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Spectrophotometric determination of isoniazid using 6,7-dichloroquinoline-5,8-dione.

A spectrophotometric procedure for the determination of isoniazid and a number of its pharmaceutical preparations has been developed that offers advantages of simplicity, speed, sensitivity and stability indication over the official BP (1980) method. The proposed method is based on the formation of a green condensation product by the reaction between isoniazid and 6,7-dichloroquinoline-5,8-dione in an alkaline aqueous ethanolic medium. At the maximum absorption of 645 nm, Beer's law is obeyed in the range 2–25 µg ml–1. The structure of the condensation product was confirmed by ultraviolet, infrared and microanalysis data and the mechanism of the reaction is discussed. Congenial drugs, common carbohydrates and excipients used as additives in pharmaceutical formulations did not interfere in the proposed method. The method can be used as a selective stability-indicating assay of isoniazid.