The electrodes used for measurement of partial pressure of CO2 in blood are based on measuring the pH as demonstrated in the figure below:
The measurement is based on the fact that when CO2 is dissolved in water, it forms a weakly dissociated carbonic acid (H2CO3) that subsequently forms free hydrogen and bicarbonate ions per the following chemical reaction:
CO2 + H2O ⟷ H2CO3 ⟷ H+ + HCO3–
As a consequence of this chemical reaction, the pH of the solution is changed. This change generates a potential between the glass pH and a reference e.g. Ag/AgCl electrode that is proportional to the negative logarithm of the pCO2.
Related: How Estimation of Oxygen and Carbon dioxide is Carried out in Biological Solutions
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