Theory of concentrated solutions of strong electrolytes. Part 2.—Thermodynamic properties of mixed electrolytes. Theoretical basis of the Harned rule

Abstract

The lattice-like model of electrolyte solutions has been used to interpret the thermodynamic properties of mixtures of strong electrolytes. Activity coefficients and the partial molar enthalpy content of several pairs of electrolytes have been calculated by assuming a cubic lattice of the common ion in which the two sorts of counter ions occupy body-centred positions. Comparison with experiment shows fairly good agreement. The theoretical treatment yields a correlation between Harned rule coefficients and the activity coefficients of the pure components at the total concentration of the mixture. According to this correlation the absolute value of the Harned rule coefficients, α_1,2 and α_2,1, must be the same, which is in agreement with measured values in case of acid mixtures, while the difference in their absolute value points to local hydrolysis in salt solutions.