(Official January 1, 2007)

Standard preparation— Transfer about 10 mg of USP Mebendazole RS, accurately weighed, to a 100-mL volumetric flask, and add 90 mL of chloroform, 7 mL of isopropyl alcohol, and 2 mL of 96 percent formic acid. Agitate until the solid has dissolved, add isopropyl alcohol to volume, and mix. Transfer 5.0 mL of this solution to a second 100-mL volumetric flask, dilute with isopropyl alcohol to volume, and mix to obtain a solution having a known concentration of about 5 µg per mL.

Assay preparation 1— Transfer an accurately measured quantity of Oral Suspension, equivalent to about 1000 mg of mebendazole, to a 100-mL volumetric flask, dilute with 96 percent formic acid to volume, and mix. Transfer 10.0 mL of this mixture to a second 100-mL volumetric flask, add 40 mL of 96 percent formic acid, and heat in a water bath at a temperature of 50 for 15 minutes. Cool, add water to volume, mix, and pass through a medium-porosity, sintered-glass filter. Transfer 10.0 mL of the filtrate to a 250-mL separator, and add 50 mL of water and 50 mL of chloroform. Shake for about 2 minutes, allow the phases to separate, and transfer the chloroform layer to a second 250-mL separator. Wash the aqueous layer with two 10-mL portions of chloroform, add the chloroform washings to the second separator, and discard the aqueous layer. Wash the combined chloroform solutions with a mixture of 4 mL of 1 N hydrochloric acid and 50 mL of a 1 in 10 solution of 96 percent formic acid in water, and transfer the chloroform layer to a 100-mL volumetric flask. Extract the aqueous washing with two 10-mL portions of chloroform, add these chloroform extracts to the chloroform solution in the volumetric flask, add 2 mL of 96 percent formic acid and 7 mL of isopropyl alcohol, dilute with chloroform to volume, and mix. Transfer 5.0 mL of this solution to another 100-mL volumetric flask, dilute with isopropyl alcohol to volume, and mix.

Assay preparation 2 (where the Oral Suspension is packaged in syringes calibrated to deliver stated increments of mebendazole)—Express an increment of Oral Suspension to a volumetric flask of an appropriate nominal volume so that when diluted with 96 percent formic acid to volume a mixture containing about 10 mg of mebendazole per mL is obtained. Transfer 10.0 mL of this mixture to a 100-mL volumetric flask, add 40 mL of 96 percent formic acid, and heat in a water bath at a temperature of 50 for 15 minutes. Proceed as directed for Assay preparation 1 beginning with “Cool, add water to volume.”

Procedure— Mix 90 mL of chloroform with 2 mL of 96 percent formic acid in a 100-mL volumetric flask, add isopropyl alcohol to volume, and mix. Transfer 5.0 mL of this solution to a second 100-mL volumetric flask, dilute with isopropyl alcohol to volume, and mix to obtain a reagent blank. Concomitantly determine the absorbances of the relevant Assay preparation and the Standard preparation at the wavelength of maximum absorbance at about 247 nm with a spectrophotometer, using the reagent blank to set the instrument. Calculate the quantity, in mg, of mebendazole (C16H13N3O3) in the portion of Oral Suspension taken to prepare Assay preparation 1 by the formula: in which C is the concentration, in µg per mL, of USP Mebendazole RS in the Standard preparation; and AU and AS are the absorbances of Assay preparation 1 and the Standard preparation, respectively. Where appropriate, calculate the quantity, in mg, of mebendazole (C16H13N3O3) in the increment of Oral Suspension taken to prepare Assay preparation 2 by the formula: in which V is the volume, in mL, of the volumetric flask into which the increment of Oral Suspension was expressed; AU is the absorbance of Assay preparation 2; and the other terms are as defined above.