Adaptation of a color reaction for spectrophotometric determination of diclofenac sodium and piroxicam in pure form and in pharmaceutical formulations

A simple, sensitive, and convenient spectrophotometric method for the determination of diclofenac sodium and piroxicam in pure form and in pharmaceutical formulations was developed. The method is based on the oxidation of diclofenac sodium or piroxicam by iron(HI) in the presence of o-phenanthroline. The formation of tris(o-phenanthroline) iron(II) complex (ferroin) upon the reaction of diclofenac sodium or piroxicam with an iron(III)-o-phenanthroline mixture in acetate buffer solution of pH 4.4 and 4.8, respectively, was investigated. The ferroin complex is measured at 510 nm against a reagent blank prepared in the same manner. The optimum experimental parameters for the color production are selected. Beer's law is valid within a concentration range of 1.0-32 mug mL(-1) for diclofenac sodium and 1.0-28 mug mL-1 for piroxicam. For more accurate results, Ringbom optimum concentration ranges are 2.0-30 and 2.0-26 mug mL(-1) for diclofenac sodium and piroxicam, respectively. The molar absorptivities are 1.15 x 10(4) and 1.63 x 10(4) L mol(-1) cm(-1), whereas Sandell sensitivities are 2.78 and 2.03 ng cm(-2) for diclofenac sodium and piroxicam, respectively. The method gave a mean percentage recoveries 99.8 +/- 1.2\% for diclofenac sodium and 100.3 +/- 0.8\% for piroxicam. The developed method is applied for the determination of diclofenac sodium and piroxicam in bulk powder and in their pharmaceutical formulations without any interference from tablet fillers.