Carbon dioxide sensing electrode

Fonong and Rechnitz (1984a) entrapped the enzyme physically with a dialysis membrane at the sensing tip of a carbon dioxide selective electrode. The sensor was suitable for salicylate concentrations in the range 0.04-2.2 mmol/l. 

Figure 5.9. Pn 3 m cubic phase matrix containing the catalyst. Reprinted from Scheme 5.1 R Rowinski, R. Bilewicz, M.J. Stebe and E. Rogalska, A concept for immobilizing catalytic complexes on electrodes cubic phase layers for carbon dioxide sensing, Analytical Chemistry, 74(7), 1554-1559 (2002). Copyright 2002, with permission of American Chemical Society.

Such changes in the pH are sensed by the inner glass electrode. The overall cell potential is thus determined by the carbon dioxide concentration in the sample ... 

D.L. Simpson and R.K. Kobos, Potentiometric microbiological assay of gentamicin, streptomycin, and neomycin with a carbon dioxide gas-sensing electrode. Anal. Chem. 55, 1974-1977 (1983). 

When the sample solution containing glutamic acid was injected into the system, the potential of the carbon dioxide gas-sensing electrode increased with time. The enzyme reaction was carried out at pH 4.4, which was sufficiently below the pK value (6.34 at 2 fc) of carbon dioxide. 

Fig. 9. Schematic design of a carbon dioxide partial pressure (pC02) electrode. C02 diffuses through the membrane into or out of the electrolyte where it equilibrates with HCO, thus generating or consuming protons. The respective pH change of the electrolyte is sensed with a pH electrode and is logarithmically proportional to the pC02 in the measuring solution. Since the electrolyte may become exhausted, one can replace it through in/out lines. These can also be used to re-calibrate the pH electrode. Therefore, the electrode is retractable by means of a mechanical positioner...

The first gas-sensing electrode based on a similar principle was the carbon dioxide electrode, developed to determine CO2 in blood. Later, sensors for other gases (e.g., SO2, NOx, and HCN, etc.) appeared on the market. 

Gas-sensing membrane electrodes use a gas-permeable membrane that allows measured species (as a dissolved gas) to pass through and be measured within the electrode. These probes are very selective and sensitive for gases such as ammonia and carbon dioxide. For carbon dioxide, the working electrode is usually a modified glass pH electrode covered in a teflon membrane. Carbon dioxide in solution forms carbonic acid which lowers the pH. 

Electrodes for the measurement of carbon dioxide are of the gas permeable membrane type. The membrane of the electrode is permeable to carbon dioxide which diffuses into a layer of sodium hydrogen carbonate solution that is trapped between a glass electrode and the membrane. The solution reacts with the carbon dioxide producing a pH change that is sensed by the glass electrode. The probe measures the free carbon dioxide in the sample. If the sample is acidified to pH 5 on the introduction of the probe, then the total carbon dioxide in the sample can be determined. 

Potentiometric electrodes Ammonia or carbon dioxide gas sensing probes can be employed in the same way as the oxygen electrode, with the immobli-zed microbial layer held in place on the surface of the gas-permeable membrane. The ammonia probe is commonly used to determine amino acids by detection of ammonia production by microbial deamination of amino acids. Carbon dioxide probes are alternatives to oxygen electrodes for monitoring respiratory activity. 

Figure 1 Schematic diagram of the arrangement of a tissue-based membrane biosensor (A) slice of tissue (B) bovine serum albumin conjugate layer (C) carbon dioxide gas permeable membrane (D) 0-ring (E) internal electrolyte solution (F) pH-sensing glass membrane (G) plastic electrode body. (Reproduced with permission from Kurlyama S and Rechnitz GA (1981) Analytica Chimica Acta 131 91.)...

---