On the Effect of Organic Solvent Composition on the pH of Buffered HPLC Mobile Phases and the pKa of Analytes—A Review

A review about the analyte pKa and buffer pH variations in RP-HPLC mobile phases with the changes in the organic modifier content (acetonitrile or methanol) is presented. A model to accurately predict the pH of particular mobile phases for several commonly used buffers (acetic, citric and phosphoric acid and ammonia systems) in acetonitrile-water and methanol-water mixtures is described. Linear relationships are also presented for several families of acid-base compounds (aromatic and aliphatic carboxylic acids, phenols, amines and pyridines) to estimate pKa values of analytes in methanol-water and acetonitrile-water from their corresponding aqueous pKa. From both, the estimated pH of the mobile phase and the estimated pKa of acid-base analytes, it is possible to predict their degree of ionization and, therefore, the analyte chromatographic retention.