Transition metal-catalysed oxidation of 3,3'-diaminobenzidine [DAB] in a model system

The oxidation of 3,3'-diaminobenzidine [DAB] by H2O2 catalysed by 4 transition metals, Cu++, Co++, Fe+++ and Mn++, was investigated spectrophotometrically in a model system. The velocity of the reactions was higher with higher concentrations of DAB, H2O2 and a metal as well as with higher temperature. Optimal pH found for reactions catalysed by different transition metals was pH = 7.0 for Cu++ and Co++, pH - 6.0 for Fe+++ and pH - 9.0 for Mn++. A rapid spontaneous DAB oxidation by H2O2 in the absence of any transition metal was observed at pH less than 5.0.