air without any mattress has also been measured (curve 4 in figure <figref linkend="mst322926fig60">60</figref>).
<figure id="mst322926fig60"><graphic><graphic-file version="print" format="EPS" filename="images/mst322926fig60.eps" width="18pc"></graphic-file><graphic-file version="ej" format="JPEG" filename="images/mst322926fig60.jpg"></graphic-file></graphic><caption id="mst322926fc60" label="Figure 60"><p indent="no">Course of the CO<sub>2</sub> concentration in the opened measurement chamber with different mattresses (curves 1 to 3) and without mattress (curve 4).</p></caption></figure></p><p>According to table <tabref linkend="mst322926tab15">15</tabref> the rate of the CO<sub>2</sub> concentration decay can be used to characterize and to compare the CO<sub>2</sub> permeability of baby mattresses. Of course, this measuring method and setup can also be utilized for measurements with other gases as well as for the investigation of other textile materials.
<table id="mst322926tab15" frame="topbot"><caption id="mst322926tc15" label="Table 15"><p indent="no">CO<sub>2</sub> flow rates through different mattresses and without mattress from the measurement chamber into the surrounding air.</p></caption><tgroup cols="4"><colspec colnum="1" colname="col1" align="left"></colspec><colspec colnum="2" colname="col2" align="left"></colspec><colspec colnum="3" colname="col3" align="left"></colspec><colspec colnum="4" colname="col4" align="left"></colspec><spanspec spanname="2to3" namest="col2" nameend="col3" align="center"></spanspec><thead><row><entry></entry><entry></entry><entry spanname="3to4">CO<sub>2</sub> flow rate (mL min<sup>−1</sup>)</entry></row><row><entry></entry><entry></entry><entry spanname="3to4"></entry></row><row><entry>Curve</entry><entry>Mattress</entry><entry>At 0.6 vol.%</entry><entry>At 0.2 vol.%</entry></row></thead><tbody><row><entry>1</entry><entry>Mattress M1</entry><entry>0.22</entry><entry>0.072</entry></row><row><entry>2</entry><entry>Mattress M2</entry><entry>0.22</entry><entry>0.063</entry></row><row><entry>3</entry><entry>M2 with 2 cm cover</entry><entry>0.17</entry><entry>0.051</entry></row><row><entry>4</entry><entry>Without mattress</entry><entry>0.29</entry><entry>0.101</entry></row></tbody></tgroup></table></p></sec-level3></sec-level2></sec-level1><sec-level1 id="mst322926s7" label="7"><heading>Conclusions</heading><p indent="no">CO<sub>2</sub> sensors are important tools with increasing relevance to measure the gas concentration in air, in bioprocesses and as dissolved CO<sub>2</sub> in real process liquids. Their applications can provide basic information for a better understanding of processes in chemistry as well as in medicine, biology, geology and in climate research.</p><p>For the measurement of the carbon dioxide concentration in liquids and gases numerous methods have been described in the scientific literature, but only few have gained wide acceptance in practice. The basic principles and applications of CO<sub>2</sub> measurements were reviewed with the focus on sensors as <italic>in situ</italic> devices, which are applicable directly in the process. Among all principles the electrochemical sensors and optical IR detectors are widely introduced in the measuring practice. For monitoring in air, solid electrolyte electrochemical sensors and IR-detectors are preferred tools.</p><p>This review of the measurement of dissolved and gaseous carbon dioxide concentration does not claim to be complete. Due to the very wide variety of measuring principles and applications for CO<sub>2</sub> sensors the authors could not cover all aspects, but they reported preferentially those fields of development where they have own experiences and achieved results. Thus, some upcoming new principles, which are still in an early state, are not mentioned within the text. This concerns e.g. tunable laser diode spectroscopy, which was used for studies of ecosystem–atmosphere CO<sub>2</sub> exchange [<cite linkend="mst322926bib199">199</cite>, <cite linkend="mst322926bib200">200</cite>] and gas-sensitive FET devices. The latter can be used with a wide range of receptor materials allowing, among other gases, also the determination of CO<sub>2</sub> [<cite linkend="mst322926bib201">201</cite>, <cite linkend="mst322926bib202">202</cite>].</p><p>Electrochemical CO<sub>2</sub> sensors measure the partial pressure of CO<sub>2</sub>, while with analytical methods and physical sensor principles its concentration is determined. This difference should be noted especially if measurements are carried out in liquid media. According to Henry's law in thermodynamic equilibrium the concentration of a gas in a liquid is proportional to its partial pressure in the gaseous phase above the liquid. But in most cases under natural environmental or technical conditions this equilibrium does not establish. Furthermore, Henry's law only applies for dilute, ideal solutions and for solutions where the liquid solvent does not react chemically with the gas being dissolved. In narrower sense the so-called Henry's law constant is not really a constant but depends considerably on temperature and on the ingredients in the measuring solution and is not exactly known in many cases. For those reasons a profound knowledge of the basic detection principles is necessary for the correct interpretation of the measuring results of dissolved CO<sub>2</sub>.