A: Triturate a quantity of finely powdered Tablets, equivalent to about 500 mg of theophylline, with 10-mL and 5-mL portions of solvent hexane, and discard the solvent hexane. Triturate the residue with two 10-mL portions of a mixture of equal volumes of 6 N ammonium hydroxide and water, and filter each time. Evaporate the combined filtrates to about 5 mL, neutralize, if necessary, with 6 N acetic 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 105 for 2 hours: the theophylline so obtained melts between 270 and 274, the procedure for Class I being used (see Melting Range or Temperature 741). Retain the remaining portion of the theophylline for use in Identification test B.

B: The IR absorption spectrum of a potassium bromide dispersion of the residue obtained in Identification test A exhibits 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 preparation corresponds to that in the chromatogram of the Standard preparation, both relative to the internal standard, as obtained in the Assay.

Medium: water; 900 mL.

Apparatus 2: 50 rpm.

Time: 45 minutes.

Procedure— Determine the amount of C7H8N4O2 dissolved from UV absorbances at the wavelength of maximum absorbance at about 272 nm 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 RS in the same medium.

Tolerances— Not less than 80% (Q) of the labeled amount of C7H8N4O2 is dissolved in 45 minutes.

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

Assay preparation— Place 10 Tablets in a 500-mL volumetric flask, add 50 mL of water, and when the Tablets have disintegrated add 50 mL of 6 N ammonium 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 20 mL of the filtrate. Transfer an accurately measured aliquot portion of this concentrate, equivalent to about 10 mg of theophylline, to a 100-mL volumetric flask. Add 20.0 mL of Internal standard solution, dilute with Mobile phase to volume, and mix.

Chromatographic system— Proceed as directed in the Assay under Theophylline.

Procedure— Proceed as directed for Procedure in the Assay under Theophylline. Calculate the quantity, in mg, of C7H8N4O2 per Tablet taken by the formula: in which C is the concentration, in mg per mL, of USP Theophylline RS in the Standard preparation, V is the volume, in mL, of concentrate taken for the Assay preparation, and RU and RS are the response ratios of the theophylline peak to the internal standard peak obtained from the Assay preparation and the Standard preparation, respectively.