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

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

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 Suitabilityunder Chromatography á621ñ).

Standard preparation— Prepare a solution of USP Benzocaine RSin Diluenthaving a known concentration of about 0.02mg per mL.

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

Chromatographic system (see Chromatography á621ñ)—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 1.5mLper 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 50µL)of the Assay preparationand the Standard preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of C9H11NO2in the portion of Gel taken by the formula: in which Cis the concentration,in mg per mL,of USP Benzocaine RSin the Standard preparation;and rUand rSare the benzocaine peak responses obtained from the Assay preparationand the Standard preparation,respectively.