A: Transfer a quantity of finely powdered Tablets, equivalent to about 250 mg of naproxen sodium, to a centrifuge tube, and add 12 mL of water and 1 mL of hydrochloric acid: a dense white precipitate is formed. Centrifuge the mixture: the clear, supernatant responds to the identification test for Sodium 191.

B: Prepare a mixture of the Standard preparation and the Assay preparation (1:1), prepared as directed in the Assay, and chromatograph as directed in the Assay: the chromatogram thus obtained exhibits two main peaks, corresponding to naproxen and the internal standard.

Medium: 0.1 M phosphate buffer (pH 7.4), prepared by dissolving 2.62 g of monobasic sodium phosphate and 11.50 g of anhydrous dibasic sodium phosphate in water to make 1000 mL; 900 mL.

Apparatus 2: 50 rpm.

Time: 45 minutes.

Standard preparation— Dissolve an accurately weighed portion of USP Naproxen Sodium RS in Medium to obtain a solution having a known concentration of about 50 µg per mL.

Procedure— Dilute a filtered portion of the solution under test quantitatively with Medium as necessary to obtain a solution having a concentration of about 50 µg per mL of C14H13NaO3. Determine the amount of C14H13NaO3 dissolved from UV absorbances at the wavelength of maximum absorbance at about 332 nm of this solution in comparison with the Standard preparation.

Tolerances— Not less than 80% (Q) of the labeled amount of C14H13NaO3 is dissolved in 45 minutes.

Mobile phase , Solvent mixture, Internal standard solution, and Chromatographic system—Prepare as directed in the Assay under Naproxen Tablets.

Standard preparation— Dissolve an accurately weighed quantity of USP Naproxen Sodium RS in Solvent mixture to obtain a solution having a known concentration of about 2.75 mg per mL. Transfer 1.0 mL of the resulting solution and 2.0 mL of Internal standard solution to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix. This solution contains about 27.5 µg of USP Naproxen Sodium RS per mL.

Assay preparation— Weigh and finely powder not less than 20 Tablets. Transfer an accurately weighed quantity of the powder, equivalent to about 275 mg of naproxen sodium, to a 100-mL volumetric flask. Add 10 mL of water, and shake until the material is completely dispersed. Dilute with acetonitrile to volume, and mix. Allow any insoluble matter to settle, then transfer 1.0 mL of the clear supernatant to a 100-mL volumetric flask, add 2.0 mL of Internal standard solution, dilute with Mobile phase to volume, and mix.

Procedure— Proceed as directed for Procedure in the Assay under Naproxen Tablets. Calculate the quantity, in mg, of C14H13NaO3 in the portion of Tablets taken by the formula: in which C is the concentration, in µg per mL, of USP Naproxen Sodium RS in the Standard preparation, and RU and RS are the ratios of the response of the naproxen peak to the response of the internal standard peak obtained from the Assay preparation and the Standard preparation, respectively.