Benzoquinone imines. Part I. p-Phenylenediamine–ferricyanide and p-aminophenol–ferricyanide redox systems

Abstract

The effect of pH on the position of equilibrium in the p-phenylenediamine–ferricyanide and p-aminophenol–ferricyanide redox systems is discussed. The equilibria are established very rapidly at all pH. At high pH (>7), the equilibria lie in favour of the quinone imine and the instability of the systems is due to hydrolysis of these imines. At low pH (<4), the equilibria lie in favour of the amines and the systems are relatively stable. At intermediate pH, both the amine and imine are present in significant amounts, resulting in rapid coupling reactions with the formation of coloured products. The p-phenylenediamine–ferricyanide system is complicated by the formation of significant quantities of the p-benzosemiquinone di-imine radical cation. Equilibrium data are used to calculate the oxidation potentials of the imines and the equilibrium constant for the formation of the semiquinone.