In the PCO2 electrode, CO2 diffuses into the internal electrolyte and forms carbonic acid, which shifts the pH of the internal layer. What is the primary composition of this internal electrolyte?

• A. Bicarbonate solution
• B. Sulfuric acid
• C. Saline
• D. Potassium hydroxide

Answer: (A)

A bicarbonate buffer solution inside the electrode. CO2 diffuses into this internal electrolyte and reacts with water to form carbonic acid, which then dissociates to H+ and HCO3-. The bicarbonate/ carbonic acid equilibrium provides buffering, so the CO2-induced production of H+ produces a measurable pH change that the electrode detects. Using sulfuric acid would create a strongly acidic, non-buffered environment; saline lacks buffering capacity; potassium hydroxide would be strongly basic and would dampen or alter the intended pH shift. The bicarbonate buffer is what allows the CO2 signal to be translated into a stable, measurable pH change.