A new spectrophotometric method for the determination of promethazine–HCl from pure and pharmaceutical preparations

Promethazine–HCl reacts with potassium persulphate to give a pinkish red color complex exhibiting maximum absorbance at 515 nm. The reaction is selective for promethazine–HCl, with 1 μg/mL as visual limit of identification and provides a basis for a new spectrophotometric determination method. The color forming reaction obeys Beerʹs Law from 0.001 to 0.125 mg/mL of promethazine–HCl. The relative standard deviation does not exceed 0.005 mg/mL. The method is successfully applied to pure and pharmaceutical formulations of promethazine–HCl. The quantitative assessment of tolerable amounts of possible interferants was also studied. The results are reproducible within ±1% and in good agreement with those obtained by the standard procedure.