Indirect Flow-Injection Analysis of Ascorbic Acid by Photochemical Reduction of Methylene Blue

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Abstract
Ascorbic acid was determined indirectly by spectrophotometry at 666 nm based on the photochemical reduction of methylene blue using flow-injection analysis. The carrier stream was 12.7 μgml−1 methylene blue in phthalate-HCl buffer solution at pH 3.2. The reactor was irradiated with a 500-Watt halogen lamp to facilitate the development of the photochemical reaction. The system allows determination of ascorbic acid in the range 0.18–6.12 μgml−1 with relative standard deviations of 2.09 and 0.31% for 1.97 and 4.92 μgml−1 samples, respectively, and a sampling frequency of 50–55 h−1. The proposed method was applied successfully to the determination of ascorbic acid in vitamin C tablets.