Packaging and storage—
Preserve in tight, light-resistant containers.
Identification—
B:
Mix about 100 mg with 500 mg of anhydrous sodium carbonate, and ignite the mixture. Cool, add 5 mL of hot water, heat for 5 minutes on a steam bath, filter, and neutralize the filtrate with nitric acid: the solution responds to the test for
Chloride 
191
.
Ordinary impurities 466—
Test solution:
a mixture of methanol and chloroform (1:1).
Standard solution:
a mixture of methanol and chloroform (1:1).
Eluant:
a mixture of n-propyl alcohol, glacial acetic acid, and water (8:1:1).
Visualization:
2.
Organic volatile impurities, Method V 467:
meets the requirements.
Solvent—
Use dimethyl sulfoxide.
Assay—
Dissolve about 200 mg of Pyrimethamine, accurately weighed, in 25 mL of glacial acetic acid, warming slightly to effect solution. Cool the solution to room temperature, add 4 drops of
quinaldine red TS, and titrate with 0.1 N perchloric acid VS. Perform a blank determination, and make any necessary correction. Each mL of 0.1 N perchloric acid is equivalent to 24.87 mg of C
12H
13ClN
4.
;)
for 4 hours: it loses not more than 0.5% of its weight.