Coulometric-potentiometric determination of the autoprotolysis constant and the relative acidity scale of water

Abstract

The autoprotolysis constant and the relative acidity scale of water were determined by applying the coulometric-potentiometric method and a hydrogen/palladium (H2/Pd) generator anode. In the described procedure for the evaluation of the autoprotolysis constant, a strong base, coulometrically generated in situ at the platinum cathode in the electrolytic cell, in the presence of sodium perchlorate as the supporting electrolyte, was titrated with hydrogen ions obtained by the anodic oxidation of hydrogen dissolved in the palladium electrode. The titration was performed with a glass-SCE electrode pair at 25.0±0.1 °C. The obtained value, pK w = 13.91±0.06, is in agreement with literature data. The range of the acidity scale of water was determined from the difference between the half-neutralization potentials of the electrogenerated perchloric acid and that of sodium hydroxide in a sodium perchlorate medium. The half-neutralization potentials were measured using both a glass-SCE and a (H2/Pd) ind-SCE electrode pair. A wider range of the relative acidity scale of water was obtained with the glass-SCE electrode pair. 2010 Copyright (CC) SCS.