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Purpose: To investigate the visible spectrophotometric and thermodynamic parameters of diclofenac with regard to charge-transfer complexation with 2, 3-dichloro-5, 6-dicyano-1, 4-benzoquinone. Methods : The wavelength of maximum absorption, stoichiometry, time-absorbance relationship, effect of pH, Beer’s plot and Beer’s law limit for the complex were evaluated for diclofenac. In order to validate an assay method based on the complexation reaction, the limit of detection and quantitation, precision, accuracy and selectivity were determined. The developed assay method was applied to the assay of diclofenac tablet and capsule brands. Results : A 1:1 complex that absorbed maximally at 506 nm and a Beer’s plot with a regression coefficient of 0.9970 ± 0.0010 were obtained. Beer’s law was obeyed in the concentration range of 0.0017 to 0.2111 mg/ml and precision was within acceptable limits. The limits of detection and of quantitation of the method was 0.000801 and 0.002426 mg/ml, respectively. The spontaneity of the complexation reaction was demonstrated by negative Gibb’s free and positive entropy energy changes. The drug content of commercial diclofenac products to which the assay method was applied was between 94 and 102 %. Conclusion : Charge-transfer complexation of diclofenac with 2, 3-dichloro-5, 6-dicyano-1, 4- benzoquinone is straightforward, and the assay method based on it is simple and robust for the determination of diclofenac in tablet or capsule formulation. Keywords : Spectrophotometry, Charge transfer complex, Diclofenac, Tablets, Capsules, Validation

Visible Spectrophotometric and Thermodynamic Studies of Diclofenac: Charge-Transfer Complexation with 2, 3 Dichloro-5, 6-dicyano, 1, 4-benzoquinone