Effect of pressure on dissociation of nickel sulfate ion pair in water

Abstract

The ionic conductivities of aqueous solutions of nickel sulfate have been measured as a function of pressure up to 1600kg/cm^2 for the concentrations from 1×10^-4 M to 3×10^-4 M at 15, 25, and 40℃. The Kohlrausch relationship between Λ and √<C> was satisfied at each pressure and temperature. The molar conductivity, Λ(NiSO_4) is affected by the variations of mobility of free ions and of the degree of dissociation of ion pair by pressure. The mobility of Ni^2+ ion and SO_4^2- ion increases with increasing pressure up to definite pressure in this temperature range. The initial increase in the mobility of free ions at infinite dilution with pressure would be due to the destruction of the water structure and the decrease of the effective radii of hydrated ion by pressure. Analyzed with the Robinson-Stokes method, the hydration number of Ni^2+ ion was 12 and that of SO_4^2- ion was 4. The dissociation of Ni^2+・SO_4^2- ion pair proceeded with increasing pressure and decreasing temperature. The volume changes in the dissociation process of Ni^2-・SO_4^2- ion pair, ΔV^^-° were -10.1cc/mol at 15℃, -8.6cc/mol at 25℃ and -7.4cc/mol at 40℃, and the entropy change was -26cal/deg.mol. These volume contraction and the decrease of entropy may be ascribed to the electrostriction due to the ion-dipole interaction and the free ions would be hydrated much more (by 4 molecules of water) than ion pair. The partial molal volume of Ni^2+・SO_4^2- ion pair and the entropy of hydration of this ion pair were found to be -1.4 cc/mol and -69.6ca1/deg.mol at 25℃.