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

Abstract

A review about the analyte pK _a 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 pK _a values of analytes in methanol‐water and acetonitrile‐water from their corresponding aqueous pK _a. From both, the estimated pH of the mobile phase and the estimated pK _a of acid‐base analytes, it is possible to predict their degree of ionization and, therefore, the analyte chromatographic retention.