Which blood gas electrode is composed of a silver/silver chloride reference electrode and a glass sensing membrane?

• A. PO2
• B. pH
• C. PCO2
• D. HCO3

Answer: (C)

The key idea is how CO2 is measured in blood gas instruments: by converting CO2 into a change in hydrogen ion concentration and detecting that with a glass pH-sensing membrane, while using a stable reference electrode. In Severinghaus-type PCO2 electrodes, CO2 diffuses through a gas-permeable barrier into a bicarbonate buffer, forming carbonic acid and lowering the pH. The glass membrane of the pH sensor detects this pH change, and the silver/silver chloride reference electrode provides a stable reference potential. This combination—a glass sensing membrane with a silver/silver chloride reference—is characteristic of the PCO2 electrode, which is why it’s the best choice.