Clark Electrode — FRCA Primary MCQ
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Correct answer: E — Platinum cathode; silver/silver chloride anode; oxygen is reduced and the current is proportional to PO2
Explanation lettering: E = shown as C · C = shown as D · D = shown as E
D is correct. The Clark electrode is an amperometric, polarographic sensor containing a platinum cathode and silver/silver chloride anode in electrolyte, separated from the sample by an oxygen-permeable membrane. An external polarising potential of approximately 0.6 V is applied. Oxygen diffuses through the membrane and is reduced at the cathode: O2 + 2H2O + 4e− → 4OH−. The diffusion-limited current is proportional to PO2. Option B describes the potentiometric Severinghaus CO2 electrode. Option E describes the principle and typical metals of a galvanic fuel cell, which generates its own electromotive force. Options A and C incorrectly reverse the electrode roles or describe oxygen as undergoing oxidation; reduction occurs at the cathode.
Reference: Royal College of Anaesthetists. Guide to the FRCA Examination: The Primary, Fourth Edition, section 'Measuring FiO2 and PO2'. 2013. https://www.rcoa.ac.uk/media/49221