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Issue 3, 2003
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Kinetics and mechanism of a fast leuco-Methylene Blue oxidation by copper(ii)–halide species in acidic aqueous media

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Abstract

The kinetics of a fast leuco-Methylene Blue (LMB) re-oxidation to Methylene Blue (MB) by copper(II)–halide (Cl, Br) complexes in acidic aqueous media has been studied spectrophotometrically using a stopped-flow technique. The reaction follows a simple first order rate expression under an excess of the copper(II) species (and H+(aq)), and the pseudo-first order rate constant (kobs) is largely independent of the atmosphere used (air, oxygen, argon). The rate law, at constant Cl (Br) anion concentration, is given by the expression: (d[MB+])/dt = ((kaK[H+] + kb)/(1 + K[H+]))·[CuII][LMB] = kobs[LMB], where K is the protonation constant, and ka and kb are the pseudo-second order rate constants for protonated and deprotonated forms of LMB, respectively. The rate law was determined based on the observed kobsvs. [CuII] and [H+] dependences. The rate dramatically increases with [Cl] over the range: 0.1–1.5 M, reflecting the following reactivity order: Cu2+(aq) ≪ CuCl+(aq) < … < CuCl42−. The slow re-oxidation of LMB by oxygen has also been briefly examined at different [H+]. ESR results provide clear evidence for the formation of an intermediate radical. The mechanistic consequences of all these results are discussed.

Graphical abstract: Kinetics and mechanism of a fast leuco-Methylene Blue oxidation by copper(ii)–halide species in acidic aqueous media

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Publication details

The article was received on 14 Jun 2002, accepted on 16 Oct 2002 and first published on 23 Dec 2002


Article type: Paper
DOI: 10.1039/B205786G
Dalton Trans., 2003, 348-353

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    Kinetics and mechanism of a fast leuco-Methylene Blue oxidation by copper(II)–halide species in acidic aqueous media

    O. Impert, A. Katafias, P. Kita, A. Mills, A. Pietkiewicz-Graczyk and G. Wrzeszcz, Dalton Trans., 2003, 348
    DOI: 10.1039/B205786G

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