Isotachophoresis in mixed solvents consisting of water, methanol and dimethyl sulphoxide : I. The acid—base and migration behaviour of substituted benzenecarboxylic acids

Abstract

The acid—base properties of 30 neutral acids (benzenecarboxylic acids were investigated in methanol—water and dimethyl sulphoxide—methanol—water. The results of the estimation of the apparent p K values are discussed on the basis of the concept of transfer activity coefficient and interpreted by anion solvation effects. Different changes in the apparent p K values in the solvents were utilized to enhance the selectivity characteristics of isotachophoresis. For this purpose, a number of analytes were characterized by their relative step heights in two aqueous electrolyte systems with different pH values of the leading electrolyte. The corresponding properties of the analytes were also determined in systems with equal apparent pH values but different solvent compositions. The similarity of two aqueous two mixed aqueous—organic electrolyte systems was quantified by numerical taxonomy, using the effective ionic mobilities as parameters. The mobilities were determined from the relative step heights. As a result, the systems with (apparent) pH values of 5.7 and dimethyl sulphoxide—methanol—water and water as solvents, were found to have the lowest degree of resemblance. The increase in separation selectivity which can be generated by methanol dimethyl sulphoxide, is demonstrated for benzenecarboxylic acids.