Measurement of pH and Redox Potential in Boric Acid/Lithium Hydroxide Buffer Solutions at Elevated Temperatures

A palladium‐hydride electrode (Pd black on Pt) was used for the measurement of pH in high temperature aqueous systems (25°–275°C). The electrode potential was calibrated against boric acid/lithium hydroxide buffer solutions, whose vs. temperature profiles have been calculated (1) using known dissociation constants for the components. The plateau potential for the α+β phase was observed up to 150°C, but was not observed above 200°C. The potential measured above 200°C was identified to be the redox potential and to correspond to the hydrogen electrode potential in the case of a hydrogenated system. The redox potential measured in hydrogenated systems were found to exhibit linear Nernstian responses to temperatures up to 275°C, but deviations of the measured potentials from equilibrium values were detected at temperatures above 200°C. The deviations are most probably due to uncertainties in the thermal and isothermal liquid junction potentials associated with the external reference electrode, and to uncertainties in the thermodynamic properties of high temperature aqueous solutions.