Summary
The kinetics of oxidation ofl-ascorbic acid (H2A) by the mononuclear complex [Co(NH3)5(OH2)] (ClO3)3 have been studied spectrophotometrically at 490 nm. The mechanism involves single one-electron transfers involving the pentaamminehydroxo complex and the ascorbate anions (HA− and A2−), subsequent formation of ascorbate radicals and > 90% CoII. The appropriate rate law in the 7.00 ⩽ pH ⩽ 8.40 and 0.005 ⩽ [A T] ⩽ 0.05 mol dm−3 ranges, has been established as:
The rate is slower at lower pH as the less important reactionk 1 (ROH sup3inf2 + HA− → products) becomes dominant, where R is (NH3)5CoIII. The more significant rate constantk 4 (ROH2+ + A2− → products) was calculated at 25.7 °C as (2.7 + 0.5) × 102 dm−3mol−1 s−1, with ΔH # = 105 ± 11 kJ mol−1 and ΔS # = 93 +- 45 J mol−1 K −1.
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Dixon, D.A., Sadler, N.P. & Dasgupta, T.P. Mechanism of the oxidation ofL-ascorbic acid by the pentaammineaquacobalt(III) ion in aqueous solution. Transition Met Chem 20, 295–299 (1995). https://doi.org/10.1007/BF00143496
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DOI: https://doi.org/10.1007/BF00143496