[Infrared spectrometry determination of polymorphic form C in mebendazole]

Mebendazole displays polymorphism. There are three polymorphic forms (A, B and C) which were identified by means of IR and X-ray diffraction techniques. Procedures have been worked out to determine the amount of the polymorphic form C in pure mebendazole and in tablets. The sample (30 mg) and internal standard (K3Fe(CN)6, 6 mg) were mulled in 0.25 ml of mineral oil and placed between KBr plates separated by a 0.1 mm lead spacer. The spectra were recorded by the absorbance mode between 850-800 cm-1 and 2150-2050 cm-1. Both of the base lines are the horizontal line from 2140 cm-1 peak valley and from 842 cm-1 peak valley. The values of the absorbance difference were measured at 834 and 842 cm-1, 2114 and 2140 cm-1 respectively. The percent content of the polymorphic form C was calculated by the method of internal standard. This analytical approach is simple and the results are satisfactory.