Some response characteristics of CO2 electrodes.

Radiometer carbon dioxide electrodes and probably other electrodes of similar design do not give the same readings, except under certain very limited conditions, when used to measure carbon dioxide concentrations in solution and the gas phase with which the liquid is in equilibrium. Blood samples of high P(CO2), when introduced into the sample chamber of an electrode at low P(CO2), lose CO2 to the electrode resulting in a reduction of sample P(CO2) and hence, because the sample size is very small, a recorded value much below the real P(CO2) of the sample is obtained. The reverse occurs if the P(CO2) of the sample is below that of the electrode. To avoid these problems the electrode should be held at the same P(CO2) as the sample, or sample replacement should be used to establish the real P(CO2). The differences in response of the electrode to gases and liquids should also be taken into account.