Oxidative degradation pharmaceutically important phenothiazines II: Quantitative determination of promethazine and some degradation products.

Methods for the determination of promethazine and several degradation products, which can be used in kinetic studies, were developed. All determinations were carried out after isolation of the compounds by TLC. Promethazine was determined by oxidation with vanadyl sulfate in an acidic medium. The method was suitable for approximately 3 mg of the compound. 3H-Phenothiazine-3-one was determined spectrophotometrically at 492 nm in acetone. The compound could be determined in amounts of 30-40 microgram. Promethazine 5-oxide was determined spectrophotometrically using the acid dye method; 100-200 microgram could be determined. 7-Hydroxy-3H-phenothiazine-3-one could be determined spectrophotometrically at 600 nm after extraction of the other compounds from the solution. The medium had to be alkaline and contain water in a fixed ratio. The method was suitable for 15-30 microgram of the compound.