A: Digest a quantity of powdered Tablets,equivalent to about 100mg of chlorthalidone,with 10mLof acetone on a steam bath for about 5minutes.Filter the solution into a 50-mLbeaker,add 20mLof water,and boil on the steam bath for about 5minutes,passing a gentle current of air above the solution to remove the acetone.Cool in an ice bath,filter,and dry the crystals at 105for 4hours:the crystals so obtained respond to Identificationtest Aunder Chlorthalidone.

B: The retention time of the major peak in the chromatogram of the Assay preparationcorresponds to that of the Standard preparation,both relative to the internal standard,as obtained in the Assay.

Medium: water;900mL.

Apparatus 2: 75rpm.

Time: 60minutes.

Standard preparation— Dissolve an accurately weighed quantity of USP Chlorthalidone RSin methanol to obtain a solution having a known concentration of about 5mg per mL.

Procedure— Determine the amount of C14H11ClN2O4Sdissolved from UVabsorbances at the wavelength of maximum absorbance at about 275nm of filtered portions of the solution under test,suitably diluted with water,in comparison with a quantitative dilution in water of the Standard preparationhaving a known concentration of USP Chlorthalidone RScomparable to the concentration of the solution under test.

Tolerances— Not less than 70%(Q)of the labeled amount of C14H11ClN2O4Sis dissolved in 60minutes.

Mobile phase and Internal standard solution—Prepare as directed in the Assayunder Chlorthalidone.

Standard preparation— Prepare as directed in the Assayunder Chlorthalidone,except to substitute 10.0mLof methanol for the CCAsolution.

Assay preparation— Weigh and finely powder not less than 20Tablets.Transfer an accurately weighed portion of the powder,equivalent to about 100mg of Chlorthalidone,to a 100-mLvolumetric flask.Dissolve in about 50mLof methanol,shake for 30minutes,dilute with methanol to volume,and mix.Transfer about 30mLof this solution to a 50-mLcentrifuge tube,and centrifuge for 10minutes.Pipet 5mLof the clear supernatant into a 50-mLvolumetric flask containing 5.0mLof Internal standard solutionand 10.0mLof methanol.Dilute with water to volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm ×25-cm column that contains packing L7.The flow rate is about 1.0mLper minute.Chromatograph five 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%,and the resolution factor between chlorthalidone and the internal standard is not less than 1.5.The tailing factors for the chlorthalidone and internal standard peaks are not more than 2.0.

Procedure— Separately inject equal volumes (about 25µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.The relative retention times are about 0.8for chlorthalidone and 1.0for the internal standard.Calculate the quantity,in mg,of C14H11ClN2O4Sin the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Chlorthalidone RSin the Standard preparation,and RUand RSare the peak response ratios of chlorthalidone and the internal standard obtained from the Assay preparationand the Standard preparation,respectively.