Ionic association in aqueous solutions of bivalent sulphates

Abstract

Osmotic coefficient data for the sulphates of Mg, Ca, Zn, Ni and Cu in aqueous solution have been analysed to obtain the dissociation constants (d.c.) of ion pairs, triplets and quadruplets, as well as information on their state of hydration. The d.c.’s of the sulphate ion pairs, though of similar magnitude, show distinct individualities, and the trend is similar to that of the d.c.’s calculated for the nitrates of di-valent ions, though the d.c.’s of the latter are about 200 times larger. The d.c.’s of the ion triplets show no correlations, but their absolute values are similar to those calculated from the theory of Fuoss & Kraus (1933, 1935) based on electrostatic considerations. The hydration parameters of the associated sulphates (both pairs and triplets), as characterized in the linear (statistical) terms of the osmotic coefficients, are only slightly smaller than those of the fully dissociated electrolytes, indicating that there is only little loss of hydration water in the process of association, in spite of the heavy hydration of the divalent cations and the relatively close approach of the sulphate ion. The complete analysis of the osmotic coefficient curves makes it possible to calculate the absolute values of the activity coefficients of the divalent sulphates (previously not known to any accuracy).