A: The UV absorption spectrum of the Assay Preparation exhibits maxima at the same wavelengths as that of a similar solution of USP Pyrimethamine RS.

B: To a quantity of powdered Tablets, equivalent to about 250 mg of pyrimethamine, add 25 mL of acetone, boil for 2 minutes, and filter through a sintered-glass crucible. Repeat this treatment three times with 25-mL portions of acetone. Evaporate the combined filtrates carefully on a steam bath with the aid of a current of air to dryness: the residue responds to Identification test A under Pyrimethamine, and melts between 237 and 242 (see Melting Range or Temperature 741).

Medium: 0.1 N hydrochloric acid; 900 mL.

Apparatus 2: 50 rpm.

Time: 45 minutes.

Procedure— Determine the amount of C12H13ClN4 dissolved by employing UV absorption at the wavelength of maximum absorbance at about 273 nm on filtered portions of the solution under test, suitably diluted with Dissolution Medium, if necessary, in comparison with a Standard solution having a known concentration of USP Pyrimethamine RS in the same Medium.

Tolerances— Not less than 75% (Q) of the labeled amount of C12H13ClN4 is dissolved in 45 minutes.

Procedure for content uniformity— Transfer 1 Tablet to a 100-mL volumetric flask, add 25 mL of 0.1 N hydrochloric acid, warm the flask on a steam bath for 5 minutes, cool, dilute with 0.1 N hydrochloric acid to volume, mix, and filter, rejecting the first few mL of the filtrate. Pipet a portion of the clear filtrate, equivalent to about 2.5 mg of pyrimethamine, into a 250-mL volumetric flask, dilute with 0.1 N hydrochloric acid to volume, and mix. Dissolve an accurately weighed quantity of USP Pyrimethamine RS in 0.1 N hydrochloric acid, and dilute quantitatively and stepwise with 0.1 N hydrochloric acid to obtain a Standard solution having a known concentration of about 10 µg per mL. Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 273 nm, with a suitable spectrophotometer, using 0.1 N hydrochloric acid as the blank. Calculate the quantity, in mg, of C12H13ClN4 in the Tablet taken by the formula: in which T is the labeled quantity, in mg, of pyrimethamine in the Tablet; D is the concentration, in µg per mL, of pyrimethamine in the solution from the Tablet, on the basis of the labeled quantity per Tablet and the extent of dilution; C is the concentration, in µg per mL, of USP Pyrimethamine RS in the Standard solution; and AU and AS are the absorbances of the solution from the Tablet and the Standard solution, respectively.

(Official January 1, 2007)

50C(AU / AS),