Packaging and storage—
Preserve in well-closed containers.
Identification—
A:
Ultraviolet Absorption 
197U
—
Solution:
5 µg per mL.
Medium:
0.1 N hydrochloric acid.
Absorptivities at 325 nm, calculated on the anhydrous basis, do not differ by more than 3.0%. The ratio A255/A325 does not exceed 0.06.
B:
To a solution of 600 mg in 6 mL of sodium hydroxide solution (1 in 33) add slowly, with shaking, 0.5 mL of methyl iodide. Allow the mixture to stand for 2 hours at room temperature, cool in an ice bath, and adjust with acetic acid to a pH of about 5. Collect the crystals, and recrystallize from hot water: the methylmercaptopurine trihydrate so obtained, dried at 120
for 30 minutes, melts between 218
and 222
, with decomposition.
Phosphorus—
Digest 200 mg with 2 mL of 15 N sulfuric acid in a large test tube, periodically adding nitric acid, dropwise and with caution. Continue heating until practically all of the liquid has evaporated and the residue is colorless. Transfer the residue, with the aid of small portions of water, to a 25-mL volumetric flask, add 1 mL of 15 N sulfuric acid, 0.5 mL of nitric acid, 0.75 mL of
ammonium molybdate TS, and 1 mL of
aminonaphtholsulfonic acid TS, then dilute with water to volume, and mix. Allow to stand for 5 minutes, and determine the absorbance of this solution at 750 nm, with a suitable spectrophotometer, using as a blank a solution of the same quantities of the same reagents, prepared in the same manner. The absorbance of this solution is not greater than that of a solution obtained by treating as directed above, beginning with “add 1 mL of 15 N sulfuric acid,” a 2-mL portion of a standard phosphate solution prepared to contain, in each mL, 43.96 µg of dried monobasic potassium phosphate, equivalent to 10 µg of P (0.010%).
Organic volatile impurities, Method V 467:
meets the requirements.
Solvent—
Use dimethyl sulfoxide.
Assay—
Dissolve about 300 mg of Mercaptopurine, accurately weighed, in 80 mL of dimethylformamide. Add 5 drops of a 1 in 100 solution of thymol blue in dimethylformamide, and titrate with 0.1 N sodium methoxide VS, using a magnetic stirrer and taking precautions against absorption of atmospheric carbon dioxide. Perform a blank determination, and make any necessary correction. Each mL of 0.1 N sodium methoxide is equivalent to 15.22 mg of C5H4N4S.
;)
Mercaptopurine contains not less than 97.0 percent and not more than 102.0 percent of C5H4N4S, calculated on the anhydrous basis.