Solvent isotope effect on mobilities of potassium and chloride ions in water at high pressure. III. A high temperature study

Limiting molar conductances of the K⁺ and Cl⁻ ions in heavy and light water have been determined at 45 °C as a function of pressure up to 2000 kgf cm⁻² (1 kgf cm⁻² =0.9807×10⁵ Pa) from the measured conductances and transference numbers of KCl. The residual friction coefficients (Δζ_obs ) of the K⁺ and Cl⁻ ions obtained by using their limiting molar conductances and the bulk viscosity of solvent are compared up to 1000 kgf cm⁻² with the corresponding values (Δζ_HO ) predicted by the Hubbard–Onsager dielectric friction theory. As predicted, Δζ_obs for the cation in H₂ O is smaller than that in D₂ O and slightly decreases with increasing pressure, while its value in D₂ O is almost invariant in the pressure range studied. The failure of the continuum theory in D₂ O indicates that the effect of the open structure of water on the cation migration cannot be neglected even at 45 °C because of stronger hydrogen bonds in D₂ O than in H₂ O. For the anion, on the other hand, the continuum theory shows more serious limitations: (i) Δζ_obs (Cl⁻) in H₂ O is not smaller than that in D₂ O even at 45 °C with a large difference at high pressures and (ii) Δζ_obs (Cl⁻) becomes negative in both types of water at high pressures. The difference in Δζ(Cl⁻) between theory and experiment at 45 °C, however, becomes much smaller than that at lower temperatures.