Determination of pK_a values of some hydroxylated benzoic acids in methanol-water binary mixtures by LC methodology and potentiometry

摘要

An accurate estimation of pK_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 pK_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 pK_a values of all the studied compounds,as they increase with increasing concentration of organic modifier,which allows a linear relationship between pK_a values and mole fraction of methanol to be obtained.The pK_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 pK_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 pK_a values and the organic cosolvent fraction in the mixtures.