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 proposed that the chemical step of the CE reaction, namely protonation of the azo bond, could occur either intramolecularly, via azo/hydrazone tautomerism, or intermolecularly from bulk solution. At low pH the protonation from free H+ predominated while above neutral pH, until deprotonation of the ortho hydroxyl group involved in the tautomerism, the intramolecular mechanism was more prevalent. The kinetic parameters for the chemical steps in each of the two protonation mechanisms were determined.