Mechanism of the oxidation of L-ascorbic acid by the bis(pyridine-2,6-dicarboxylate)cobaltate(III) ion in aqueous solution

A detailed investigation of the oxidation of L-ascorbic acid (H(2)A) by the title complex has been carried out using conventional spectrophotometry at 510 nm, over the ranges: 0.010 less than or equal to [ascorbate](T) less t han or equal to 0.045 mol dm(-3), 3.62 less than or equal to pH less than o r equal to 5.34, and 12.0 less than or equal to theta less than or equal to 30.0 degrees C, 0.50 less than or equal to I less than or equal to 1.00 mo l dm(-3), and at ionic strength 0.60 mol dm(-3) (NaClO4). The main reaction products are the bis(pyridine-2,6-dicarboxylate)cobaltate(II) ion and l-de hydroascorbic acid. The reaction rate is dependent on pH and the total asco rbate concentration in a complex manner, i.e., k(obs) = (k(1)K(1))[ascorbat e](T)/(K-1 + [H+]). The second order rate constant, k(1) [rate constant for the reaction of the cobalt(III) complex and HA(-)] at 25.0 degrees C is 2. 31 +/- 0.13 mol(-1) dm(3) s(-1). Delta H-double dagger = 30 +/- 4 kJ mol(-1 ) and Delta S-double dagger = -138 +/- 13 J mol(-1) K-1. K-1, the dissociat ion constant for H(2)A, was determined as 1.58 x 10(-4) mol dm(-3) at an io nic strength of 0.60 mol dm(-3), while the self exchange rate constant, k(1 )1 for the title complex, was determined as 1.28 x 10(-5) dm(3) mol(-1) s(- 1). An outer-sphere electron transfer mechanism has been proposed.