Retention of ionizable compounds on high-performance liquid chromatography: III. Variation of p K values of acids and pH values of buffers in acetonitrile–water mobile phases

Abstract

A critical compilation of the literature p K data for acetonitrile–water is presented and equations and parameters that allow estimation of the p K of these compounds for any acetonitrile–water mixture up to 60% of acetonitrile by volume are proposed. The data and equations have been used to calculate the pH of different buffers, which have been prepared and used to calibrate a potentiometric system for several acetonitrile–water mixtures. The measured potential vs. pH plots follow the Nernst equation for all solvent mixtures after elimination of some outliers. This procedure allows us to identify the most reliable p K data and from them to propose buffered solutions of accurate pH. These solutions have been used for every-day calibration of the pH-meter in solute retention–mobile phase pH relationships. A model developed in earlier works has been used to relate the retention of benzoic acid in a C 18 column with the measured pH of the mobile phase (60% acetonitrile). The results obtained demonstrate that the best relationships are obtained when the retention is related with the true pH of the buffer in the mobile phase, instead of the pH value of the buffer in water as it is usually done.