The CO2 Electrode

P = O2 tension at open surface of layer, mm Hg Q = O2 consumption rate, ml 02/ml layer/sec X = distance variable, cm L = layer thickness, cm 

If we combine the equilibrium equations for the first and second dissociations of carbonic acid, and invoke the principle of electroneutrality, the relationship between pH and PCO2 can be developed (Severinghaus, 1968)  

[ + (2K2Mh)] a = activity coefficient = hydrogen activity = sodium activity, X2 = dissociation constants = ion product of water 

This equation applies to an aqueous solution which contains NaHC03. In 1957, Stow et al indicated the possibility of measuring PCO2 by measuring the pH of a film of water separated from a test solution by a membrane permeable to CO2- If the sensitivity of a membrane electrode based on this principle is defined by 

Various membrane materials can be used including rubber, silastic. Teflon, and polyethylene. Membrane thickness determines electrode response time, with thin membranes (0.001 in.) being the most responsive. Response time is similar to that of pOj electrodes (1-2 min) which is to be expected as the same sorts of processes are involved. 

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Because the electrical circuit is closed inside the sensor, no external reference electrode is necessary and the Severinghaus-type electrode can be used for measurement in either gaseous or liquid samples. It is important to remember, however, that the potential of the internal reference electrode must remain constant. In principle, it would be possible to use a liquid junction but it would add to the complexity of the design. Because the counterion resulting from the dissociation equilibrium is the only interfering ion, and because it is present in a very low concentration, it is possible to ascertain the constancy of the reference potential by careful choice of the internal electrolyte. Thus, for example, in the CO2 electrode the internal electrolyte is O.lMNaHCOs and 0.1 M NaCl and Ag/AgCl is used as an internal reference element. 

The optode showed good reproducibility in the 0.1-10 kPa CO2 range, with a response time of 3-4 s for 90% of the final value. It can be accelerated by addition of the enzyme carbonic anhydrase, which catalyzes the establishment of equilibrium (1). The short response is in striking contrast to the 0.5-2.0 min for the CO2 electrode. When coupled to a fiber-optic system it allows the transcutaneous measurement of CO2 pressure [128]. 

NH4, and CO2 are both measured with a modified ion-selective electrode. They use a pH electrode modified with a thin layer of a solution (sodium bicarbonate for CO2 and ammonium chloride for NH J) whose pH varies depending on the concentration of ammonium ions or COj it is equilibrated with. A thin membrane holds the solution against the pH glass electrode and provides for equilibration with the sample solution. Note that the CO2 electrode in Figure 27.4 is a combination electrode. This means that both the reference and indicating electrodes have been combined into one unit. Most pH electrodes are made as combination electrodes. 

The membrane-covered NH3 electrode also has wide applicability in the field of enzyme electrodes to be discussed in the next chapter. Ruzicka and Hansen [170] reported the successful application of the air-gap NH3 electrode to the determination of NJHj in serum and plasma samples. With the help of a Technicon Autoanalyzer , 60 determinations per hour were possible. With the CO2 electrode, the total inorganic and organic carbon content of water samples could be easily and inexpensively obtained... 

The pCOj electrode was first described in 1957 by Stow and later improved to its presently used form by Severinghaus The basic principle of operation relies on equilibration of COj with an aqueous solution. The change in pH in the aqueous solution associated with the equilibration due to carbonic acid formation (H2CO3) is measured and varies with log pCO2] It should be pointed out here that these measurements (and likewise for the pO electrode described below) give CO2 tension not concentration. To obtain concentration, Henry s Law of gas solubUity must be applied. However, for most medical and biological applications the knowledge of the gas tension is sufficient. 

The electrolytes Na", and Cl are second only to glucose in being the most frequently run hospital tests. Many clinical chemistry analyzers now contain an ISE module for electrolyte analysis. Most commonly the module will consist of a Na -glass electrode, a valinomycin/PVC electrode, a Ag/AgCl pellet or a quaternary ammonium ion/PVC electrode and a reference electrode. A selective electrode for the bicarbonate ion continues to elude workers in the field. An indirect measurement of HCOf must be made. The sample is usually reacted with acid to evolve carbon dioxide gas which is measured with a traditional Severinghaus type CO2 electrode. Alternatively, the sample is treated with base to convert HCO to CO3 and a carbonate ion-selective electrode is used In this manner, the complete primary electrolyte profile is obtained electrochemically. 

