A: The UVabsorption spectrum of the Assay Preparationexhibits 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 250mg of pyrimethamine,add 25mLof acetone,boil for 2minutes,and filter through a sintered-glass crucible.Repeat this treatment three times with 25-mLportions 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 Identificationtest Aunder Pyrimethamine,and melts between 237and 242(see Melting Range or Temperature á741ñ).

Medium: 0.01Nhydrochloric acid;900mL.

Apparatus 2: 50rpm.

Time: 45minutes.

Procedure— Determine the amount of C12H13ClN4dissolved by employing UVabsorption at the wavelength of maximum absorbance at about 273nm 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 RSin the same Medium.

Tolerances— Not less than 75%(Q)of the labeled amount of C12H13ClN4is dissolved in 45minutes.

Procedure for content uniformity— Transfer 1Tablet to a 100-mLvolumetric flask,add 25mLof 0.1Nhydrochloric acid,warm the flask on a steam bath for 5minutes,cool,dilute with 0.1Nhydrochloric acid to volume,mix,and filter,rejecting the first few mLof the filtrate.Pipet a portion of the clear filtrate,equivalent to about 2.5mg of pyrimethamine,into a 250-mLvolumetric flask,dilute with 0.1Nhydrochloric acid to volume,and mix.Dissolve an accurately weighed quantity of USP Pyrimethamine RSin 0.1Nhydrochloric acid,and dilute quantitatively and stepwise with 0.1Nhydrochloric acid to obtain a Standard solution having a known concentration of about 10µg per mL.Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 273nm,with a suitable spectrophotometer,using 0.1Nhydrochloric acid as the blank.Calculate the quantity,in mg,of C12H13ClN4in the Tablet taken by the formula: in which Tis the labeled quantity,in mg,of pyrimethamine in the Tablet,Dis 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,Cis the concentration,in µg per mL,of USP Pyrimethamine RSin the Standard solution,and AUand ASare the absorbances of the solution from the Tablet and the Standard solution,respectively.

50C(AU/AS),