Determination of the thermodynamic equilibrium constants of water-insoluble (sparingly soluble) compounds by capillary electrophoresis

A method based on migration time estimation in capillary electrophoresis for the determination of aqueous pK(a)(th) values of water-insoluble compounds of pharmacological (or environmental) interest is presented. The method requires apparent pK(a) estimation at least at two different pH values and at different methanol concentrations in the background electrolyte (usually more points of the methanol concentration vs. pK(a)(th) dependence are determined). Two automated operational modes were used; in the first, a stepped pH change was applied in the ascending mode (starting with low pH - usually 3.5) reaching, in 0.5 pH units steps, the final value of 5.5, followed by a similar descending pH sequence. This procedure was repeated for different methanol concentrations (w/w). The other operational mode was shortened in the sense that only two buffers were used, typically pH 5.5 and 3.0 (the former being assumed to yield the pH at which the solutes investigated are fully ionized). In all cases, the apparent pH of the methanolic solutions was estimated and the pK, value was determined by extrapolating the set of data obtained to zero methanol concentration.