In a partial pressure of oxygen (PCO2) electrode, carbon dioxide does which of the following?

1. It reacts with potassium chloride.
2. It reacts with water to form carbonic acid.
3. It diffuses through a polyethylene membrane.
4. It diffuses through a semipermeable Teflon membrane.
a. 2 and 3 c. 1 and 4
b. 1 and 3 d. 2 and 4

ANS: D
A PCO2 electrode is basically a pH electrode covered with a carbon dioxide–permeable Teflon or silicone (Silastic) membrane. A bicarbonate-buffered solution is held between the Teflon mem-brane and the pH glass electrode by a nylon spacer. CO2 from the blood diffuses across the sem-ipermeable Teflon membrane and reacts with the water to form carbonic acid, which dissociates into hydrogen ions and bicarbonate.