The paper extends applicability of the internal standard method published in 2009 (Fuguet E. et al., J. Chromatogr. A 2009, 1216(17), 3646). Although the original capillary zone electrophoresis method was suggested to determine thermodynamic acidity constants of compounds sparingly soluble in aqueous solutions by carrying out only runs at two different pH values (i.e., without the need to perform many experiments over the appropriate pH range including the form of a low-ionized analyte), we proved that the approach also virtually overcomes any interactions of the analyte in mixed solvents, so that the experiments can be carried out in a methanol-water buffer where the solubility is much better. Applicability of the extended method is illustrated on six selected β-blockers.

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