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: 30minutes.

Mobile phase,Solvent mixture,and Aspirinand codeine phosphate standard preparation— Prepare as directed in the Assay for aspirin and codeine phosphate and limit of free salicylic acid.

Internal standard solution— Dissolve phenacetin in methanol to obtain a solution having a concentration of about 0.07mg per mL.

Standard solution A— Prepare a solution of USP Aspirin RSin Solvent mixturehaving an accurately known concentration of about 0.36mg per mL.

Standard solution B— Transfer about 12mg of USP Codeine Phosphate RSand 25mg of USP Salicylic Acid RS,each accurately weighed,to a 50-mLvolumetric flask,add 2.5mLof methanol,and mix.Add Mediumto volume,and mix.Pipet 10mLof the resulting solution into a 100-mLvolumetric flask,add Mediumto volume,and mix.

Standard preparations Aand B— Pipet 10mLof Standard solution Aand 10mLof Standard solution Binto separate containers,add 3.0mLof the Internal standard solutionto each container,and mix.

Test preparation— Withdraw a portion of the solution under test and filter,discarding the few mLof the filtrate.Pipet 10mLof the filtrate and 3.0mLof the Internal standard solutioninto a suitable container,and mix.

Chromatographic system— Proceed as directed for Chromatographic systemin the Assay for aspirin and codeine phosphate and limit of free salicylic acid,except to use only the Aspirin and codeine phosphate preparationfor evaluation of the suitability of the system.

Procedure— Proceed as directed in the Assay for aspirin and codeine phosphate and limit of free salicylic acid,except to inject about 50µLof the Standard preparationsand the Test preparation.The relative retention times are 0.3for salicylic acid,0.6for aspirin,0.8for codeine phosphate,and 1.0for phenacetin.Calculate the amount of codeine phosphate dissolved by comparison of the relative peak response ratios for the codeine phosphate peaks,obtained from Standard preparation Band the Test preparation.Calculate the percentage of aspirin dissolved by the formula: in which Cis the concentration,in µg per mL,of USP Aspirin RSin Standard solution A;RUand RSare the peak response ratios for the aspirin component obtained from the Test preparationand Standard preparation A,respectively;C¢is the concentration,in µg per mL,of USP Salicylic Acid RSin Standard solution B;R¢Uand R¢Sare the peak response ratios for the salicylic acid component obtained from the Test preparationand Standard preparation B,respectively;and 180.16and 138.12are the molecular weights of aspirin and salicylic acid,respectively.

Tolerances— Not less than 75%(Q)of the labeled amounts of aspirin (C9H8O4)and codeine phosphate hemihydrate (C18H21NO3·H3PO4·½H2O)is dissolved in 30minutes.

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.Pass through a membrane filter,and degas.[NOTE—The amounts of sodium 1-octanesulfonate,methanol,and acetonitrile may be varied to obtain acceptable chromatography.]

Solvent mixture— To 15g of anhydrous citric acid add 200mLof methanol and 20mLof glacial acetic acid,dilute with chloroform to 1000mL,and mix until the citric acid is dissolved.

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 an accurately weighed quantity of USP Salicylic Acid RSin Solvent mixture,and quantitatively dilute with Solvent mixtureto obtain a solution having a known concentration of about 1mg per mL.

Salicylic acid standard preparation— Transfer 5.0mLof Salicylic acid stock standard solutionto a 50-mLvolumetric flask,add 5.0mLof Internal standard solution,dilute with Solvent mixtureto volume,and mix.

Codeine phosphate stock standard solution— Transfer about 325Jmg of USP Codeine Phosphate RS,accurately weighed,to a 25-mLvolumetric flask,Jbeing the ratio of the labeled amount,in mg,of codeine phosphate to the labeled amount,in mg,of aspirin per Tablet.Dissolve in and dilute with Solvent mixtureto volume,and mix.

Aspirinand codeine phosphate standard preparation— Transfer about 65mg of USP Aspirin RS,accurately weighed,to a 10-mLvolumetric flask.Add 5.0mLof Codeine phosphate stock standard solution,1.0mLof Salicylic acid stock standard solution,and 1.0mLof Internal standard solution,dilute with Solvent mixtureto volume,and mix.

Assay preparation— Weigh and finely powder not fewer than 20Tablets.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,mix,and sonicate for 2to 5minutes.Centrifuge,and use a portion of the resultant clear solution as the Assay preparation.Use on the day prepared.

Chromatographic system(see Chromatography á621ñ)— The liquid chromatograph is equipped with a 280-nm detector and a 3.9-mm ×30-cm column that contains 10-µm packing L1.The flow rate is about 2mLper minute.Chromatograph replicate injections of the Salicylic acid standard preparationand the Aspirin and codeine phosphate standard preparation,and record the peak responses as directed for Procedure:the relative retention times for salicylic acid,aspirin,codeine,and phenacetin are about 0.3,0.5,0.8,and 1.0,respectively;the resolution,R,between salicylic acid and aspirin,between aspirin and codeine,and between codeine and phenacetin is not less than 2.0;the tailing factor for each analyte peak is not more than 2.0;and the relative standard deviation of the ratios of the peak responses of salicylic acid,aspirin,and codeine to the peak response of phenacetin is not more than 3.0%.

Procedure— Separately inject equal volumes (about 5µL)of the Salicylic acid standard preparation,Aspirin and codeine phosphate standard preparation,and 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 Aspirin and codeine phosphate standard preparation;and RUand RSare the ratios of the peak responses of aspirin and phenacetin obtained from the Assay preparationand the Aspirin and codeine phosphate standard preparation,respectively.Calculate the quantity,in mg,of codeine phosphate hemihydrate (C18H21NO3·H3PO4·½H2O)in the portion of Tablets taken by the formula: in which 406.37and 397.37are the molecular weights of codeine phosphate hemihydrate and anhydrous codeine phosphate,respectively;Cis the concentration,in mg per mL,of USP Codeine Phosphate RSin the Aspirin and codeine phosphate standard preparation;and RUand RSare the ratios of the peak responses of codeine phosphate and phenacetin obtained from the Assay preparationand the Aspirin and codeine phosphate 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 Salicylic acid standard preparation;ais the quantity,in mg,of aspirin in the portion of powdered 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 Salicylic acid standard preparation,respectively:not more than 3.0%is found.