Spectrophotometric determination of ethionamide in pharmaceuticals using Folin-Ciocalteu reagent and iron(III)-ferricyanide as chromogenic agents

Two simple and sensitive spectrophotometric methods are described for the determination of ethionamide (ETM) in pure drug and tablets. The first method is based on the reduction of Folin-Ciocalteu (F-C) reagent by ETM in sodium carbonate medium to form a blue coloured complex, which was measured at 760 nm (Molybdenum-tungsten blue method). In the second method (Prussian blue method), iron(III) was reduced to iron(II) by ETM in HCl medium, in which iron(II) was complexed with ferricyanide, and the resulting Prussian blue was also measured at 760 nm. The absorbance measured in each case was related to the ETM concentration. The experimental conditions were carefully studied and optimised. Beer's law was obeyed in concentration ranges of 1-40 μg/ml and 0.2-4.0 μg/ml with the Molybdenum-tungsten blue method and the Prussian blue method, respectively, with corresponding molar absorptivity values of 5.72 × 103 and 3.18 × 104 l/(mol·cm). The limits of detection (LOD) and quantification (LOQ) were 0.09 and 0.27 μg/ml for the Molybdenum-tungsten blue method and 0.01 and 0.04 μg/ml for the Prussian blue method. Within-day and between-day relative standard deviations (%RSD) at three different concentration levels were <3%, and the respective relative errors (%RE) were ≤2%, implying good accuracy and precision of the methods. The proposed methods were successfully applied to the determination of ETM in bulk powder and tablets, and the results demonstrated that the methods were as accurate and precise as the official method.