Selective and sensitive assay of mebendazole in pharmaceuticals using bromocresol green by spectrophotometry

Three new, simple, rapid and sensitive spectrophotometric methods have been developed for the assay of mebendazole (MBD) in bulk drug, tablets and suspension. First method (method A) is based on the formation of a colored ion-pair complex (1:1 drug/dye) of MBD with bromocresol green (BCG) at pH 1.99 ± 0.01 and extraction of complex into chloroform followed by measurement of yellow ion-pair complex at 430 nm. In second and third methods, drug-dye ion-pair was dissolved either in ethanolic sulphuric acid and resulting acid form of dye was measured at 440 nm (method B) or in ethanolic potassium hydroxide and resulting base form of dye was measured at 600 nm (method C). Under optimized conditions, Beerûs law was obeyed over 1.0-20.0, 0.5-10 and 0.2-8.0 μg mL-1 for method A, method B and method C, respectively, and corresponding molar absorptivity values are 1.55 x 104, 2.95 x 104 and 3.64 x 104 L mol-1 cm-1. Sandell sensitivity, limits of detection and quantification values are also reported for all three methods. Molar ratio of formed ion-pair complex was found to be 1:1 as deduced by Jobûs method for method A, and calculated stability constant was also reported. Over linear ranges applicable, accuracy and precision of methods were evaluated on intra-day and inter-day basis. Application of proposed methods to bulk powder, commercial pharmaceutical tablet, suspension and spiked human urine was presented.