Effect of pressure on dissociation of ion pairs : MnSO4, CuSO4 and ZnSO4

Abstract

Conductivity at 15～40℃ of aqueous solutions of MnSO_4, CuSO_4 and ZnSO_4 has been measured as a function of pressure up to 1600kg/cm^2 for the concentrations from 1×10^-4 to 4×10^-4 M. The effect of pressure on the dissociation constant was calculated and the volume changes in the dissociation process of these ion pairs at 25℃ have been estimated to be -8.5 ml/mol for MnSO_4, -8.1 ml/mol for CuSO_4 and -6.0 ml/mol for ZnSO_4, respectively. Since the partial molal volumes at infinite dilution were -3.7 ml/mol for MnSO_4, -8.1 ml/mol for CuSO_4 and -8.0 m1/mol for ZnSO_4, the partial molal volumes of the ion pairs at 25℃ would be 4.8 ml/mol for Mn^2+ ・SO_4^2-, 0.0 ml/mol for Cu^2+ ・SO_4^2- and -2.0 ml/mol for Zn^2+ ・SO_4^2-, respectively, From these results, we have obtained the order of Zn>Cu>Mn for the tendency of metal ion to form outersphere ion pair with sulfate ion.