A: The chromatogram of the Assay preparationobtained as directed in the Assay for aspirin and limit of free salicylic acidexhibits a major peak for aspirin,the retention time of which corresponds with that exhibited in the chromatogram of the Standard preparation,obtained as directed in the Assay for aspirin and limit of free salicylic acid.

B: To a 0.7-g portion of finely powdered Tablets add 20mLof 3Nhydrochloric acid and 5drops of methyl red TS,heat to boiling,and add 6Nammonium hydroxide until the color of the solution changes to deep yellow.Continue boiling for 2minutes,and filter:the filtrate so obtained responds to the tests for Magnesium á191ñ.

C: Wash the precipitate obtained in Identificationtest Bwith a hot solution of ammonium chloride (1in 50),and dissolve the precipitate in hydrochloric acid:the solution so obtained responds to the tests for Aluminum á191ñ.

Medium: 0.05Macetate buffer,prepared by mixing 2.99g of sodium acetate (trihydrate)and 1.66mLof glacial acetic acid with water to obtain 1000mLof solution having a pHof 4.50±0.05;900mL.

Apparatus 2: 75rpm.

Time: 45minutes.

Procedure— Determine the amount of aspirin (C9H8O4)dissolved from UVabsorbances at the wavelength of the isosbestic point of aspirin and salicylic acid at 265±2nm of 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 Aspirin RSin the same medium.[NOTE—Prepare the Standard solution at the time of use.An amount of methanol not to exceed 1%of the total volume of the Standard solution may be used to bring the Reference Standard into solution prior to dilution with Medium.]

Tolerances— Not less than 75%(Q)of the labeled amount of C9H8O4is dissolved in 45minutes.

Mobile phase— Dissolve 225mg of tetramethylammonium hydroxide pentahydrate and 200mg of sodium 1-octanesulfonate in 700mLof water.Add 150mLof methanol,150mLof acetonitrile,and 1.0mLof glacial acetic acid,and stir.[NOTE—The composition of the Mobile phase may be adjusted if necessary (see System Suitabilityunder Chromatography á621ñ)].

Solvent mixture— To 2g of anhydrous citric acid add 990mLof acetonitrile,990mLof chloroform,and 20mLof formic acid,and stir for about 30minutes.Allow to settle,and decant the clear solution into a suitable container.Use the clear solution as the Solvent mixture.

Internal standard solution— Dissolve phenacetin in Solvent mixtureto obtain a solution having a concentration of about 2mg per mL.

Salicylic acid stock standard solution— Dissolve a suitable quantity of USP Salicylic Acid RSin Solvent mixtureto obtain a solution having a known concentration of about 1mg per mL.

Standard preparation— Transfer about 325mg of USP Aspirin RS,accurately weighed,to a 50-mLvolumetric flask.Add 10.0mLof Salicylic acid stock standard solutionand 5.0mLof Internal standard solution,dilute with Solvent mixtureto volume,and mix.

Assay preparation— Weigh and finely powder not fewer than 20Tablets.Immediately transfer an accurately weighed portion of the powder,equivalent to about 325mg of aspirin,to a screw-capped,120-mLbottle,add 5.0mLof Internal standard solutionand 45.0mLof Solvent mixture,cap the bottle,mix,and sonicate for 2to 5minutes.Centrifuge,and use a portion of the resultant clear solution as the Assay preparation.

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a 280-nm detector and a 4-mm ×30-cm column that contains 10-µm packing L1.The flow rate is about 2mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the relative retention times are about 0.3for salicylic acid,0.6for aspirin,and 1.0for phenacetin.[NOTE—Record each chromatogram until the chloroform peak appears at a relative retention time of about 1.8.];the resolution,R,between the salicylic acid,aspirin,and internal standard peaks is not less than 2.0;the tailing factor for any of these peaks is not more than 2.0;and the relative standard deviation for replicate injections is not more than 3.0%.

Procedure— Separately inject equal volumes (about 5µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of aspirin (C9H8O4)in the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Aspirin RSin the Standard preparation;and RUand RSare the ratios of the peak responses of aspirin and phenacetin obtained from the Assay preparationand the Standard preparation,respectively.Calculate the percentage of free salicylic acid in the Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Salicylic Acid RSin the Standard preparation;ais the quantity,in mg,of aspirin in the portion of Tablets taken,based on the labeled amount;and RUand RSare the ratios of the peak responses of salicylic acid and phenacetin obtained from the Assay preparationand the Standard preparation,respectively:not more than 3.0%is found.

Edetate disodium titrant— Prepare and standardize as directed in the Assayunder Ammonium Alum.

Assay preparation— Weigh and finely powder not fewer than 20Tablets.Transfer an accurately weighed portion of the powder,equivalent to about 250mg of aluminum hydroxide,to a 150-mLbeaker,add 20mLof water,stir,and slowly add 30mLof 3Nhydrochloric acid.Heat gently,if necessary,to aid solution,cool,and transfer to a 200-mLvolumetric flask.Wash the beaker with water,adding the washings to the flask,add water to volume,and mix.

Procedure— Pipet 50mLof Assay preparationinto a 250-mLbeaker,then add,in the order named and with continuous stirring,25.0mLof 0.05MEdetate disodium titrantand 20mLof acetic acid-ammonium acetate buffer TS,and heat the solution near the boiling temperature for 5minutes.Cool,add 50mLof alcohol and 2mLof dithizone TS,and mix.Titrate with 0.05Mzinc sulfate VSuntil the color changes from green-violet to rose-pink.Perform a blank determination,substituting 50mLof water for the Assay preparation,and make any necessary corrections.Each mLof 0.05MEdetate disodium titrantconsumed is equivalent to 3.900mg of Al(OH)3.

Assay preparation— Prepare as directed in the Assay for aluminum hydroxide.

Procedure— Pipet a volume of Assay preparation,equivalent to about 80mg of magnesium hydroxide,into a 400-mLbeaker,add 200mLof water and 20mLof triethanolamine,and mix.Add 50mLof ammonia-ammonium chloride buffer TSand 2drops of eriochrome black indicator solution (prepared by dissolving 200mg of eriochrome black Tin a mixture of 15mLof triethanolamine and 5mLof dehydrated alcohol,and mixing).Cool the solution to between 3and 4by immersion of the beaker in an ice bath,then remove,and titrate with 0.05Medetate disodium VSuntil the color changes to pure blue.Perform a blank determination,substituting for the Assay preparation,a volume of water equal to the volume of Assay preparationused,and make any necessary corrections.Each mLof 0.05Medetate disodium is equivalent to 2.916mg of Mg(OH)2.