A: Dissolve a quantity of powdered Lozenges, equivalent to about 20 mg of benzocaine, in 10 mL of water with the aid of a few drops of 3 N hydrochloric acid, and filter, if necessary, to obtain a clear solution. Add 5 drops of a solution of sodium nitrite (1 in 10), followed by 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 for benzocaine in the chromatograms of the Assay preparations corresponds to that in the chromatograms of the respective Standard preparations, as obtained in the Assay.

(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed mixture of water, acetonitrile, and 1.0 M monobasic potassium phosphate solution previously adjusted with phosphoric acid to a pH of 3.0 (700:250:50). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation 1— Prepare a solution of USP Benzocaine RS in 0.1 N hydrochloric acid having a known concentration of about 0.01 mg per mL.

Standard preparation 2— Prepare a solution of USP Benzocaine RS in a mixture of acetonitrile and water (1:1) having a known concentration of about 0.01 mg per mL.

Assay preparations— Weigh and finely powder not fewer than 20 Lozenges. Transfer accurately weighed portions of the powder, each equivalent to about 40 mg of benzocaine, to two separate 200-mL volumetric flasks. To one flask add about 150 mL of 0.1 N hydrochloric acid, and stir for not less than 2 hours. Dilute with 0.1 N hydrochloric acid to volume, and mix. Transfer 5.0 mL of this solution to a 100-mL volumetric flask, dilute with 0.1 N hydrochloric acid to volume, and mix (Assay preparation 1). To the second flask add about 150 mL of a mixture of acetonitrile and water (1:1), and stir for not less than 30 minutes. Dilute with the mixture of acetonitrile and water (1:1) to volume, and mix. Transfer 5.0 mL of this solution to a 100-mL volumetric flask, dilute with the mixture of acetonitrile and water (1:1) to volume, and mix (Assay preparation 2).

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 280-nm detector and a 4.6-mm × 25-cm column that contains packing L7. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparations, and record the peak responses as directed for Procedure: the tailing factor is not more than 1.5, and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20 µL) of Assay preparation 1, Assay preparation 2, Standard preparation 1, and Standard preparation 2 into the chromatograph, record the chromatograms, and measure the area responses for the major peaks. Calculate the quantity, in mg, of total C9H11NO2 in the portion of Lozenges taken to prepare Assay preparation 1 by the formula: in which C is the concentration, in mg per mL, of USP Benzocaine RS in Standard preparation 1, and rU and rS are the benzocaine peak areas obtained from Assay preparation 1 and Standard preparation 1, respectively. Calculate the quantity, in mg, of free C9H11NO2 in the portion of Lozenges taken to prepare Assay preparation 2 by the formula: in which C is the concentration, in mg per mL, of USP Benzocaine RS in Standard preparation 2, and rU and rS are the benzocaine peak responses obtained from Assay preparation 2 and Standard preparation 2, respectively.