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redox reactions of methylene blue: A pulse radiolysis study

1989; Elsevier BV; Volume: 34; Issue: 4 Linguagem: Inglês

10.1016/1359-0197(89)90084-2

1878-1020

Kamal Kishore, S.N. Guha, Jagan Mahadevan, P.N. Moorthy, J.P. Mittal,

Analytical chemistry methods development

One-electron oxidation of methylene blue (MB+ has been studied using specific oxidizing radicals such as Ċl-2, Ḃr-2, Ṅ3 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 Ṅ3 radical as the oxidant, the equilibrium: Ṅ3 + 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 λm = 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.