PREFERENTIAL SOLVATION IN ACETONITRILE-WATER MIXTURES - RELATIONSHIP BETWEEN SOLVATOCHROMIC PARAMETERS AND STANDARD PH VALUES

Standard pH values; pH(s), for five reference buffer solutions, KH tartrate, KH2 citrate, KH phthalate, acetate buffer and phosphate buffer in acetonitrile-water mixtures containing 10, 30, 40, 50, 70 and 100 wt.% acetonitrile at 298. 15 K have been determined using the IUPAC standardization rules. The relationship between pH(s) and solvent composition, expressed as a fraction, have been studied, with a view to assessing the presence of preferential solvation effects. In order to obtain pH(s) values for all possible acetonitrile-water mixtures, the linear solvation energy relationships method, LSER, has been applied. The pH(s) values were then correlated with the Kamlet-Taft, pi*, alpha and beta solvatochromic parameters of the acetonitrile-water mixtures. The equation obtained permits the standardization of potentiometric sensors in these mixtures.