THE MECHANISM OF THE ELECTRO-REDUCTION OF SOME AZO DYES

The pH dependence of the reduction mechanism of the azo dyes Orange G and Sunset Yellow FCF was investigated using a mercury plated rotating disk electrode. The reduction was shown to proceed via a CE mechanism except at very high pH, where direct reduction was observed. It is pr oposed that the chemical step of the CE reaction, namely protonation o f the azo bond, could occur either intramolecularly, via azo/hydrazone tautomerism, or intermolecularly from bulk solution. At low pH the pr otonation from free H+ predominated while above neutral pH, until depr otonation of the ortho hydroxyl group involved in the tautomerism, the intramolecular mechanism was more prevalent. The kinetic parameters f or the chemical steps in each of the two protonation mechanisms were d etermined.