Based on recently developed hydration analysis of solubility isotherms, a method for calculating activity coefficients of solute in ternary saturated solution is suggested. The method needs no adjustable parameters; it takes only solubility data and the value of the activity coefficient of the respective solute in its binary saturated solution under identical conditions as starting point.
In the NaCl - KCl - H2O model system, a comparison between the calculated activity coefficients and those obtained by de Lima and Pitzer from solution properties and from the solubility shows a good agreement over a wide temperature range.