A: Infrared Absorption 197K.

B: Ultraviolet Absorption 197U—

C: Transfer about 500 mg of Monobenzone, previously dried, to a 150-mL flask fitted with a reflux condenser, employing a suitable glass joint. Add 5 mL of pyridine and 3 mL of acetic anhydride, reflux for 10 minutes, and cool. Add 100 mL of water and 6 mL of acetone to the flask, and insert a stopper. Cool the contents of the flask in a refrigerator for 1 hour, collect the precipitate in a sintered-glass crucible, and wash the precipitate with water until no odor of pyridine remains. Dry the precipitate for 16 hours in a vacuum desiccator over phosphorus pentoxide. The monobenzone acetate so obtained melts between 110 and 113 when determined as directed for Class I (see Melting Range or Temperature 741).

Standard preparation— Dissolve an accurately weighed quantity of USP Monobenzone RS in methanol, and dilute quantitatively, and stepwise if necessary, with methanol to obtain a solution having a known concentration of about 40 µg per mL.

Assay preparation— Transfer about 100 mg of Monobenzone, accurately weighed, to a 100-mL volumetric flask, dissolve in and dilute with methanol to volume, and mix. Pipet 4 mL of this solution into a 100-mL volumetric flask, dilute with methanol to volume, and mix.

Procedure— With a suitable spectrophotometer, using methanol as a blank, concomitantly determine the absorbances of the Standard preparation and the Assay preparation at the wavelength of maximum absorbance at about 292 nm. Calculate the quantity, in mg, of C13H12O2 in the portion of Monobenzone taken by the formula: 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.