Mechanism of Voltammetric Determination of pKa of Brønsted–Lowry Acids in Aprotic Solvent by Quinone Reduction

Abstract The determination of pKa of Bronsted–Lowry acids in aprotic solvent is a meaningful work. We observed that a small concentration of acids caused a new cathodic peak during quinone reduction in acetonitrile. The new peak is independent of the quinone concentration, but its height increases with increasing acid concentration and its potential is dependent on the pKa of the acids. Moreover, the substituent of quinones strongly affects the potential difference between the new peak and the original peak. The new peak is produced by a sharp increase in pH on the electrode surface. Therefore, the new peak is independent of the concentration of quinone. We suggest a new mechanism of voltammetric determination of pKa of Bronsted acids in acetonitrile. In addition, the pKa values obtained in this work are consistent with those in literature.