The measurement of CO2 in blood by means of a PCO2 electrode is dependent on the

• A. Passage of H+ ions through the membrane that separates the sample and the electrode
• B. Change in pH because of increased carbonic acid in the electrolyte surrounding the electrodes
• C. Movement of bicarbonate across the membrane that separates the sample and the electrode
• D. Linear relationship between PCO2 in the sample and measured pH

Answer: (B)

The measurement hinges on the pH change caused by carbonic acid formation in the electrolyte. In a PCO2 electrode, CO2 from the blood diffuses through a CO2-permeable membrane into an electrolyte buffer containing bicarbonate. There, CO2 reacts with water to form carbonic acid, which rapidly dissociates to bicarbonate and H+, increasing proton concentration and lowering pH. The pH sensor detects this shift and the instrument converts it to a CO2 partial pressure. So the signal depends on the rise in H+ (the pH change) produced by carbonic acid in the electrolyte. The other ideas—direct H+ movement across the membrane, bicarbonate movement, or relying on a direct linear pH–PCO2 relationship—do not describe what drives the measurement.