Identification—
B:
Ultraviolet Absorption 
197U
—
Solution:
10 µg per mL.
Medium:
methanol.
Absorptivities at 292 nm, calculated on the dried basis, do not differ by more than 3.0%.
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).
Assay—
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 C
13H
12O
2 in the portion of Monobenzone taken by the formula:
2500C(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.
;)
