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The kinetics of the intra-molecular electron transfer of an adduct of L-ascorbic acid and the [Fe 3 III O(CH 3 -COO) 6 (H 2 O) 3 ] ? cation in aqueous acetate buffer was studied spectrophotometrically, over the ranges 2.55 B pH B 3.74, 20.0 B h B 35.0°C, at an ionic strength of 0.50 and 1.0 mol dm -3 (NaClO 4 ). The reaction of L-ascorbic acid and the complex cation involves the rapid formation of an adduct species followed by a slower reduction in the iron centres through consecutive one-electron transfer processes. The final product of the reaction is aqueous iron(II) in acetate buffer. The proposed mechanism involves the triaqua and diaqua-hydroxo species of the complex cation, both of which form adducts with L-ascorbic acid. At 25°C, the equilibrium constant for the adduct formation was found to be 86 ± 15 and 5.8 ± 0.2 dm 3 mol -1 for the triaqua and diaquahydroxo species, respectively. The kinetic parameters derived from the rate expression have been found to be: k 0 = (1.12 ± 0.02) 9 10 -2 s -1 for the combined spontaneous decomposition and k 1 = (4.47 ± 0.06) 9 10 -2 s -1 (DH 1 à = 51.0 ± 2.3 kJ mol -1 , DS 1 à = -100 ± 8 J K -1 mol -1 ), k 2 = (4.79 ± 0.38) 9 10 -1 s -1 (DH 2 à = 76.5 ± 0.8 kJ mol -1 , DS 2 à = 6 ± 3 J K -1 mol -1 ) for the triaqua and diaqua-hydoxo species, respectively. Abbreviation Trimer [Fe 3III O(CH 3 COO) 6 (H 2 O) 3 ] ?

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