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 Leucomethylene Blue formed in the reaction is detected at +0.050 V. At 2.2 ml min −1 and using a sample loop of 43 μl, the method allows the determination of AA in the range 5.0–90.0 μg ml −1, with a relative standard deviation of 1.3–4.8%, a detection limit of 1.9 μg ml −1 and a sampling frequency of 45–50 h −1.