A new approach to the determination of pKa and pKb of Brønsted–Lowry weak acid and weak base in aprotic solvent

Abstract

• Developing a method of determining the dissociation constants of weak acid and base in acetonitrile. • Unambiguous voltammetric peak potentials were used to exactly measure pK a and pK b . • Compared with using potentiometry, the method is simple and rapid because of using cyclic voltammetry. The pK a and pK b of Bronsted–Lowry weak acids and weak bases in aprotic solvents are difficult to accurately determine. Herein, we found that in an aprotic solvent, the conjugate base of weak acid can produce a new, unambiguous anodic peak of hydroquinone, and the conjugate acid of weak base can produce a new cathodic peak of p-benzoquinone. These new peaks were due to the drastic change in pH near the electrode surface, and their potentials depended on the strength of conjugate acids and bases but not on the concentration of hydroquinone and quinone. Therefore, the conjugate acid–base dissociation constant can be measured by the new peak potential. Then, the weak acid–base dissociation constant can be obtained by the difference between the autoprotolysis constant of the aprotic solvent and the conjugate acid–base dissociation constant. The experimental results were consistent with those in literature.