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Publication Type
Journal Article
UWI Author(s)
Author, Analytic
Beckford, F.A.,; Dasgupta, Tara, P.; Stedman, G.
Author Affiliation, Ana.
Chemistry; Faculty of Pure and Applied Sciences
Article Title
Reaction of [Fe2(CN)10]4- with L-ascorbic acid
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Journal Title
Journal of the Chemical Society. Dalton Transactions
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Reprint Status
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Date of Publication
1995
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Issue ID
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Page(s)
2561-2563
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Abstract
The oxidation of L-ascorbic acid by the ion(III) dimeric complex [Fe2(CN)10]4- in acid aqueous solution at an ionic strength of 1.0 mol dm-3 (NaC1O4) has been studied by stopped-flow spectrophotometry. The overall reaction, which entails reduction of both ion (III) centres followed by aquation resulting in the formation of 2 mol of [Fe(CN)5(OH2)]3-, takes place in three stages. The first one is a one-electron reduction to form the mixed-valance iron (III, II) dimer followed by a second one-electron reduction of the other iron(III) centre to form the iron (II) dimer. The third stage which occurs with a very small absorbance change was not studied. The mechanisms for both stages of the reaction are similar and involve formation of an ion triplet [Fe(CN)10]n-.M+.Ap- where n = 4 or 5 and Ap- is the reacting ascorbate species with p = 1 or 2. The rate law was found to be of the form (i) where [A]T is the total concentration of added ascorbate, K1 and K2 the acid dissociation constants k1 K1 [H+] + k' K1K2 Rate = __________________________ [A]T[complex] (i) K1K2 + K1 [H+] + [H+]2 of ascorbic acid and k1 and k' the pseudo-second-order rate constants. They were found to be 6.44 x 103, 2.48 x 109 and 1.25 x 103, 2.98 x108 dm3 mol-1 s-1at 19C for HA- and A2- for the first and second stages respectively. The reaction is affected by alkali-metal cations, increasing rate with increasing size of the metal ions: Li+ < Na+ < K+....
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