Redox reactions of methylene blue: a pulse radiolysis study

Abstract

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 pK_a of ≈ 8.6, and hence it is inferred that OH radicals do not bring about one-electron oxidation of the molecule.