» Monobenzone Cream contains not less than 94.0 percent and not more than 106.0 percent of the labeled amount of monobenzone (C13H12O2).
Packaging and storage Preserve in tight containers, and avoid exposure to temperatures higher than 30.
Identification Transfer a quantity of Cream, equivalent to about 500 mg of monobenzone, to a centrifuge bottle, add 100 mL of water, and shake until the cream is completely dispersed. Centrifuge the suspension, decant the supernatant, wash the residue with water, again centrifuge, and decant the water. Transfer the residue to a separator with the aid of water, and adjust the volume to about 100 mL. Extract with four 25-mL portions of chloroform, filtering the extracts through a pledget of cotton into a 150-mL flask. Evaporate the chloroform in a current of warm air, and add 5 mL of pyridine and 3 mL of acetic anhydride to the dry residue. Connect the flask to a reflux condenser, reflux for 10 minutes, cool, and proceed as directed in Identification test C under Monobenzone, beginning with Add 100 mL of water. It meets the requirements of Identification test C under Monobenzone.
Standard preparation Prepare as directed in the Assay under Monobenzone.
Assay preparation Transfer an accurately weighed portion of Cream, equivalent to about 200 mg of monobenzone, to a suitable container, add 100 mL of methanol, and shake for about 30 minutes. Transfer the mixture to a 200-mL volumetric flask. Rinse the container with two 25-mL portions of methanol, and add the rinsings to the 200-mL volumetric flask. Dilute with methanol to volume, mix, and filter, discarding the first 20 mL of the filtrate. Pipet 4 mL of this solution into a 100-mL volumetric flask, dilute with methanol to volume, and mix.
Procedure Proceed as directed in the Assay under Monobenzone, but use the Assay preparation under Monobenzone Cream. Calculate the quantity, in mg, of monobenzone (C13H12O2) in the portion of Cream taken by the formula:
5000C(AU / AS)in which C is the concentration, in mg per mL, of USP Monobenzone RS in the Standard preparation and AU and AS are the absorbances obtained from the Assay preparation and the Standard preparation, respectively.
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USP32NF27 Page 3004