Internal standard solution, Mobile phase, System suitability preparation, Solvent blank solution, and Chromatographic system— Proceed as directed in the Assay under Anthralin.

Standard preparation— Dissolve an accurately weighed quantity of USP Anthralin RS in dichloromethane to obtain a solution having a known concentration of about 0.25 mg per mL. Pipet 2 mL of this solution and 2 mL of Internal standard solution into a 25-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Assay preparation— Accurately weigh approximately 5 g of Cream into a tared 100-mL beaker. Add about 20 mL of dichloromethane and 10 mL of glacial acetic acid, and stir to disperse the Cream. Transfer the contents of the beaker to a filter paper (Whatman No. 4, or equivalent) with the aid of dichloromethane, and filter into a 100-mL volumetric flask. Thoroughly wash the precipitate with dichloromethane, and allow the washings to drain into the flask. Dilute with dichloromethane to volume, and mix. Pipet a volume of this solution, equivalent to about 0.5 mg of anthralin, and 2 mL of Internal standard solution into a 25-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Procedure— Proceed as directed in the Assay under Anthralin. Calculate the quantity, in mg, of anthralin (C14H10O3) in the portion of Cream taken by the formula: in which C is the concentration, in mg per mL, of USP Anthralin RS in the Standard preparation; V is the volume, in mL, of the filtrate taken for the Assay preparation;, and RU and RS are the response ratios of the anthralin peak to the o-nitroaniline peak obtained from the Assay preparation and the Standard preparation, respectively.