Evaluation of acidity constants and preferential solvation in tetrahydrofuran-water mixtures

Dissociation constant values for reference materials were determined in tetrahydrofuran-water (THF-water) mixtures containing 0, 10, 20, 30, 40, 50, 60, 70 and 100% (w/w) THF. The values obtained were: pK(1) and pK(2) for phthalic acid; pK(1), pK(2) and pK(3) for citric acid; pK(a) for acetic and boric acids. In order to explain the variation of the pK values obtained over the whole composition range studied, the quasi-lattice quasi chemical (QLQC) theory of preferential solvation was applied. The results are discussed in terms of average macroscopic properties of the mixed solvent and in terms of the solvation shells around the solute in order to identify the solvent characteristics affecting the pK(a) values. Thus, the methodology of linear solvation energy relationships (LSER) was used to relate pK(a) values with solvatochromic parameters of THF-water mixtures. The equations obtained allow calculation of the pK(a) values of substances in the widely used THF-water mixtures. (C) 1999 Elsevier Science Ltd. All rights reserved.