Blood PCO2 may be measured by which type of electrode?

• A. Direct colorimetric measurement of dissolved CO2
• B. A self-contained potentiometric electrode
• C. Measurement of CO2-saturated hemoglobin
• D. Measurement of CO2 consumed at the cathode

Answer: (B)

Measuring pCO2 in blood is done with a potentiometric sensor that is self-contained, such as the Severinghaus-type electrode. CO2 diffuses through a gas-permeable membrane into a buffered chamber, where it forms carbonic acid and lowers the hydrogen ion activity. This change alters the electrical potential read by an internal pH-sensing electrode; the instrument converts that voltage into a pCO2 value. This setup is inherently potentiometric, meaning it relies on measuring a potential (voltage) rather than drawing current or producing a color change.

Direct colorimetric methods would require a visible color change to indicate CO2 levels, which is not how modern blood gas analyzers operate. Measuring CO2 bound to hemoglobin reflects a different, less direct aspect of CO2 transport rather than the partial pressure of dissolved CO2. Measuring CO2 consumption at the cathode describes an amperometric approach, which is not the basis for pCO2 determination in routine practice.