Gas sensing electrodes

1. GAS-SENSING ELECTRODES Tapeshwar Yadav (Lecturer) BMLT, DNHE, M.Sc. Medical Biochemistry

2. Gas- sensing electrodes Two gas electrodes are widely used in biology The oxygen electrode The carbon dioxide electrode O 2 electrode is used in many diverse branches of biology CO 2 electrode is chiefly used to measure CO 2 in the blood.

3. 1. Oxygen electrode It is used in many diverse branches of biology Different anode- cathode combinations for oxygen electrode are available, the platinum with silver/silver chloride is the most used cathode-anode combination. Arrangement of these electrodes is annular with the anode enclosing the platinum cathode. Electrodes dip into an electrolyte soln. (usually buffered KCl soln.) which is held inside an electrode by an oxygen permeable membrane. Membrane might be a very thin polypropylene. Polarization of electrodes at 0.6v is achieved with the help of a mercury cell.

4. Contd… The oxygen electrodes measure the partial pressure of a gas in solution rather than its concentration. The partial pressure is often abbreviated to PX, where x stands for the gas whose partial pressure is being indicated. Thus, the partial pressure of oxygen is indicated as pO 2 & that of carbon dioxide as pCO 2

5. Mechanism of Oxygen (pO 2 ) Electrode When the oxygen electrode is dipped into a solution containing oxygen, the following reactions happen: i . The oxygen molecules from the sample diffuse through the membrane into the electrolyte so that within a short time the electrolyte and the sample come to equilibrium with respect to the pO 2 .

6. Contd… ii . The outermost valency shell of each oxygen atom has a vacancy for two electrons upon acceptance of which it can be turned into an oxygen ion. These electrons are supplied by the platinum cathode. The reaction at the cathode is: 4e - +O 2 + 2H 2 O = 4OH - iii. The hydroxyl ions so produced at the cathode then react with KCl in the electrolyte soln. The reaction in the electrolyte is: OH - + KCl = KOH + Cl -

7. Contd… iv. The negatively charged chloride ion produced in the electrolyte soln. are attracted to the positive anode and donate their electrons. The reaction at the anode is: Cl - = Cl + e - Ag + Cl = AgCl Thus, deposition of AgCl on to the anode . v. The current through the system is directly proportional to the pO2 and can be recorded directly after amplification into pO2.

8. 2. Carbon dioxide electrode It is used to measure carbon dioxide in the blood. Constituents of a carbon dioxide electrodes are A conventional glass PH electrode with a calomel reference electrode. A thin plastic or teflon membrane which is permeable to carbon dioxide and not to other ions.

9. Contd… A standard bicarbonate solution, usually 0.005 M NaHCO 3 between the membrane and the glass electrode. When the electrode is dipped into a sample containing dissolved CO 2 , the CO 2 is allowed to diffuse into the bicarbonate solution by the permeable membrane.

10. Contd… The PH of the bicarbonate solution changes, and this change is read by the glass electrode. This pH change is reflected by the pH meter which is directly calibrated for pCO 2. The response time of a CO 2 electrode is higher because the standard bicarbonate soln. has to come into equilibrium with the sample.

11. Mechanism of Carbon dioxide (pC0 2 ) Electrode It contains pH sensitive glass electrode and a reference electrode immersed in a bicarbonate buffer system. This is separated from the solution under test (mainly blood) by a plastic membrane permeable to gaseous CO 2, but not permeable to dissolved ions. The CO2 in blood diffuses through the plastic membrane and reacts with the buffer system to change the pH.

12. Contd… The pCO2, electrode takes advantage of the fact that pH has a linear relationship to the log pCO2 over the range of 10-90 mmHg. The H+ ion conc. Change due to the dissolution of CO2 is detected by the pH-sensitive glass electrode. A potential difference exists bet n the glass electrode & reference electrode and it is measured on the meter. The meter’s scale is usually calibrated for pCO 2 in a semi logarithmic fashion , since PH is inversely proportional to the log of the pCO2.

13. Applications gas-sensing electrodes The O 2 electrode is being widely used in many different biological experiments wherever there is a need of measuring oxygen. The CO 2 electrode on the other hand is mainly used for clinical purposes, often for measuring the CO 2 in blood or plasma.