A simple method to derive the water activities of highly supersaturated binary electrolyte solutions from ternary solution data

Using mixing rules in electrolyte models for estimating the thermodynamic properties of multicomponent mixtures requires knowledge of data for each binary solution. Very often, a ternary solution in a single-particle levitation experiment can achieve a higher ionic strength as compared with the ionic strengths derived from any of the binary solutions alone. Using water activity data of ternary solutions and the Zdanovskii-Stokes-Robinson and the Kusik and Meissner (KM) equations, water activities of binary solutions of (NH₄)₂SO₄, NaCl, and NH₄Cl at supersaturations higher than any previously reported are calculated. With available data from the literature on aqueous supersaturated droplets of NH₄NO₃-(NH₄)₂SO₄ ternary solutions and letovicite at ambient temperatures, water activities (>0.32) of (NH₄)₂SO₄ solutions for ionic strengths up to 112 mol kg^-1 are obtained. The new water activity relation enables the use of the KM and Pitzer models at higher ionic strengths than in previous research. Applying the same method to the data on (NH₄)₂SO₄-NaCl and NH₄NO₃-NaCl systems yields water activities (>0.28) of NaCl solutions for ionic strengths up to 25 mol kg^-1. Similarly, water activities (>0.34) of NH₄Cl solutions up to 42 mol kg^-1 are calculated from the measured data on NH₄Cl-NH₄NO₃ and NH₄Cl-(NH₄)₂SO₄ systems. New water activity measurements for the NH₄Cl-NH₄NO₃, NH₄Cl-(NH₄)₂SO₄, and NH₄NO₃-NaCl systems are also reported.