Variation of acidity constants and pH values of some organic acids in water—2-propanol mixtures with solvent composition. Effect of preferential solvation

A general equation which relates the dissociation constant of an acid with the solvent composition in binary solvent mixtures is derived. The equation considers that the two solvents interact to form mixed structures. The electrolyte interacts with the mixed solvent formed and with the two pure solvents mixed. The degree of these interactions determines the preferential solvation of the electrolyte by any of the three solvents which can be present in the solvent mixture. Dissociation constants of 3-nitrophenol, 4-nitrophenol, acetic, benzoic, 3-bromobenzoic, 3,5-dinitrobenzoic and o-phthalic acid have been determined in several water—2-propanol mixtures and the proposed equation applies successfully to these acids. The parameters obtained allow an easy calculation of the dissociation constant of these acids at any solvent composition. The calculated constant can be directly used, without any experimental determination, wherever an accurate pK or pH value in water—2-propanol is needed, such as in some liquid chromatographie applications.