Thermodynamic examination of pH measurements

The electrometric determination of pH is essentially an evaluation of the thermodynamic quantity -loga(H(3)O)(+) from a measurement of an electrical potential difference between two electrodes. It is accordingly necessary to examine the electrometric method in light of thermodynamic concepts in order to disclose the meaning of experimental pH values and their limitations. In this paper it is shown by thermodynamic considerations that the evaluation of -loga(H(3)O)(+) is actually a special case of the evaluation of the thermodynamic quantity The operational definition of pH prescribes a general method to evaluate this quantity, but not -loga(H(3)O(+)) as commonly believed.