A definition of the primary medium effect for individual ionic species in solution

Abstract

By using concepts arising from the Kirkwood-Buff theory of solutions, a definition of the effects of pressure and solvent composition on the standard chemical potential and hence the partial molecular entropy of individual ionic species in solution in given. This definition involves no assumptions or approximations and is entirely in accord with corresponding concepts for solutions of non-electrolytes. It is equivalent to the statement: “The medium effects are equal for ionic species whose interactions with any solvent are identical.” It enables the electrical potential difference between ideal solutions of electrolytes in different media to be defined in terms of physically significant quantities. An interpretation of the reported medium effect for the proton in ethanol + water mixtures is given.