INDIRECT FLOW-INJECTION ANALYSIS OF ASCORBIC-ACID BY PHOTOCHEMICAL REDUCTION OF METHYLENE-BLUE

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 mugml-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 mugml-1 with relative standard deviations of 2.09 and 0.31% for 1.97 and 4.92 mugml-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.