Oxidized diphenylamine is newly utilized as a redox spectrophotometric reagent for the
determination of six pharmaceutically important thiol and thioamide drugs named:
acetylcystiene, captopril, carbimazole, propylthiouracil, thiopental sodium, and tiopronin. The
method is based on measurement of the decrease in absorption intensify of the oxidized
diphenylamine (diphenylbenzidine violet, Lmax= 580 nm) reagent as a result of the reduction
effect of the analysed drugs, This reagent was instantaneously prepared by the oxidation of
diphenylamine using ferric sulphate in sulphuric acid medium. The molar ratio of the
chromogen reagent was determined to be 2: 1; diphenylamine: iron (111). The decrease in
colour intensity was found to be quantitatively dependant on drug concentration. Experimental
variables including reagent concentration, acid type and concentration, dilution solvent,
reaction time, temperature and stability were studied and optimized. Validation parameters
including linearity range, detection and quantitation limits, precision, selectivity and robustness
were evaluated. The proposed method was found to be simple, sensitive and accurate one
indicated by the studied validation parameters. Good recoveries (98.0:tO.14 - 100%, :to.98)
were obtained by the suggested method and it was applied for the determination of the studied
drugs in many pharmaceutical dosage forms available in the local market. Good agreement,
indicated by acceptable t- & F- tests, was found between results obtained by the suggested
method and those obtained by the reported or pharmacopoeial methods