Zinc(II)-Chloride Complexes in Acetonitrile-Water Mixtures

Abstract

Equilibrium constants for the formation of ZnCln^2-n complexes have been measured in some acetonitrile ( MeCN )-water mixtures ranging from 0 to 100% MeCN at 25°C and I = 0.1 M (Et₄NClO₄), by potentiometric titration. The equilibria shift strongly in favour of the formation of the higher-order complexes (n = 3, 4) at high MeCN concentrations. This is shown to be due to differences in the magnitude of unfavourable solvation of the Zn²⁺, Cl ^- and ZnCln^2-n ions in MeCN-H₂O solutions. The Gibbs free energies of transfer of all of these ions from water to MeCN-H₂O mixtures are strongly positive, but the net change in the transfer function for the formation of the higher-order complexes is favourable. Thus the formation of these complexes is predominant in MeCN -rich aqueous solutions. Approximate measurements in propiononitrile indicate that the ZnCln^2-n complexes are even stronger than in acetonitrile.