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

Abstract

Dissociation constants of acid components of pH reference materials in tetrahydrofuran–water mixtures containing 0, 10, 20, 30, 40, 50, 60, 70 and 100% (w/w) tetrahydrofuran, THF, were obtained. Dissociation constant values determined were: pK₁ and pK₂ for tartaric and phthalic acid; pK₁, pK₂ and pK₃ for citric acid; pK_a for acetic and boric acid and pK₂ for phosphoric acid. These values are essential to the determination of the reference pH_S values of the standard buffer solutions, which are needed for accurate pH measurements in THF–water mixtures. In order to explain the variation of the pK_a values obtained over the whole composition range studied, the quasi-lattice quasi chemical (QLQC) theory of preferential solvation was applied. To identify the solvent characteristics affecting the pK_a values, the results are discussed in terms of average macroscopic properties of the mixed solvent and in terms of the solvation shells around the solute. Thus, the methodology of linear solvation energy relationships (LSER) was used to relate pK_a values with parameters of THF–water mixtures. The equations obtained allow calculation of the pK_a values of substances in THF–water mixtures.