Kinetic studies on oxidations of leucomethylene blue and leucothionine by iron (III) in aqueous solution

Abstract

The reaction kinetics and mechanism of the oxidations of leucomethylene blue (MBH) and leucothionine (TH) by iron (III) sulfate in aqueous solution were studied spectrophotometrically by the stopped-flow technique. MBH and TH, which were freshly prepared by photoreduction of methylene blue and thionine, respectively, with ascorbic acid were used in the kinetic measurements. The pseudo-first-order rate constants (k obsd ) show kinetic saturation at high initial concentrations of iron (III) sulfate for MBH and TH. It was found that the reciprocal of k obsd increases linearly with increase in the reciprocal of [Fe 3 ] 0 . A broad absorption band was observed on mixing MBH and Fe 3 solutions at low temperatures, and this was attributed to a charge-transfer complex between MBH and Fe 3 . The effects of Fe 2 ion and L-()-ascorbic acid on the rates of oxidation were also investigated. A small kinetic isotope effect on the oxidation rate for MBH was observed. The results can be explained by a general mechanism with stepwise electron-proton-electron transfers with the formation of a complex between reactants.

Hydro-ionothermally Generated Paramagnetic Fe State-Embedded Ferromagnetic Iron Oxide with a 2D Hexagonal Nanostructure: A Conceptual Intervention on the Methylene Blue Degradation Mechanism

“…This reversible formation of MB + during the UV irradiation process in the presence of A Fe x O y can be attributed to the oxidation by Fe 3+ . 55 A similar phenomenon was also observed with B Fe x O y (in Figure 10d) but with an alternate backward reaction rate. The reversible reaction of MB with the Fe state is shown in Figure S3 in Supporting Information, along with a surface profile of hexagonal B Fe x O y .…”

“…Previous investigations on the hydride ion transfer reactions of TH and MBH with two electron oxidants in a closed system proved this to be a stepwise electron–proton–electron (EPE) transfer process . Under ordinary pH, the monovalent cations of thionine and MB, T + and MB + , respectively, are not efficiently reduced by one-electron systems like the Fe 2+ ion . The Fe 3+ state formed by the oxidation of Fe 2+ ions will re-oxidize the photo-reduced products to T and MB, leading to a reversible equilibrium.…”

“…62 Under ordinary pH, the monovalent cations of thionine and MB, T + and MB + , respectively, are not efficiently reduced by one-electron systems like the Fe 2+ ion. 55 The Fe 3+ state formed by the oxidation of Fe 2+ ions will reoxidize the photo-reduced products to T and MB, leading to a (…”

“…The nature of FM was also confirmed from the Arrott plot, where spontaneous magnetization (M sp ) is shown in Table 4. It is interesting to say that the highest spontaneous magnetization is observed for C Fe x O y , followed by A Fe x O y 55 The observed photo-bleaching of MB (oxidized form) to LMB (reduced form) form is also indicative of the low dissolved oxygen content 56 since the UV irradiation experiments were carried out in a closed system. The LMB form is very reactive and unstable.…”

Greening the Blue Bottle

“…Of these, only iron with copper gave positive kinetic results but the overall color was less satisfactory. Studies of iron in related systems have also been reported (11,16,17).…”