Solvent effects on acid-base behavior: Five uncharged acids in water-sulfolane solvents

Abstract

An electrometric titration method utilizing glass electrodes and silver-silver bromide electrodes in a cell without liquid junction has been applied to a determination of the dissociation constants of five uncharged weak acids (HA) in four mixtures of water and sulfolane (tetramethylene sulfone) at 25°C. The acids studied were monochloroacetic, formic, benzoic, and acetic acids andp-nitrophenol, and the mole fractions (x2) of sulfolane in the mixed solvents were 0.2, 0.4, 0.6, and 0.8. All measurements were made at a constant ionic strength of 0.02 mole-kg−1 in a solution containing NaBr at a molality of 0.01 mole-kg−1. The cell was standardized by measurements of HClO4 at molalities from 0.002 to 0.01 mole-kg−1 and, in subsequent measurements, a solution of NaA was titrated with HClO4. To obtain the thermodynamic pK, an activity correction derived from the Debye-Huckel theory was applied. The pK of all five acids was found to vary linearly withx2 in the range 0 to 0.8. By comparison of data for acetic acid in water-methanol, water-dioxane, and water-sulfolane solvents, it was shown that the results are consistent with the known properties of sulfolane (low acidity, basicity, and solvating power), but the linear variation of pK with solvent composition remains unexplained.