The Severinghaus electrode is essentially a slightly modified glass electrode. The CO2 dissolved in the sample diffuses into the middle compartment of the electrode via a thin membrane. Once inside, the CO2 finds itself in an aqueous solution. For convenience, there may or may not be a bicarbonate solution added to this chamber. The reaction which takes place is an old familiar carbonic acid dissociation equilibrium. Thus, the pH of the solution in the middle chamber changes. The change in pH is completely dependent on the pCO2, provided the temperature and pressure remain constant. This results in a change in potential difference in the glass electrode; and the function of this item has already been discussed at some length in another chapter. Thus, from the change in pH, one can calculate the pCO2.