RETENTION OF IONIZABLE COMPOUNDS ON HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY - III - VARIATION OF PK VALUES OF ACIDS AND PH VALUES OF BUFFERSIN ACETONITRILE-WATER MOBILE PHASES

A critical compilation of the Literature pK data for acetonitrile-wate r is presented and equations and parameters that allow estimation of t he pK of these compounds for any acetonitrile-water mixture up to 60% of acetonitrile by volume are proposed. The data and equations have be en used to calculate the pH of different buffers, which have been prep ared and used to calibrate a potentiometric system for several acetoni trile-water mixtures. The measured potential vs. pH plots follow the N ernst equation for all solvent mixtures after elimination of some outl iers. This procedure allows us to identify the most reliable pK data a nd from them to propose buffered solutions of accurate pH. These solut ions have been used for every-day calibration of the pH-meter in solut e retention-mobile phase pH relationships. A model developed in earlie r works has been used to relate the retention of benzoic acid in a C-1 8 column with the measured pH of the mobile phase (60% acetonitrile). The results obtained demonstrate that the best relationships are obtai ned when the retention is related with the true pH of the buffer in th e mobile phase, instead of the pH value of the buffer in water as it i s usually done. (C) 1998 Elsevier Science B.V. All rights reserved.