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

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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 (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.