Capillary zone electrophoresis in mixed aqueous-organic media: effect of organic solvents on actual ionic mobilities, acidity constants and separation selectivity of substituted aromatic acids - II. Ethanol

The influence of the composition of mixed aqueous-ethanolic solvents on the actual mobilities and the apparent ionization constants of 26 aromatic acids with methyl, nitro, chloro and hydroxy substitution was investigated, with concentrations ranging up to 80% (v/v) ethanol. Mobilities were determined by capillary zone electrophoresis (phosphate buffer, pH 7 in water, ionic strength 20 mmol/l), the mixed-mode, apparent pK(a) values by potentiometric titration. Increase of the pK(a) values by about 1.8-2.6 units was found for 80% EtOH, about 0.5 units higher compared to methanolic solvents with the same composition. A drastic decrease of the actual mobilities with increasing ethanol concentration was found; this decrease is less pronounced considering the change of the viscosity of the mixed solvent. Taking into account the product of actual mobility with viscosity of the solvent (the Walden product), only considerably small changes of this parameter with solvent composition are observed on the other hand. Based on these changes, and also based on the selectivity coefficients (the ratios of the actual mobility of benzoate to that of the respective ionic solute) a classification of the solutes into two distinct groups is found: one class consists of all non-hydroxy-substituted solutes; the second contains all analytes with OH substituents. (C) 1998 Elsevier Science B.V. All rights reserved.