Indirect spectrophotometric determination of some biologically important phenothiazines using potassium dichromate, iron(II) and 1,10-phenanthroline

A sensitive method is presented for the spectrophotometric determination of phenothiazine derivatives of biological importance. The drugs namely, chlorpromazine hydrochloride (CPH), promethazine hydrochloride (PH), trifluoperazine hydrochloride (TFPH), prochlorperazine maleate (PCPM) and fluphenazine hydrochloride (FPH) are reacted with a fixed amount of dichromate in acidic conditions. After the reaction is complete, the unreacted dichromate is determined by treating with iron(II) and ortho-phenanthroline at a raised pH and measuring the absorbance at 510 nm. The amount of dichromate reacted corresponds to the drug content. The linearity ranges are found to be 5-30, 2.5-25, 5-45, 7.5-60 and 5-50 μg mL for CPH, PH, TFPH, PCPM and FPH, respectively. The apparent molar absorptivity values are in the range 3.46×10-6.47×10 L mol cm and the limits of determination range from 0.49 to 1.87 μg mL and the relative standard deviation is less than 2%. The proposed method has been applied for the determination of these drugs in pure form and in pharmaceutical formulations with recoveries in the range 96.28-103.24%. The method was further validated by parallel determination by the official British Pharmacopoeial procedure and by recovery studies.