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

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.