A: Transfer 5 mL to a separator containing 25 mL of water, and extract the solution with two 25-mL portions of a mixture of equal volumes of ether and solvent hexane. Combine the extracts, and retain the water solution for Identification test B. Extract the ether-hexane solution with 50 mL of water, and discard the water layer. Evaporate the ether-hexane solution to dryness, dry the residue in vacuum at 40 to 50 for 1 hour, and dissolve the residue in 1 mL of chloroform: the IR absorption spectrum of this solution exhibits maxima at the same wavelengths as that of a similar solution of USP Benzocaine RS, concomitantly measured.

B: Add 5 mL of 1 N sodium hydroxide solution to the water solution retained from Identification test A, and extract with two 25-mL portions of chloroform. Evaporate the combined extracts to dryness, dry the residue in vacuum at 40 to 50 for 1 hour, and dissolve the residue in 3 mL of chloroform: the IR absorption spectrum of this solution exhibits maxima at the same wavelengths as that of a similar solution of USP Antipyrine RS, concomitantly measured.

C: The retention times of the major peaks in the chromatogram of the Assay preparation correspond to those in the chromatogram of the Standard preparation, as obtained in the Assay.

(Official January 1, 2007)

Ammonium acetate solution— Dissolve 7.7 g of ammonium acetate in water, dilute with water to 1000 mL, and mix.

Mobile phase— Prepare a filtered and degassed mixture of Ammonium acetate solution and acetonitrile (3:1). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Transfer about 15 mg of USP Benzocaine RS, accurately weighed, to a 100-mL volumetric flask, add 15J mg of USP Antipyrine RS, accurately weighed, J being the ratio of the labeled amount, in mg, of antipyrine to the labeled amount, in mg, of benzocaine per mL of Otic Solution. Add 50 mL of methanol, dissolve in methanol, dilute with methanol to volume, and mix. Transfer 10.0 mL of this solution to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Assay preparation— Transfer an accurately measured volume of Otic Solution, equivalent to about 15 mg of benzocaine, to a 100-mL volumetric flask. Dissolve in 50 mL of methanol, dilute with methanol to volume, and mix. Transfer 10.0 mL of this solution to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 280-nm detector and a 4-mm × 15-cm column that contains 5-µm packing L15. The flow rate is about 1.0 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the tailing factor for the benzocaine peak is not more than 2.5, the column efficiency for the benzocaine peaks is not less than 1500 theoretical plates, and the relative standard deviation for replicate injections is not more than 2%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. The relative retention times are about 0.35 for antipyrine, and 1.0 for benzocaine. Calculate the quantity, in mg, of benzocaine (C9H11NO2) in each mL of the Otic Solution taken by the formula: in which C is the concentration, in mg per mL, of USP Benzocaine RS in the Standard preparation; V is the volume, in mL, of Otic Solution taken; and rU and rS are the peak responses due to benzocaine in the Assay preparation and the Standard preparation, respectively. Calculate the quantity, in mg, of antipyrine (C11H12N2O) in each mL of the Otic Solution taken by the same formula, changing the terms to refer to antipyrine instead of benzocaine.