Isotachophoresis in mixed solvents, consisting of water, methanol and dimethyl sulphoxide : II. Solvation effects on the acid—base and transport properties of cation acids of the ammonium ion type

Abstract

The isotachophoretic behaviour of seventeen ammonium ions was investigated in water, methanol—water and dimethyl sulphoxide—methanol—water as solvents. The apparent p K values of the solutes were estimated and the changes with solvent composition are discussed, on the basis of the transfer activity coefficient. The pH of the electrolyte systems for isotachophoresis was chosen such that all the analytes were totally protonated. A direct measurement of the mobilities of the leading and reference ions was carried out by a moving boundary technique with an instrument normally used for isotachophoresis. With the aid of these values, the actual mobilities of the analytes were derived from their relative step heights. The changes in the ionic mobilities of the analytes with solvent composition are attributed to cation solvation effects. The ammonium ions were classified, according to their mobilities and the changes in the mobilities, respectively, in the different solvents. The physico-chemical significance of the resulting resemblances are discussed.