Determination of p Ka values of some hydroxylated benzoic acids in methanol–water binary mixtures by LC methodology and potentiometry

Abstract

An accurate estimation of p K a values in methanol–water binary mixtures is very important for several separation techniques such as liquid chromatography and capillary electrophoresis that use these solvent mixtures. In this study, the p K a values of 11 polyphenolic acids have been determined in methanol–water binary mixtures (10%, 20% and 30% (v/v)) by potentiometry, liquid chromatography (LC) and LC-DAD methodology. The results show a similar trend for the p K a values of all the studied compounds, as they increase with increasing concentration of organic modifier, which allows a linear relationship between p K a values and mole fraction of methanol to be obtained. The p K a values obtained in aqueous medium have been compared with those given in the literature, and also with the values predicted by the SPARC on-line p K a calculator. The data obtained have been used to test the feasibility of an estimation of dissociation constants in a methanol–water medium from the relationship between p K a values and the organic cosolvent fraction in the mixtures.