</p><p>The further development of electrochemical CO<sub>2</sub> sensors based on the Severinghaus principle will focus on the miniaturization of these devices, e.g. for medical purposes. A promising approach is to use the ISFET technology, but there a lot of technological problems concerning the gas permeable membrane and packaging have to be solved, yet. Other materials such as pH-sensitive metals and metal oxides have been investigated intensively, too. Besides the diminution of the CO<sub>2</sub> sensor size the potential stable reference element is still a crucial component for miniaturization and the groundbreaking step is still missing. Other improvements of Severinghaus-type sensors will cover the field of membranes. They will be optimized for the aimed purpose e.g. modified with anticoagulants for the use in blood or with antimicrobial active compounds for their application in biotechnological processes. For the measurement of dissolved CO<sub>2</sub> in biotechnological processes and foodstuffs production the development is to be aimed at sterilizable and even CIP (<underline>c</underline>leaning <underline>i</underline>n <underline>p</underline>rocess)-resistant sensors.</p><p>CO<sub>2</sub> sensors that use solid electrolytes allow long-term stable and calibration-free measurements as well as short response times. In the near future improvements concerning lifetime and detection limits of the analytical devices as well as a further shortening of the measuring time will be in the focus of the developments. For that aim the preparation and systematic investigation of new or modified electrode materials is a challenge of innovation.</p><p>In the area of optical IR detectors the research will be directed to an increase of the long-term stability of the infrared radiators and to an enhancement of the IR detectors with respect to a shift of the specific detectivity <italic>D</italic>* towards the theoretical limit (given by the radiation noise) of 1.81 × 10<sup>10</sup> cm Hz<sup>1/2</sup> W<sup>−1</sup>. A remarkable improvement of the detection limit for the investigation of CO<sub>2</sub> traces in small gas samples will be achieved by miniaturization of measuring cuvettes including advanced solutions for multiple reflections to prolong the absorption path. 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Electrochemical CO2 sensors based on the Severinghaus principle and solid electrolyte sensors operating at high temperatures have been manufactured and widely applied already for a long time. Besides these, nowadays infrared, non-dispersive infrared and acoustic CO2 sensors, which use physical measuring methods, are being increasingly used in some fields of application. The advantages and drawbacks of the different sensor technologies are outlined. Electrochemical sensors for the CO2 measurement in aqueous media are pointed out in more detail because of their simple setup and the resulting low costs. A detailed knowledge of the basic detection principles and the windows for their applications is necessary to find an appropriate decision on the technology to be applied for measuring dissolved CO2. In particular the pH value and the composition of the analyte matrix exert important influence on the results of the measurements.</abstract><subject><genre>keywords</genre><topic>CO2 sensor</topic><topic>Severinghaus electrode</topic><topic>solid electrolyte sensor</topic><topic>IR</topic><topic>NDIR</topic><topic>sensor application</topic></subject><relatedItem type="host"><titleInfo><title>Measurement Science and Technology</title></titleInfo><titleInfo type="abbreviated"><title>Meas. Sci. Technol.</title></titleInfo><genre type="journal">journal</genre><identifier type="ISSN">0957-0233</identifier><identifier type="eISSN">1361-6501</identifier><identifier type="PublisherID">mst</identifier><identifier type="CODEN">MSTCEP</identifier><identifier type="URL">stacks.iop.org/MST</identifier><part><date>2011</date><detail type="volume"><caption>vol.</caption><number>22</number></detail><detail type="issue"><caption>no.</caption><number>7</number></detail><extent unit="pages"><start>1</start><end>45</end><total>45</total></extent></part></relatedItem><identifier type="istex">1518AFDAF6F3212D5984FCD25E802E024B790A6A</identifier><identifier type="DOI">10.1088/0957-0233/22/7/072001</identifier><identifier type="PII">S0957-0233(11)22926-6</identifier><identifier type="articleID">322926</identifier><identifier type="articleNumber">072001</identifier><accessCondition type="use and reproduction" contentType="copyright">2011 IOP Publishing Ltd</accessCondition><recordInfo><recordContentSource>IOP</recordContentSource><recordOrigin>2011 IOP Publishing Ltd</recordOrigin></recordInfo></mods></metadata><serie></serie></istex></record>Pour manipuler ce document sous Unix (Dilib)
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