Ion-coupled transport of solvent in a binary solvent mixture: A new source of thermodynamic data

Abstract

This work reports the development of a method of determining the standard Gibbs energy of transfer of an electrolyte between two different solvents, using a combination of ion-assisted solvent transport data from diffusion experiments and emf data from Wagner cells. The changes of solvent composition accompanying the diffusion of electrolyte in an initially homogeneous binary solvent mixture have been determined with a diaphragm diffusion cell using differential refractometry and conductivity. Theoretical analysis shows that the observed transport rates depend both on the composition dependence of the Gibbs energy of transfer of the electrolyte from a pure solvent component to the mixed solvent and on the solvent transport properties of the ions. The information obtained is closely related to that which is obtainable from emf studies of galvanic cells in which the half-cells contain solvents of different composition (Wagner cells). When combined with appropriate emf data the diffusion data can give values of the relative solvation intensities of the ions and, in appropriate cases, the standard Gibbs energies of transfer of electrolytes from one solvent to another. The apparatus used and the results of a study of sodium chloride in a range of methanol + water mixtures are described. The relative solvation intensities of the chloride ion, the alkali metal cations and the hydrogen ion have been determined as a function of solvent composition in methanol + water mixtures from emf data. It is shown how these measurements provide a new tool for the determination of thermodynamic data in mixed solvents.