Medium: pH 4.5 acetate buffer, prepared as directed in the Dissolution test under Propoxyphene Hydrochloride, Aspirin, and Caffeine Capsules; 500 mL.

Apparatus 1: 100 rpm.

Time: 60 minutes.

Diethylamine phosphate buffer, Mobile phase, and Chromatographic system— Proceed as directed in the Assay.

Standard solution— Dissolve an accurately weighed quantity of USP Propoxyphene Napsylate RS in Dissolution Medium to obtain a solution having a known concentration of about 0.1 mg per mL.

Test solution— Quantitatively dilute a filtered portion of the solution under test with Dissolution Medium to obtain a solution having a concentration of about 0.1 mg of propoxyphene napsylate per mL.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the areas for the propoxyphene peaks. Calculate the quantity, in mg, of C22H29NO2·C10H8O3S·H2O dissolved by the formula: in which 565.72 and 547.72 are the molecular weights of propoxyphene napsylate and anhydrous propoxyphene napsylate, respectively; C is the concentration, in mg per mL, of USP Propoxyphene Napsylate RS in the Standard solution; D is the dilution factor used to prepare the Test solution; and rU and rS are the peak responses obtained from the Test solution and the Standard solution, respectively.

Tolerances— Not less than 75% (Q) of the labeled amount of C22H29NO2·C10H8O3S·H2O is dissolved in 60 minutes.

Diethylamine phosphate buffer— Mix 5.0 mL of diethylamine with 995 mL of water, and adjust with phosphoric acid to a pH of 3.2.

Diluent— Prepare a mixture of Diethylamine phosphate buffer and acetonitrile (4:1).

Mobile phase— Prepare a mixture of Diethylamine phosphate buffer and acetonitrile (3:2). Sonicate for 15 minutes, and pass through a filter having a 0.5-µm or finer porosity. Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Propoxyphene Napsylate RS in Diluent to obtain a solution having a known concentration of about 0.1 mg per mL.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 5 Tablets, to a 1000-mL volumetric flask. Dissolve in and dilute with Diluent to volume, and mix. Further dilute 10.0 mL of the resulting solution with Diluent, mix, and filter or centrifuge to obtain a clear solution having a concentration of about 0.1 mg per mL.

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 217-nm detector and a 3.9-mm × 30-cm column containing packing L1. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the 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 the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the areas for the propoxyphene peaks. Calculate the quantity, in mg, of propoxyphene napsylate (C22H29NO2·C10H8O3S·H2O) in the portion of the Tablets taken by the formula: in which 565.72 and 547.72 are the molecular weights of propoxyphene napsylate monohydrate and anhydrous propoxyphene napsylate, respectively; C is the concentration, in mg per mL, of USP Propoxyphene Napsylate RS in the Standard preparation; D is the dilution factor used to prepare the Assay preparation; and rU and rS are the responses obtained from the Assay preparation and the Standard preparation, respectively.