Multicomponent diffusion of double salts. Sodium hydrogen sulphate in aqueous solution

Abstract

Diffusion coefficients for the double salt sodium hydrogen sulphate in aqueous solution have been determined at 25 °C from infinite dilution to 2 mol dm^–3 using limiting ionic conductances, the Harned restricted-diffusion method and Gouy optical data reported previously for aqueous sodium sulphate–sulphuric acid mixtures. Because the total flux of hydrogen ions exceeds the flux of less-mobile sodium co-ions, diffusion of aqueous NaHSO₄ forms a ternary mixture of NaHSO₄–H₂SO₄ in front of the diffusion boundary while a ternary mixture of NaHSO₄–Na₂SO₄ remains behind. Diffusion in aqueous solutions of NaHSO₄ can be described by a coupled flow of H₂SO₄ in addition to the flow of NaHSO₄. At concentrations below 0.1 mol dm^–3 the coupled flow of H₂SO₄ exceeds the flow of NaHSO₄. Expressions are developed to predict ternary transport coefficients for diffusion of double salts.