Determination of Autoprotolysis Constants of Water-Organic Solvent Mixtures by Potentiometry

This paper describes a potentiometric titration method for the determination of stoichiometric autoprotolysis constants (pKs) in binary mixtures of water with methanol, ethanol, 1-propanol, 2-propanol, dioxane, and acetonitrile. A combined pH electrode modified by replacing its aqueous KCl solution with 0.01 M NaCl + 0.09 M NaClO4 saturated with AgCl was used for the first time in aqueous organic solvent mixtures. The potentiometric cell was calibrated before each experiment so that the hydrogen ion concentration rather than the activity was measured. The calibration constants E° and k were determined by titrating a solution of perchloric acid with sodium hydroxide solution for each medium studied at 25 °C and under nitrogen atmosphere. During each titration the concentration of NaClO4 was maintained at 0.1 M and the pKs values were calculated by using this titration data. It is shown that the combined electrode system could be successfully used to determine autoprotolysis constants in water-organic solvent mixtures. Furthermore, the variation of the pKs values with solvent composition is discussed.