A: Triturate a quantity of finely powdered Tablets,equivalent to about 500mg of theophylline,with 10-mLand 5-mLportions of solvent hexane,and discard the solvent hexane.Triturate the residue with two 10-mLportions of a mixture of equal volumes of 6Nammonium hydroxide and water,and filter each time.Evaporate the combined filtrates to about 5mL,neutralize,if necessary,with 6Nacetic acid,using litmus,and then cool to about 15,with stirring.Collect the precipitate on a filter,wash it with cold water,and dry at 105for 2hours:the theophylline so obtained melts between 270and 274,the procedure for Class Ibeing used (see Melting Range or Temperature á741ñ).Retain the remaining portion of the theophylline for use in Identificationtest B.

B: The IRabsorption spectrum of a potassium bromide dispersion of the residue obtained in Identificationtest Aexhibits maxima only at the same wavelengths as that of a potassium bromide dispersion of USP Theophylline RS.

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

Medium: water;900mL.

Apparatus 2: 50rpm.

Time: 45minutes.

Procedure— Determine the amount of C7H8N4O2dissolved from UVabsorbances at the wavelength of maximum absorbance at about 272nm of filtered portions of the solution under test,suitably diluted with water,if necessary,in comparison with a Standard solution having a known concentration of USP Theophylline RSin the same medium.

Tolerances— Not less than 80%(Q)of the labeled amount of C7H8N4O2is dissolved in 45minutes.

Mobile phase ,Internal standard solution,and Standard preparation—Prepare as directed in the Assayunder Theophylline.

Assay preparation— Place 10Tablets in a 500-mLvolumetric flask,add 50mLof water,and when the Tablets have disintegrated add 50mLof 6Nammonium hydroxide.Shake until no more dissolves,dilute with water to volume,mix,and filter through a dry filter with the aid of suction,if necessary,into a dry flask,discarding the first 20mLof the filtrate.Transfer an accurately measured aliquot portion of this concentrate,equivalent to about 10mg of theophylline,to a 100-mLvolumetric flask.Add 20.0mLof Internal standard solution,dilute with Mobile phaseto volume,and mix.

Chromatographic system— Proceed as directed in the Assayunder Theophylline.

Procedure— Proceed as directed for Procedurein the Assayunder Theophylline.Calculate the quantity,in mg,of C7H8N4O2per Tablet taken by the formula: in which Cis the concentration,in mg per mL,of USP Theophylline RSin the Standard preparation,Vis the volume,in mL,of concentrate taken for the Assay preparation,and RUand RSare the response ratios of the theophylline peak to the internal standard peak obtained from the Assay preparationand the Standard preparation,respectively.