The Measurement of pH in Aqueous Systems at Elevated Temperatures Using Palladium Hydride Electrodes

The use of palladium hydride electrodes (Pd black on Pd, Pt, and Au) for the measurement of pH in high temperature aqueous systems (298°–548°K) is described. The electrodes were calibrated against boric acid/lithium hydroxide buffer solutions, whose vs. temperature profiles have been calculated using known dissociation constants for the components. Linear potentialvs. pH correlations are observed for all temperatures studied. However, the gradients of the correlations deviate from the predicted Nernstian values. Furthermore, thermodynamic analyses of the systems demonstrate that above 373°K the observed potential deviates significantly from the equilibrium values calculated on the supposition that the potential‐determining reaction is . Possible reasons for the observed deviation from ideal behavior are identified.