A: Dissolve a quantity of Cream, equivalent to about 50 mg of pramoxine hydrochloride, in a mixture of 25 mL of methanol and 75 mL of ether, and extract with three 25-mL portions of a mixture of equal volumes of 3 N hydrochloric acid and water. Discard the methanol-ether solution, render the combined extracts alkaline with 25 mL of 5 N sodium hydroxide, and extract the pramoxine with 50 mL of chloroform. Evaporate the clear chloroform extract with the aid of a current of air to dryness: the UV absorption spectrum of a 1 in 100,000 solution of the residue so obtained, in 0.1 N hydrochloric acid, exhibits maxima and minima at the same wavelengths as that of a similar solution of the residue similarly obtained from USP Pramoxine Hydrochloride RS, concomitantly measured.

B: To a 5-mg portion of the pramoxine obtained in Identification test A add 1 drop of nitric acid. To the yellow solution cautiously add 5 drops of ammonium hydroxide: a red-brown precipitate is formed.

pH 7.5 phosphate buffer— Dissolve 3.5 g of dibasic potassium phosphate in 100 mL of water, and adjust the solution by the addition of phosphoric acid solution (1:1) to a pH of 7.5 ± 0.1.

Mobile phase— Prepare a suitable degassed and filtered mixture of acetonitrile, water, and pH 7.5 phosphate buffer (22:17:1).

Internal standard solution— Prepare a solution of dibutyl phthalate in methanol having a final concentration of about 4 µL per mL.

Standard preparation— Prepare a solution of USP Pramoxine Hydrochloride RS in methanol having a known concentration of about 2 mg per mL. Pipet 10 mL of this solution and 5 mL of Internal standard solution into a 100-mL volumetric flask, dilute with methanol to volume, mix, and filter.

Assay preparation— Transfer an accurately weighed portion of Cream, equivalent to about 18 mg of pramoxine hydrochloride, to a glass-stoppered, 250-mL conical flask. Add 15.0 mL of isopropyl alcohol and 40.0 mL of methanol, heat on a steam bath, with swirling, to dissolve the Cream, add 40.0 mL of methanol and 5.0 mL of Internal standard solution, and mix. Cool the flask to a temperature of 10 or less to precipitate the waxes, and filter the solution.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 224-nm detector, a 4.6-mm × 3-cm guard column that contains packing L1, and a 4-mm × 30-cm analytical column that contains packing L1. The flow rate is about 2 mL per minute. Chromatograph three replicate injections of the Standard preparation, and record the peak responses as directed for Procedure: the relative standard deviation is not more than 2.0%, and the resolution factor between pramoxine hydrochloride and dibutyl phthalate is not less than 2.4.

Procedure— Separately inject equal volumes (about 20 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 0.8 for pramoxine hydrochloride and 1.0 for dibutyl phthalate. Calculate the quantity, in mg, of pramoxine hydrochloride in the portion of Cream taken by the formula: in which C is the concentration, in mg per mL, of USP Pramoxine Hydrochloride RS in the Standard preparation, and RU and RS are the peak response ratios of pramoxine hydrochloride and internal standard obtained from the Assay preparation and the Standard preparation, respectively.