ELECTROOXIDATION OF ASCORBIC ACID ON THE DISPERSED PLATINUM GLASSY CARBON ELECTRODE AND ITS AMPEROMETRIC DTERMINATION IN FLOW INJECTION ANALYSIS.

Cyclic voltammetry was used to investigate the electrochemical behavior of ascorbic acid at a platinum modified glassy carbon electrode (Pt-CME) in perchloric acid solutions. Tafel slopes, reaction orders and pH effects were evaluated and a possible electrooxidation mechanism is reported. Experiments in flow injection analysis were performed to characterize the electrode as an amperometric sensor for the determination of ascorbic acid. The effects of several common interferences on the amperometric signal were estimated. The electrode stability, precision, limit of detection and linear range were evaluated at low and high potentials (i.e., 0.55 V and 1.0 V). In particular, at 0.55 V the limit of detection was 25 ng mL(-1) and the calibration graph was linear in the range 60 ng mL(-1)-350 mu g mL(-1) (correlation coefficient 0.999). The mechanism of the oxidation of ascorbic acid on the Pt-CME is rather similar to the bulk platinum electrode, but in FIA a different behavior was observed. In particular, the Pt-CME, after repetitive injections of ascorbic acid, shows very reproducible amperometric signals without recourse to pulsed waveform potentials. In contrast, bulk platinum electrodes shows a significant poisoning effect towards the electrooxidation of ascorbic acid, thus pulsed amperometric techniques are necessary to preserve the catalytic performance. Moreover, the Pt-CME in comparison with the bulk platinum electrode, shows significant oxidation currents in the wide range of potentials (i.e., 0.4 V-1.5 V). Examples of the determination of ascorbic acid in fruit juice were achieved by flow injection analysis. Good levels of recovery (99-103%) were obtained. For comparison, the ascorbic acid was also determined in these samples by using a chromatographic method with UV detection at 254 nm.