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; pKa for acetic and boric acid and pK2 for phosphoric acid. These values are essential to the determination of the reference pHS 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 pKa 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 pKa 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 pKa values with parameters of THF–water mixtures. The equations obtained allow calculation of the pKa values of substances in THF–water mixtures.