Preferential solvation in the THF-water mixtures. Dissociation constants of acid components of pH reference materials

Dissociation constants of acid components of pH reference materials in tetr ahydrofuran-water mixtures containing 0, 10, 20, 30, 40, 50, 60, 70 and 100 % (w/w) tetrahydrofuran, THF, were obtained. Dissociation constant values d etermined were: pK(1) and pK(2) for tartaric and phthalic acid; pK(1), pK(2 ) and pK(3) for citric acid; pK(a) for acetic and boric acid and pK(2) for phosphoric acid. These values are essential to the determination of the ref erence pH(s) values of the standard buffer solutions, which are needed for accurate pH measurements in THF-water mixtures. In order to explain the var iation of the pK(a) values obtained over the whole composition range studie d, the quasi-lattice quasi chemical (QLQC) theory of preferential solvation was applied. To identify the solvent characteristics affecting the pK(a) v alues, the results are discussed in terms of average macroscopic properties of the mixed solvent and in terms of the solvation shells around the solut e. Thus, the methodology of linear solvation energy relationships (LSER) wa s used to relate pK(a) values with parameters of THF-water mixtures. The eq uations obtained allow calculation of the pK(a) values of substances in THF -water mixtures.