Significance of effects of pressure on electrode reactions. Part III. Equilibrium processes at reference electrodes and the volume of H in Pd

For studies on effects of pressure on kinetics of electrode processes (Parts I, II), reversible reference electrodes suitable for use in a completely enclosed high-pressure bomb are required. The electrodes must exhibit reversible behaviour over the pressure range employed in the experiments, i.e., their changes of emf with increasing and decreasing changes of pressure must be reproducible and correspond to the respective volume changes in the reactions.A series of reversible reference electrodes is examined over a range of pressures up to ca. 2500 bars. The Pd–H/H+ and Ag, AgCl/Cl− reference electrodes are found to behave very satisfactorily at elevated pressures; the Pt,H2/H+ electrode is, however, less satisfactory, due to problems associated with dissolved H2.The results enable the volume of Pd–H and of H sorbed into Pd to be evaluated, together with estimates of the partial molar volume of H2 in aqueous HCl. These data enable the pressure-coefficients of metal–solution potential differences at individual reference electrodes to be evaluated. Such information is required for interpretation of effects of pressure on kinetics of electrode processes.