A: Transfer a quantity of finely powdered Tablets,equivalent to about 250mg of naproxen sodium,to a centrifuge tube,and add 12mLof water and 1mLof hydrochloric acid:a dense white precipitate is formed.Centrifuge the mixture:the clear,supernatant responds to the identification test for Sodium á191ñ.

B: Prepare a mixture of the Standard preparationand 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: 0.1Mphosphate buffer (pH7.4),prepared by dissolving 2.62g of monobasic sodium phosphate and 11.50g of anhydrous dibasic sodium phosphate in water to make 1000mL;900mL.

Apparatus 2: 50rpm.

Time: 45minutes.

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

Procedure— Dilute a filtered portion of the solution under test quantitatively with Mediumas necessary to obtain a solution having a concentration of about 50µg per mLof C14H13NaO3.Determine the amount of C14H13NaO3dissolved from UVabsorbances at the wavelength of maximum absorbance at about 332nm of this solution in comparison with the Standard preparation.

Tolerances— Not less than 80%(Q)of the labeled amount of C14H13NaO3is dissolved in 45minutes.

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

Standard preparation— Dissolve an accurately weighed quantity of USP Naproxen Sodium RSin Solvent mixtureto obtain a solution having a known concentration of about 2.75mg per mL.Transfer 1.0mLof the resulting solution and 2.0mLof Internal standard solutionto a 100-mLvolumetric flask,dilute with Mobile phaseto volume,and mix.This solution contains about 27.5µg of USP Naproxen Sodium RSper mL.

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

Procedure— Proceed as directed for Procedurein the Assayunder Naproxen Tablets.Calculate the quantity,in mg,of C14H13NaO3in the portion of Tablets taken by the formula: in which Cis the concentration,in µg per mL,of USP Naproxen Sodium RSin the Standard preparation,and RUand RSare the ratios of the response of the naproxen peak to the response of the internal standard peak obtained from the Assay preparationand the Standard preparation,respectively.