New, sensitive, and reliable spectroscopic methods were constructed for the fast determination of the anthelmintic drug mebendazole. The methods depended on the reaction of the amino group in mebendazole with eosin in acidic medium forming an ion pair complex. The first method, Method I, relied on quenching of the native fluorescence of eosin after reaction with mebendazole at pH 3.7 using acetate buffer. Fluorescence quenching was measured at 538 nm after excitation at 518 nm. This method showed a linear response over the concentration range 5.0-20.0 μg/ml. The second method, Method II, was based on measuring the absorbance of the formed complex at 554 nm; the method showed good linearity from 7.0 to 22.0 μg/ml. Different parameters that influenced the formation of the reaction product were carefully investigated to reach the optimized conditions. A comparison between the proposed methods and a previous spectrophotometric method was carried out and there was no significant difference between them. The methods could be applied successfully to determine mebendazole in its tablet form. Moreover, the methods used water as diluting solvent, which made them compatible with the 'green' analytical chemistry principles. No organic solvents were used throughout the study.
Keywords: eosin; mebendazole; quenching; tablets.
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