(Official January 1, 2007)

Standard preparation— Dissolve a suitable quantity of USP Hydroquinone RS in methanol, and dilute quantitatively and stepwise with methanol to obtain a solution having a known concentration of about 10 µg per mL.

Assay preparation— Transfer an accurately weighed portion of Cream, equivalent to about 20 mg of hydroquinone, to a 100-mL beaker. Triturate the Cream with 50 mL of methanol, and pass the liquid through folded filter paper, previously washed with methanol, into a 500-mL volumetric flask. Repeat the trituration and filtration. Dilute, by washing the contents of the filter paper with methanol through the paper into the volumetric flask, to volume, and mix. Pipet 25 mL of this solution into a 100-mL volumetric flask, add methanol to volume, and mix.

Procedure— Concomitantly determine the absorbances of the Standard preparation and the Assay preparation in 1-cm cells at the wavelength of maximum absorbance at about 293 nm, with a suitable spectrophotometer, using methanol as the blank. Calculate the quantity, in mg, of hydroquinone (C6H6O2) in each g of the Cream taken by the formula: in which C is the concentration, in mg per mL, of USP Hydroquinone RS in the Standard preparation; W is the weight, in g, of Cream taken; and AU and AS are the absorbances of the Assay preparation and the Standard preparation, respectively.