Published thermodynamic approaches to calculate the yttria-stabilized zirconia (YSZ) ceramic pH sensor potential are reviewed. A thermodynamic approach related to the Gibbs free energies of reactants and products is proposed to calculate the YSZ pH sensor potential, identify the YSZ pH sensors as primary pH sensors and to determine solution pH at high temperatures. YSZ thimbles have been used with a silver powder internal element prepared in the presence of dry air to generate a redox system, Ag|O-2, sensitive to pH. YSZ (Ag|O-2) pH sensors have been tested over a temperature range of 100 to 300 degreesC in lithiated boric acid solutions which are related to pressurized water reactor coolant chemistry. The test results have good agreement with the thermodynamic calculation from 100 to 300 degreesC. The thermodynamic calculation can be applied to a variety of YSZ pH sensor internal elements including Cu|Cu2O, Hg| HgO, Ag| Ag2O, Ag|O-2, whereas earlier published calculations are not generally applicable.