Properties of Aqueous Mixtures of Pure Salts. Thermodynamics of the Ternary System: Water-Calcium Chloride-Magnesium Chloride at 25 °C

The chemical potentials of each solute in an aqueous mixture of two electrolytes have been determined for many systems. For example, the activity coefficients of both hydrochloric acid and sodium chloride in mixed aqueous solution have been derived from emf measurements [I]. The isopiestic vapor pressure method has been used [2] to determine the activity coefficients of each solute in aqueous mixtures of sodium chloride and potassium chloride. These are two examples of mixtures of uniunivalent electrolytes but considerable work has been done with mixtures of 1:1 and 1:2 electrolytes. For example, the systems water-hydrochloric acid-barium chloride and water-hydrochloric acid-strontium chloride have been studied by the emf method [3] as have the systems water-hydrochloric acid-calcium chloride and water-hydrochloric acidmanganous chloride [4], while the isopiestic method has been applied to the systems water-sodium chloridebarium chloride [5] and water-potassium chloridebarium chloride [6]. The emf method has been adapted [7] to give trace activity coefficients for the system water-hydrochloric acid-calcium perchlorate, a 'trace' activity coefficient being the limiting value of the activity coefficient of one salt in a solution where its concentration is vanishingly small and the solution contains virtually only the other salt. The sodium-sensitive glass electrode has been used [8] to study the systems water-sodium chloride-alkaline earth metal chloride. However, there appears to be no case cited in the literature in which solutions of two biunivalent electrolytes have been studied. We now present data for one system of this type, watercalcium chloride-magnesium chloride. 2. Definitions