Capillary zone electrophoresis in mixed aqueous-organic media: effect of organic solvents on actual ionic mobilities and acidity constants of substituted aromatic acids IV. Acetonitrile

The effect of the composition of binary mixtures of acetonitrile (ACN) and water (ranging up to 80%, v/v, ACN) on the apparent pK(a) values and the actual mobilities of 26 aromatic substituted benzoic acids is investigated. The (nonthermodynamic) pK(a) values are determined potentiometrically (at T = 25 degrees C and an ionic strength, I, of 5 mmol/l). The actual mobilities are measured by capillary zone electrophoresis (phosphate buffer, pH 7 in water, I = 20 mmol/l, T = 25.0 degrees C). An increase of the pK(a) values with increasing concentration of ACN is found, in a magnitude comparable to mixed aqueous-methanolic and -ethanolic solvents (1.5 to 2 pK units at 80% ACN). These changes are interpreted based on the theoretical model of the transfer activity coefficient or the medium effect. The actual mobilities change only slightly with solvent composition, which is in fair agreement with the change of the solvent viscosity. It is found that Walden's rule (supposing the constancy of the product of mobility, mu, and viscosity, eta) applied for the solvent systems investigated in the series of papers dealing with binary aqueous mixtures with methanol, ethanol, l-propanol and acetonitrile as cosolvent, and to the anions under consideration is obeyed within +/-20%. An extension of this rule (mu eta(0.9) = constant) leads to products that are constant within +5 and -9% over the concentration range of up to 60% organic solvent. The effect of the substituents (methyl, nitro, chloro, hydroxy derivatives) on the change of the actual mobilities are most pronounced for the hydroxy-substituted benzoates, especially for those with the OH group in position 3 to the carboxylate group. (C) 1999 Elsevier Science B.V. All rights reserved.