Amperometric flow-injection method for the assay of L-ascorbic acid based on the photochemical reduction of Methylene Blue

Ascorbic acid (AA) is determined by amperometric detection based on the photochemical reduction of Methylene Blue (MB(+)) in 0.1 M phthalate buffer at pH 3.8. In this medium, MB(+) using flow-injection analysis. The carrier stream is 1 mM MB(+) is reduced quasi-reversibly at a glassy carbon electrode at -0.34 V vs. Ag/AgCl, while AA is oxidized irreversibly at about 0.3 V. The reactor is irradiated with a 500 W halogen lamp to facilitate the development of the photochemical reaction. A laboratory-built wall-jet electrode system was used. The Leu-comethylene Blue formed in the reaction is detected at +0.050 V. At 2.2 ml min(-1) and using a sample loop of 43 mu l, the method allows the determination of AA in the range 5.0-90.0 mu g ml(-1), with a relative standard deviation of 1.3-4.8%, a detection limit of 1.9 mu g ml(-1) and a sampling frequency of 45-50 h(-1).