A simple, rapid, accurate, precise and sensitive spectrophotometric method for the determination of four analgesic drugs namely, tramadol (TRAM), morphine (MORF), nalbuphine (NALB) and naltrexone (NALT) in bulk sample and in dosage forms is described. The method is based on oxidation of the studied drugs by potassium permanganate in acidic medium and determination of the unreacted oxidant by measuring the decrease in the absorbance of indigo carmine (IC) dye at 610 nm. Under the optimum conditions, Beer's law limit, molar absorptivity and Sandell's sensitivity were calculated. The limits of detection and quantification were also reported. Statistical evaluation of the method was examined by determining an intra-day and inter-day precisions. The method was successfully applied to the assay of drugs in their pharmaceutical formulations. No interference was observed from common additives and the validity of the method was tested.

A novel study describes the development and validation of a selective and simple kinetic spectrophotometric method for the determination of some analgesic drugs namely, morphine (MOF), nalbuphine (NAB) and naltrexone (NAT) in bulk and in pharmaceutical formulations. This method was based on the reaction of the drugs under studying with alkaline KMnO4 to form a water-soluble bluish green colored product with a maximum absorbance at 605 nm. Although, the determination of MOF, NAB and NAT drugs by rate constant, fixed-concentration and fixed time methods were feasible with the calibration equations obtained, the fixed time method has been found to be more applicable. The fixed-time method is adopted for constructing the calibration curves, which were found to be linear over the concentration ranges of 4 - 18 μg mL-1 MOF, 2 - 20 μg mL-1 NAB and 2 - 16 μg mL-1 NAT. Different experimental parameters which can affect the development and stability of the color were carefully studied and optimized. The fixed time method of 20 min was further applied to pharmaceutical formulations of each drug and the percentage recoveries were 99.85±0.049 to 102.27±0.024. Statistical comparisons of the results with the reference methods show the excellent agreement and indicate no significant difference in accuracy and precision.