Medium: 0.1 N hydrochloric acid; 900 mL.

Apparatus 2: 50 rpm.

Time: 60 minutes.

Mobile phase— Prepare a suitable degassed solution of methanol and 0.05 M monobasic sodium phosphate (6:4).

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 232-nm detector and a 4.6-mm × 25-cm column that contains packing L9. The flow rate is about 1.9 mL per minute. Chromatograph replicate injections of a Standard solution, and record the peak responses as directed under Procedure: the relative standard deviation is not more than 2.0%.

Procedure— Inject alternately 50 µL of a filtered portion of the solution under test and a Standard solution, having a known concentration of USP Hydroxyzine Hydrochloride RS in the same medium, into the chromatograph, record the chromatogram, and measure the response for the major peak. Determine the amount of C21H27ClN2O2·2HCl dissolved from the peak response obtained in comparison with the peak response obtained from the Standard solution.

Tolerances— Not less than 75% (Q) of the labeled amount of C21H27ClN2O2·2HCl is dissolved in 60 minutes.

Mobile phase— Dissolve 7.0 g of monobasic sodium phosphate in 1000 mL of water, and adjust with phosphoric acid to a pH of 4.4. Pass the solution through a 5-µm porosity polytef membrane filter. Mix 900 mL of the filtrate with 900 mL of methanol, and degas by stirring under vacuum prior to use.

Standard preparation— Dissolve an accurately weighed quantity of USP Hydroxyzine Pamoate RS in methanol to obtain a solution having a known concentration of about 0.18 mg per mL (equivalent to about 0.1 mg of hydroxyzine hydrochloride per mL).

Assay preparation— Transfer, as completely as possible, the contents of not less than 20 Capsules to a tared beaker, and determine the average weight per capsule. Mix the combined contents, and transfer an accurately weighed portion, equivalent to about 25 mg of hydroxyzine hydrochloride, to a 250-mL volumetric flask. Add 200 mL of methanol to the flask, sonicate for 5 minutes, shake by mechanical means for 30 minutes, and sonicate for 2 minutes. Dilute with methanol to volume, and mix. Filter the solution through a 5-µm porosity polytef membrane filter equipped with a glass fiber prefilter.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 232-nm detector and a 4.6-mm × 25-cm column that contains packing L9. The flow rate is about 2.5 mL per minute. Chromatograph four replicate injections of the Standard preparation, and record the peak responses as directed under Procedure: the relative standard deviation is not more than 2.0%.

Procedure— Separately inject equal volumes (about 25 µL) of the Standard preparation and the Assay preparation into the chromatograph by means of a suitable microsyringe or sampling valve, record the chromatograms, and measure the responses for the major peaks. The retention time is about 6 minutes for hydroxyzine. Calculate the equivalent quantity, in mg, of hydroxyzine hydrochloride (C21H27ClN2O2·2HCl) in the portion of Capsules taken by the formula: in which 447.83 and 763.27 are the molecular weights of hydroxyzine hydrochloride and hydroxyzine pamoate, respectively; C is the concentration, in mg per mL, of USP Hydroxyzine Pamoate RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.