PREFERENTIAL SOLVATION IN ACETONITRILE-WATER MIXTURES - RELATIONSHIP BETWEEN SOLVATOCHROMIC PARAMETERS AND PK VALUES OF CARBOXYLIC-ACIDS

Ionization constants of several carboxy!ic acids in acetonitrile-water mixtures, usually used as mobile phases in liquid chromatography (LC) , have been obtained in order to predict the influence of pH on retent ion and selectivity in LC. In this work, the quasi-lattice quasi chemi cal (QLQC) theory of preferential solvation was applied to obtain the degree of preferential solvation of acetate ions in these media in ord er to explain the variation of the pK values obtained over the whole c omposition range studied. Also, the relationships between pK values an d various macroscopic and microscopic properties were used to calculat e pK values of the carboxylic acids in any acetonitrile-water mixture up to 70% (w/w) and, thus, to determine the acid-base behaviour of sub stances in the widely used acetonitrile-water media.