A: Transfer an amount of Gel, equivalent to about 5 mg of benzocaine, to a beaker, add 20 mL of 0.5 N hydrochloric acid, warm gently to disperse the Gel, cool, and filter, if necessary, to obtain a clear solution. To 10 mL of the clear solution add 5 drops of a solution of sodium nitrite (1 in 10) and 2 drops of methyl red TS, and neutralize with 1 N sodium hydroxide. Add 2 mL of a solution of 100 mg of 2-naphthol in 5 mL of 1 N sodium hydroxide: an orange-red precipitate is formed.

B: The retention time of the major peak of benzocaine in the chromatogram of the Assay preparation corresponds to that in the chromatogram of the Standard preparation, as obtained in the Assay.

(Official January 1, 2007)

Diluent— Prepare a mixture of methanol and water (1:1).

Mobile phase— Prepare a mixture of methanol, water, and glacial acetic acid (56:40:4). Make adjustments if necessary see System Suitability under Chromatography 621).

Standard preparation— Prepare a solution of USP Benzocaine RS in Diluent having a known concentration of about 0.02 mg per mL.

Assay preparation— Transfer an accurately weighed portion of Gel, equivalent to about 200 mg of benzocaine, to a 100-mL volumetric flask, add 80 mL of Diluent, and shake by mechanical means for 15 minutes. Dilute with Diluent to volume, mix, and filter, if necessary, to obtain a clear solution, discarding the first 5 mL of the filtrate. Transfer 1.0 mL of the clear solution to a second 100-mL volumetric flask, dilute with Diluent to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 294-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— [NOTE—Use peak areas where peak responses are indicated.] Separately inject equal volumes (about 50 µL) of the Assay preparation and the Standard preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of C9H11NO2 in the portion of Gel taken by the formula: in which C is the concentration, in mg per mL, of USP Benzocaine RS in the Standard preparation; and rU and rS are the benzocaine peak responses obtained from the Assay preparation and the Standard preparation, respectively.