Osmotic and activity coefficients of the {xZnCl(2)+(1-x)ZnSO4}(aq) system at 298.15 K

Abstract

Isopiestic vapor pressure measurements were made for {xZnCl(2)+(1-x)ZnSO4}(aq) solutions with ZnCl2 molality fractions of x=(0,0.3062,0.5730,0.7969, and 1) at the temperature 298.15 K, using KCl(aq) as the reference standard. These measurements cover the water activity range 0.901-0.919 lt = a(w) lt = 0.978. The experimental osmotic coefficients were used to evaluate the parameters of an extended ion-interaction (Pitzer) model for these mixed electrolyte solutions. A similar analysis was made of the available activity data for ZnCl2(aq) at 298.15 K, while assuming the presence of equilibrium amounts of ZnCl+(aq) ion-pairs, to derive the ion-interaction parameters for the hypothetical pure binary electrolytes (Zn2+,2Cl(-)) and (ZnCl+,Cl-). These parameters are required for the analysis of the mixture results. Although significant concentrations of higher-order zinc chloride complexes may also be present in these solutions, it was possible to represent the osmotic coefficients accurately by explicitly including only the predominant complex ZnCl+(aq) and the completely dissociated ions. The ionic activity coefficients and osmotic coefficients were calculated over the investigated molality range using the evaluated extended Pitzer model parameters.