Osmotic Coefficients and Activity Coefficients in Mixed Electrolyte Solutions

The excess free energy per mole of ionic strength in mixed electrolyte solutions is expressed as a linear function of this property of the solutions of single electrolytes at the same ionic strength, plus a quadratic term in the ionic strength fractions and plus higher deviation terms of the expansion around the midpoint, of which the cubic term is usually sufficient. Other properties are obtained by appropriate differentiations, the osmotic coefficients and activity coefficients by differentiation with respect to the logarithm of the ionic strength. The method is applied to freezing points, isopiestic, and solute activity measurements. The deviations from linearity in ionic strength fraction are relatively smooth functions of the ionic strength which may be expressed approximately as integral power series. The relatively complicated linear terms may usually be expressed approximately by the Debye-Huckel functions with a different size parameter for each electrolyte plus integral power series in the ionic strength. The relations of this method to Bronsted's principle of specific ion interaction, to Harned's rule and Friedman's application of the Mayer theory are discussed. (auth)