DETERMINATION OF THE THERMODYNAMIC-EQUILIBRIUM CONSTANTS OF WATER-INSOLUBLE (SPARINGLY SOLUBLE) COMPOUNDS BY CAPILLARY ELECTROPHORESIS

A method based on migration time estimation in capillary electrophores is for the determination of aqueous pK(a)(th) values of water-insolubl e compounds of pharmacological (or environmental) interest is presente d. The method requires apparent pK(a) estimation at least at two diffe rent pH values and at different methanol concentrations in the backgro und 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 ascen ding mode (starting with low pH - usually 3.5) reaching, in 0.5 pH uni ts steps, the final value of 5.5, followed by a similar descending pH sequence. This procedure was repeated for different methanol concentra tions (w/w). The other operational mode was shortened in the sense tha t only two buffers were used, typically pH 5.5 and 3.0 (the former bei ng assumed to yield the pH at which the solutes investigated are fully ionized). In all cases, the apparent pH of the methanolic solutions w as estimated and the pK, value was determined by extrapolating the set of data obtained to zero methanol concentration.