Voltammetric determination of the pKa of various acids in polar aprotic solvents using 1,4-benzoquinone

The effect of various proton-donors on the electrochemical reduction of 1,4-benzoquinone (BQ) was investigated in acetonitrile, dimethylsulfoxide and methylene chloride. Three representative types of acids, protonated amines, carboxylic acids and phenol derivatives, were used as proton-donors to cover a wide range of pK(a). As a proton-donor is added. a new peak appears and the magnitude of the potential shift (DeltaE(p)) is proportional to -pK(a) of the protogenic acid. Plots of DeltaE(p) versus pK(a) in the aprotic solvents employed in this study show good linear relationships regardless of the acid's functional group. An ECE process is believed to be responsible for this behavior and the linearity reflects dissociation of the Bronsted acid. As a consequence, it is possible to estimate the dissociation constants of vaious organic acids in aprotic media conveniently and reproducibly by voltammetry. A detailed interpretation of the redox behavior of quinone and some fundamental information for potentially promising analytical applications are described. (C) 2001 Elsevier Science B.V. All rights reserved.