Retention of ionizable compounds in high-performance liquid chromatography: 14. Acid–base pK values in acetonitrile–water mobile phases

Linear relationships between sspKa values in acetonitrile–water mixtures and wwpKa values in pure water have been established for five families of compounds: aliphatic carboxylic acids, aromatic carboxylic acids, phenols, amines, and pyridines. The parameters (slope and intercept) of the linear correlations have been related with acetonitrile–water composition. The proposed equations allow accurate estimation of the pKa values of any member of the studied families at any acetonitrile–water composition up to 60% of acetonitrile in volume (100% for pyridines). Conversely, the same equations can be used to estimate aqueous pKa values from chromatographic pKa values obtained from any acetonitrile–water mobile phase between the composition range studied. Estimation of pKa values have been tested with chromatographic literature data.