Packaging and storage—
Preserve in tight, light-resistant containers.
Acidity or alkalinity—
Shake 2.0 g with 100 mL of water for 15 minutes, and filter: 20.0 mL of the filtrate requires for neutralization not more than 0.10 mL of 0.020 N hydrochloric acid or not more than 0.10 mL of 0.020 N sodium hydroxide, methyl red TS being used as the indicator.
Loss on drying 
731
—
Dry it in vacuum at 105
for 5 hours: it loses not more than 1.0% of its weight.
Limit of mercaptopurine—
Prepare three solutions in 6 N ammonium hydroxide containing, respectively, 20 mg of Azathioprine per mL, 20 mg of
USP Azathioprine RS per mL, and 200 µg of
USP Mercaptopurine RS, on the anhydrous basis, per mL. Apply 5-µL volumes of the solutions at points about 2 cm from the bottom edge of a thin-layer chromatographic plate (see
Chromatography 621) coated with a 0.25-mm layer of microcrystalline cellulose. Allow the spots to dry, and develop the chromatogram in a suitable chamber, using butyl alcohol, previously saturated with 6 N ammonium hydroxide, as the solvent, until the solvent front has moved about 15 cm from the point of application. Remove the plate, air-dry, and locate the spots by viewing under short- and long-wavelength UV light: any spot in the chromatogram from Azathioprine, other than the principal spot, is not more intense than the spot in the chromatogram obtained with
USP Mercaptopurine RS (1.0%).
Organic volatile impurities, Method IV 467:
meets the requirements.
Solvent—
Use dimethyl sulfoxide.
Assay—
Dissolve about 300 mg of Azathioprine, 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 tetrabutylammonium hydroxide VS, using a magnetic stirrer, and taking precautions to prevent absorption of atmospheric carbon dioxide. Perform a blank determination, and make any necessary correction. Each mL of 0.1 N tetrabutylammonium hydroxide is equivalent to 27.73 mg of C9H7N7O2S.
;)
