ISSN:
1572-901X
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
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: $$\begin{gathered} - d [ complex] /dt = \{ (k_{\text{3}} {\text{[H}}^{\text{ + }} {\text{]}} \hfill \\ {\text{ + }}k_{\text{4}} K_{{\text{a1}}} {\text{)}}K_{\text{2}} {\text{[}}A{\text{]}}_{\text{T}} {\text{[complex]}}_T \} / \hfill \\ {\text{ \{ ([H}}^{\text{ + }} {\text{ + }}K_{{\text{a1}}} {\text{)([H}}^{\text{ + }} {\text{] + }}K_{\text{2}} {\text{)\} }} \hfill \\ \end{gathered} $$ The rate is slower at lower pH as the less important reactionk 1 (ROH inf2 sup3 + 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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00143496
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