Multicomponent diffusion in dilute solutions of mixed electrolytes

Abstract

The Onsager-Fuoss theory of ion transport can be used to study multicomponent diffusion in dilute solutions of mixed electrolytes. Precise Lij phenomenological coefficients can be calculated from a knowledge of the solvent properties and limiting ionic conductivities. If activity coefficients for dilute mixed electrolyte systems are available, precise multicomponent diffusion coefficients can also be calculated. Explicit equations are given for calculating both sets of transport coefficients. The equations apply to systems containing an arbitrary number of strong electrolytes and are valid at low total salt concentrations. To illustrate the use of the equations, ternary diffusion in a few aqueous systems is examined in detail. Aqueous mixed electrolyte systems containing H+ exhibit strongly coupled diffusion and highly variable diffusion coefficients. A modified Harned conductimetric experiment is described for the determination of ternary diffusion coefficients in dilute solutions. Results for the system HCl- KCl-H2O are in good agreement with theoretical predictions.