Autooxidation of leuco-Methylene Blue - the Role of the Methylene Blue Radicals and Reactive Oxygen Species

Autooxidation of leuco-Methylene Blue to the dye, Methylene Blue, was studied in the 1.9-7.8 pH range under a large molar excess of dioxygen dissolved in the reaction solution using electronic spectroscopy. The synproportionation of the substrate and the reaction product, yielding Methylene Blue radical, plays a key role in the overall redox process. Numerical simulation of the complicated kinetic curves shows that presence of this radical opens an effective reaction path for the autooxidation process and results in coupling of the reaction steps. Reactive oxygen species (HO2 center dot/O-2(center dot-)/ and H2O2) formed in the examined system are not effective oxidants and do not affect the autooxidation kinetics under applied conditions.