Determination of benzoate by paper chromatography with visualization due to its inhibitory activity in the reaction of the photosensitized autooxidation of pyrogallol A

Abstract

The oxidation of 1,2,4-triacetoxybenzene (Pyrogallol A) and 3,3′,5,5′-tetramethylbenzydine by air oxygen in presence of tris-(2,2′-dipyridyl)ruthenium(II) as a photosensitizer is studied upon irradiation by visible light. The photooxidation of pyrogallol A was inhibited selectively by benzoate at pH 9–11. The rate of the reaction was not affected by other carboxylic acids but was influenced by short-chained aliphatic amines and transition metals. We developed a semiquantitative procedure for the determination of benzoate by paper chromatography. Therein, a common solution of pyrogallol A and a photosensitizer were used for the visualization followed by irradiation with an incandescent bulb. Benzoate was detected as a light spot with Rf ∼ 0.3 against a brown background. The measurement of reflectance intensity for the spot by reflectometry allowed the determination of 1 × 10−5–0.01 M benzoate within a precision of half-order of magnitude. The procedure was validated by comparison to the data of capillary electrophoresis and used to test beverages containing benzoate at levels of 10−4–10−3 M.