What is measured by a pCO2 electrode as CO2 is present in the sample?

• A. Lowering of pH directly proportional to CO2 present
• B. Increase in pH with CO2 present
• C. Change in conductivity due to CO2
• D. Change in color due to CO2

Answer: (A)

The pCO2 electrode detects CO2 by observing the acid-base effect CO2 has when it dissolves in the buffer. CO2 diffuses through a membrane into a buffered solution and forms carbonic acid, which dissociates to release hydrogen ions, lowering the pH in proportion to the CO2 partial pressure. The electrode converts that pH change into a CO2 reading, so the measurement is the lowering of pH directly tied to how much CO2 is present. Conductivity or color changes aren’t what this device senses, and pH would not rise with CO2.