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 (Δζobs ) 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 (ΔζHO ) predicted by the Hubbard–Onsager dielectric friction theory. As predicted, Δζobs 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) Δζobs (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) Δζobs (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.