The role of solvent permittivity in estimation of electrolyte activity coefficients for systems with ion pairing on the basis of the mean spherical approximation

Mean ionic activity coefficient for aqueous solutions of CdCl2, CuCl2, MgCl2, Na2SO4, MgSO4 and CdSO4, systems for which dielectric relaxation data have recently been obtained, can be estimated accurately on the basis of a modified version of the mean spherical approximation (MSA) for electrolyte solutions with consideration of ion pairing. The modification makes use of the actual solution permittivity rather than the permittivity of the pure solvent. Reasonable ion pairing association constants, K-as, have been determined by fitting the theoretically determined mean ionic activity coefficients to experimentally measured values in the range of concentration for which the dielectric permittivity data exist using one additional adjustable parameter, namely, the restricted diameter for all species involved.