Washburn numbers. Part 4.—The Erdey-Grúz experiment. Relative solvent transport numbers for ion constituents in mixtures of water with raffinose, glycine, allyl alcohol, dimethylsulphoxide and dioxan

Abstract

Erdey-Grúz's diffusion experiment has been used to obtain the sum of the transport numbers of water by ions, relative to a second solvent component, Σn^°_w=(n₊)^°_w+(n_–)^°_W. The theory of the experiment is described in detail; the corrections for the effect of salt on solvent activity, lacking in Erdey-Grúz's work, have been made. The experimental method involved a diaphragm cell and analysis of changes in mixed-solvent concentration by interferometry.

The Σn^°_w are combined with Washburn numbers, w^°_w, some of which are new values obtained from cells with ion-selective electrodes. With w^°_w=(n₊)^°_wt₊–(n_–)^°_wt_–, the ionic n^°_w are obtained unambiguously for systems containing low concentrations of glycine, allyl alcohol, dimethylsulphoxide (DMSO) and dioxan. These data were also calculated for a 0.75%(w/w) raffinose + water mixture using transport and diffusion data from the literature.

The n^°_w values for the raffinose + water system, which probably approach closely the dynamic solvation number, N, of the ion in pure water, are Li⁺(16), Na⁺(10), K⁺(6), Cl^–(4) and H⁺(1). In the remaining systems these figures are reduced by the competition of the organic molecule for solvation of the ion. In the glycine + water system the ions transport glycine preferentially.