Spectrophotometric determination of diclofenac in pharmaceutical preparations

A modified. procedure for the visible spectrophotometric determination of diclofenac, in pharmaceutical preparations using as reagent an aqueous solution of copper (H), is proposed. A green color complex is formed between copper(II) and diclofenac with a maximum light absorption at 680 nm. The influences of pH, of copper(II) concentration, of the use of acetic acid in copper(II) solution, of the ionic strength and of the number of extractions with chloroform were studied. The optimal conditions were found to be 5.3 (pH of the solution to be extracted), 50.0 mg mL(-1) (copper(II) acetate in 0.01 mol L-1 acetic acid solution) and three extractions with chloroform using a total volume of 5.0 mL. It was also observed that the addition of a salt in order to increase the ionic strength does not introduce important gain in complex extraction. The method was applied to commercial pharmaceutical preparations. The results were compared with those obtained with the method recommended by the Pharmacopoeia (USP) using the statistical Student's t-test procedure. Complete agreement was found between the results obtained with the two methods. The intrinsic RSD of the proposed method was about 2.3% for sodium diclofenac and 2.7% for potassium diclofenac. When applied to tablets or liquid preparations the RSD is necessarily affected by the deviation in the masses of the individual doses. The linear correlation coefficient, R, was 0.9984 for sodium diclofenac salt and 0.9993 for potassium diclofenac salt. The linear range goes from 1.0 to 25.0 mg mL(-1) in the working solution. The detection limit is 0.2 mg mL(-1) and the determination limit is 0.7 mg mL(-1).