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 pKa. As a proton-donor is added, a new peak appears and the magnitude of the potential shift (ΔEp) is proportional to −pKa of the protogenic acid. Plots of ΔEp versus pKa in the aprotic solvents employed in this study show good linear relationships regardless of the acidʹs functional group. An ECE process is believed to be responsible for this behavior and the linearity reflects dissociation of the Brφnsted 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.