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

Assay preparation— Transfer an accurately weighed portion of Cream,equivalent to about 20mg of hydroquinone,to a 100-mLbeaker.Triturate the Cream with 50mLof methanol,and pass the liquid through folded filter paper,previously washed with methanol,into a 500-mLvolumetric 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 25mLof this solution into a 100-mLvolumetric flask,add methanol to volume,and mix.

Procedure— Concomitantly determine the absorbances of the Standard preparationand the Assay preparationin 1-cm cells at the wavelength of maximum absorbance at about 293nm,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 Cis the concentration,in mg per mL,of USP Hydroquinone RSin the Standard preparation;Wis the weight,in g,of Cream taken;and AUand ASare the absorbances of the Assay preparationand the Standard preparation,respectively.