Ion pairing and redissociation in concentrated aqueous solutions of 2:2 electrolytes. A transport coefficient study of aqueous ZnSO4
Precise self-diffusion and conductance data are reported for aqueous solutions of zinc sulfate at 25C in the general concentration range 0.05 mol dm-3 to saturation. These data, in conjunction with mutual diffusion, transference, and activity coefficient data available in the literature, enable the calculation of velocity correlation coefficients, fab, and closely related generalized transport coefficients, which may serve as sensitive probes for ion configurations in solution. In contrast to fully dissociated salts, the fab coefficients show much greater positivity, and in particular, the cation-cation and anion-anion coefficients are positive over the whole concentration range. An explanation for these positive like-ion correlations is offered in terms of higher ion associates and ion clusters, which are shown to have a profound effect on the transport coefficients. In particular, it is shown that the comparatively high conductance of concentrated aqueous solutions of 2:2 electrolytes and higher valency types, previously interpreted as a redissociation effect, results from cation-cation and anion-anion contributions that may persist to concentrations below 0.01 mol dm-3. The data are compared with the results from HNC and Monte Carlo simulations of model 2:2 electrolytes. It is concluded that the generalized transport coefficients show promise as useful probes for gauging the quality of model calculations.