Isotachophoresis in mixed solvents consisting of water, methanol and dimethyl sulphoxide : III. Influence of the solvent composition on the dissociation constants and mobilities of non- and hydroxy-substituted aliphatic carboxylic acids

Abstract

The electrophoretic properties of uncharged acids of the type HA were investigated in operational systems consisting of mixed aqueous-organic solvents. The mixed solvents applied consisted of water, methanol and dimethyl sulphoxide in which the molar fraction of water was held constant at 0.3 and those of the organic co-solvents were varied. The analytes were C 1–C 5 non- and monohydroxy-substituted n-alkanoic acids. The changes in the p K values were found to be linearly correlated with the molar fraction of dimethyl sulphoxide within the mixing range considered. The changes in the p K values can be qualitatively described by the transfer activity coefficients, which are related to the solvation abilities of the solvents involved. The mobilities and the corresponding normalized values, where the different viscosities of the particular solvents are taken into account, were determined in operational systems with apparent pH values of 8.7. No changes in the selectivities occurred within the two groups of acids. The normalized mobilities of the non-substituted acids, on the other hand, increased with increasing concentration of dimethyl sulphoxide to a smaller extent than those of the hydroxy-substituted acids. The grouping of these two classes is demonstrated by cluster analysis.