In applications where Nafion is not suitable, at temperatures above 200 °C with feed gas heavily contaminated with CO and sulfur species, a phosphoric acid fuel cell (PAFC)-based concentrator has been effective [15]. Treating the gas shown in Table 1, a H2 product containing 0.2% CO, 0.5%CO2 and only 6 ppm H2S was produced. The anode electrode was formed from a catalyst consisting basically of Pt-alloy mixed with 50% PTFE on a support of Vulcan XC-72 carbon. The cathode was... 

Complicating matters further is the fact that the platinum electrode, the standard tool for measuring Eh directly, does not respond to some of the most important redox couples in geochemical systems. The electrode, for example, responds incorrectly or not at all to the couples SO -HS-, O2-H2O, CO2-CH4, NOJ-N2, and N2-NH4 (Stumm and Morgan, 1996 Hostettler, 1984). In a laboratory experiment, Runnells and Lindberg (1990) prepared solutions with differing ratios of selenium in the Se4+ and Se6+ oxidation states. They found that even under controlled conditions the platinum electrode was completely insensitive to the selenium composition. The meaning of an Eh measurement from a natural water, therefore, may be difficult or impossible to determine (e.g., Westall, 2002). 

It is noteworthy that the adsorbed concentration of CO2 radical at various electrode potentials shows an increase in the CO2" in the more negative direction. The mechanistic significance of this observation is indicated below. Finally, in Figure 4b it... 

The information obtained can be used to give interesting information upon the CO2 reduction mechanism. Because the radical anion increases in concentration in the negative direction, it cannot be in equilibrium with the electrode. The increase in anion concentration at cathodic potentials may, however, be explained if CO2 is formed as an intermediate radical. Thus from equations 5-7... 

By 1930 the H+ electrode, which was not suitable for biological situations when CO2/HCO3 were present, was replaced by the glass electrode (Hughes, Maclnnes, and Dole). This came into routine use in biochemical laboratories in the 1940s, giving an accuracy of 0.01 pH unit compared with 0.1 unit obtained colorimetrically. 

The large change of shapes of the base line and the very large negative peak around 1600 cm are due to the decrease in the amount of water between the window and the electrode. The decrease in water seemed to be caused partly by the reaction with methanol, but mainly by CO2 gas evolution because between the window and the electrode, there is not enou solution to dissolve all the CO2 formed on the electrode. Indeed, gas bubbles were seen in these high concentration methanol experiments, which were not foimd in 0.1 M methanol experiments. 

No CO2 signal was visible on the either electrode below 500 mV. The small bumps in this region were all within the noise level and, therefore, it is impossible to judge whether CO2 formation occurred, in this potential region, where molybdate might have an effect. 

The reaction of H2 and O2 produces H2O. When a carbon-containing fuel is involved in the anode reaction, CO2 is also produced. For MCFCs, CO2 is required in the cathode reaction to maintain an invariant carbonate concentration in the electrolyte. Because CO2 is produced at the anode and consumed at the cathode in MCFCs, and because the concentrations in the anode and cathode feed streams are not necessarily equal, the Nemst equation in Table 2-2 includes the CO2 partial pressure for both electrode reactions. 

Aurbach and co-workers performed a series of ex situ as well as in situ spectroscopic analyses on the surface of the working electrode upon which the cyclic voltammetry of electrolytes was carried out. On the basis of the functionalities detected in FT-IR, X-ray microanalysis, and nuclear magnetic resonance (NMR) studies, they were able to investigate the mechanisms involved in the reduction process of carbonate solvents and proposed that, upon reduction, these solvents mainly form lithium alkyl carbonates (RCOsLi), which are sensitive to various contaminants in the electrolyte system. For example, the presence of CO2 or trace moisture would cause the formation of Li2COs. This peculiar reduction product has been observed on all occasions when cyclic carbonates are present, and it seems to be independent of the nature of the working electrodes. A single electron mechanism has been shown for PC reduction in Scheme 1, while those of EC and linear carbonates are shown in Scheme 7. ... 

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