First order derivative spectroscopy method involves the measurement of absorbance at zero crossing point of other drug. A new, rapid, precise, accurate and sensitive first derivative spectrophotometric method is proposed for the simultaneous estimation of Tizanidine hydrochloride and Mefenamic acid in pharmaceutical dosage form. The drugs obeyed the Beer’s law in the concentration range of 0.5-1.5 ug/ml (r2 = 0.9992) for Tizanidine hydrochloride and 6-19 ug/ml (r2 = 0.9994) for Mefenamic acid. In the proposed method, absorbance was measured at 252.58 nm (Zero Crossing Point of Mefenamic acid) for Tizanidine hydrochloride and 369.84 nm (Zero Crossing Point of Tizanidine hydrochloride) for Mefenamic acid. Accuracy of the method was determined by recovery studies and the average % recovery was found to be 99.63 % and 99.50 % for Tizanidine and Mefenamic acid respectively. The LOD for Tizanidine hydrochloride and Mefenamic acid were found to be 0.018 μg/ml and 0.049 μg/ml and LOQ were found to be 0.056 μg/ml and 0.1509 μg/ml respectively. Statistical analysis proves that the method is selective and sensitive for estimation of Tizanidine hydrochloride and Mefenamic acid in pharmaceutical dosage form. The results were found to be within acceptance criteria according to ICH Q2 R1 guideline.
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