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Publication Type
Journal Article
UWI Author(s)
Author, Analytic
Abdur-Rashid, Kamaluddin
Author Affiliation, Ana.
Chemistry; Faculty of Pure and Applied Sciences
Article Title
Kinetics and mechanisms of the oxidation of L-ascorbic acid by cis-diaqua cobalt (III) ammine complexes
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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
1996
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Issue ID
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Page(s)
1385-91
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Notes
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Abstract
Kinetics of oxidation of L-ascorbic acid by cis-diaquacobalt(III) complexes, [CoL4(H2O)2]3+ (L4 = (NH3)4, (en)2 or tren; en = ethane-1,2-diamine, tren = tris(2-aminoethyl)amine] was studied as a function of pH, L-ascorbic acid concentration, temperature, ionic strength and methanol content of the solvent using stopped-flow and conventional spectrophotometry. The results indicated that only the ascorbate monoanion, HA-, is involved in the redox process with the cobalt(III) species. The rate constants for the [Co(tren)(H2O)2]3+ and [Co(tren)(H2O)(OH)]2+ species (k2 and k5) are 0.26 0.09 and 1.25 0.03 dm3 mol-1 s-1 respectively at 30C and the corresponding activation parameters are DH2 = 124 9 kJ mol-1, DS2 = 137 30 JK-1 mol-1 and DH5 = 82 2 kJ mol-1, DS5 = 26 6 JK-1 mol-1. The variations in the rate constants and thermodynamic parameters for the series of complexes is discussed. The Marcus cross-relationship for electron transfer has been applied to the redox process to confirm the outer-sphere mechanism and to estimate the self-exchange rate constant for the [CoL4(H2O)(OH)]2+/+ couple....
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