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 ch annel microband electrodes in combination with variable flow rate meas urements 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 wi th a CECE mechanism, while at higher mass transport conditions an ECfa stCE mechanism is followed. At pH 6.7, the results are again consisten t with an ECE mechanism. The following mechanism is proposed: AH2 reve rsible 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 50 0 s(-1) are found for the rate constants k(CEE) and k(ECE), respective ly.