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.