ZINC(II)-CHLORIDE COMPLEXES IN ACETONITRILE-WATER MIXTURES

Equilibrium constants for the formation of ZnCln2-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 ZnCln2-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 ZnCln2-n complexes are even stronger than in acetonitrile. © 1990 ASEG.