Medium: water; 900 mL.

Apparatus 1: 50 rpm.

Time: 30 minutes.

Procedure— Determine the amount of C19H21NO dissolved from UV absorbances at the wavelength of maximum absorbance at about 292 nm on filtered portions of the solution under test, suitably diluted with Medium, if necessary, in comparison with a Standard solution having a known concentration of USP Doxepin Hydrochloride RS in the same Medium.

Tolerances— Not less than 80% (Q) of the labeled amount of C19H21NO is dissolved in 30 minutes.

Diluting solution— Prepare a mixture containing 500 mL of methanol and 500 mL of 0.05 M monobasic sodium phosphate, and filter. Adjust with 2 N sodium hydroxide to a pH of 6.7.

Standard preparation— Transfer 20 mg of USP Doxepin Hydrochloride RS, accurately weighed, to a 200-mL volumetric flask, dissolve in and dilute with Diluting solution to volume, and filter.

Test preparation— Transfer the contents of 1 Capsule into an appropriate volumetric flask, add Diluting solution to about 80% of the volume of the flask, and shake the flask by mechanical means for about 30 minutes. Dilute with Diluting solution to volume. Make further dilutions, if necessary, to obtain a solution having a known concentration of 0.1 mg per mL of doxepin hydrochloride. Individually test 9 more Capsules using the above procedure.

Procedure— Determine the amount of active ingredient in each unit of the Test preparation from UV absorbances at the wavelength of maximum absorbance at about 292 nm using 0.5-cm cells in comparison with the Standard preparation.

(Official January 1, 2007)

Mobile phase, Standard preparation, and Chromatographic system— Proceed as directed in the Assay under Doxepin Hydrochloride.

Assay preparation— Remove, as completely as possible, the contents of not fewer than 20 Capsules. Weigh the contents and determine the average weight per Capsule. Mix the combined contents, and transfer an accurately weighed quantity of the powder, equivalent to about 50 mg of doxepin hydrochloride, to a 100-mL volumetric flask. Add about 70 mL of Mobile phase, and shake by mechanical means for 30 minutes. Dilute with Mobile phase to volume, mix, and filter. Pipet 10.0 mL of this solution into a 50-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Procedure— Proceed as directed for Procedure in the Assay under Doxepin Hydrochloride. Calculate the quantity, in mg, of C19H21NO·HCl in the portion of Capsules taken by the formula: in which the terms are as defined therein.