Medium: water;900mL.

Apparatus 2: 50rpm.

Time: 45minutes.

Standard preparation— Dissolve an accurately weighed quantity of USP Thioguanine RSin 1Nsodium hydroxide to obtain a solution having a known concentration of about 4.5µg per mL.

Procedure— Determine the amount of C5H5N5Sdissolved from UVabsorbances at the wavelength of maximum absorbance at about 348nm of filtered portions of the solution under test,suitably diluted with 0.1Nhydrochloric acid,in comparison with the Standard preparation.

Tolerances— Not less than 75%(Q)of the labeled amount of C5H5N5Sis dissolved in 45minutes.

Standard preparations— Dissolve an accurately weighed quantity of USP Thioguanine RSin sodium hydroxide solution (1in 250),and dilute quantitatively and stepwise with the sodium hydroxide solution to obtain a Standard preparationhaving a known concentration of 80µg per mL.Transfer 5.0mLof this solution to a 100-mLvolumetric flask,add dilute hydrochloric acid (1in 10)to volume,and mix,to obtain the Acidic standard preparation.Transfer another 5.0-mLportion of the solution to a second 100-mLvolumetric flask,add 10.0mLof 1Nsodium hydroxide,dilute with water to volume,and mix,to obtain the Basic standard preparation.

Assay preparations— Weigh and finely powder not less than 20Tablets.Transfer an accurately weighed portion of the powder,equivalent to about 40mg of thioguanine,to a 500-mLvolumetric flask,add 50mLof 1Nsodium hydroxide,and allow to stand for 10minutes,with frequent swirling.Dilute with water to volume,mix,and filter a portion of the solution through a pledget of glass wool.Transfer 5.0mLof the filtrate to a 100-mLvolumetric flask,add dilute hydrochloric acid (1in 10)to volume,and mix,to obtain the Acidic assay preparation.Transfer another 5.0-mLportion of the filtrate to a second 100-mLvolumetric flask,add 10.0mLof 1Nsodium hydroxide,dilute with water to volume,and mix,to obtain the Basic assay preparation.

Procedure— Concomitantly determine the absorbances of the Acidic assay preparationand the Acidic standard preparationin 1-cm cells at the wavelength of maximum absorbance at about 348nm,with a suitable spectrophotometer,using the Basic assay preparationand the Basic standard preparation,respectively,as the blanks.Calculate the quantity,in mg,of C5H5N5Sin the portion of Tablets taken by the formula: in which Cis the concentration,in µg per mL,of USP Thioguanine RSin the Acidic standard preparation,and AUand ASare the absorbances of the Acidic assay preparationand the Acidic standard preparation,respectively.