Redox reactions of methylene blue: a pulse radiolysis study

Kishore, Kamal ; Guha, S. N. ; Mahadevan, J. ; Moorthy, P. N. ; Mittal, J. P. (1989) Redox reactions of methylene blue: a pulse radiolysis study International Journal of Radiation Applications and Instrumentation. Part C. Radiation Physics and Chemistry, 34 (4). pp. 721-727. ISSN 1359-0197

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One-electron oxidation of methylene blue (MB+ has been studied using specific oxidizing radicals such as Cl-2, r-2, N3 and Tl(II) in acidic and neutral aqueous solutions). The transient spectrum showed absorption maxima at 525 nm (ε = 40, 000 dm3 mol-1 cm-1) and 360 nm (ε = 9700 dm3 mol-1 cm-1) in the acidic pH region. At neutral pH also the absorption maxima were at 525 and 360 nm but the extinction coefficients were lower by 30%. A pKa of ≈4.3 was observed for the equilibrium MBH3+ MB2+ + H+. In the case of N3 radical as the oxidant, the equilibrium: N3 + MB+ N-3 + MB2+ was observed for which an equilibrium constant of 120 was estimated from the experimental data. From this as well as from cyclic voltammetric experiments, the redox potential for the MB2+/MB+ couple was calculated as 1.25 V vs NHE. The transient species produced by the reaction of OH radicals with methylene blue gave a very different spectrum with λ m = 400 nm and a pKa of ≈ 8.6, and hence it is inferred that OH radicals do not bring about one-electron oxidation of the molecule.

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