Mechanistic determination using arrays of variable-sized channel microband electrodes: The oxidation of ascorbic acid in aqueous solution

Two-dimensional voltammetry using arrays of variable-sized gold channel microband electrodes in combination with variable flow rate measurements together with independent gold rotating disk electrode (RDE) data is applied to characterize the oxidation mechanism of L-ascorbic acid (AH(2)) to dehydro-L-ascorbic acid in aqueous solutions. At pH 2.1 and very low mass transport conditions the reaction is consistent with a CECE mechanism, while at higher mass transport conditions an ECfastCE mechanism is followed. At pH 6.7, the results are again consistent with an ECE mechanism. The following mechanism is proposed: AH2 reversible arrow AH(-) + H+ (k(CEE)), AH(-) --> AH(.) + e, AH(.) - A(.-) + H+ (k(ECE)), A(.-) --> A + e (CECE); AH(2) reversible arrow AH(2)(+) + e, AH(2)(+) --> AH(.) + H+(fast), AH(.) --> A(.-) + H+ (k(ECE)), A(.-) --> A + e (ECfastCE); where, only the first pathway applies at pH 6.7, AH- being the starting species. Values of 3.4 x 10(3) s(-1) and 500 s(-1) are found for the rate constants k(CEE) and k(ECE), respectively.