Methimazole
(meth im' a zole).
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C4H6N2S 114.17

2H-Imidazole-2-thione, 1,3-dihydro-1-methyl-.
1-Methylimidazole-2-thiol [60-56-0].
» Methimazole contains not less than 98.0 percent and not more than 101.0 percent of C4H6N2S, calculated on the dried basis.
Packaging and storage— Preserve in well-closed, light-resistant containers.
USP Reference standards 11
USP Methimazole RS Click to View Structure
Identification—
B: Mercuric chloride TS produces in a solution (1 in 200) a white precipitate, but no precipitation is produced by trinitrophenol TS. The solution is colored intensely blue by molybdophosphotungstate TS.
Melting range 741: between 143 and 146.
Loss on drying 731 Dry it at 105 for 2 hours: it loses not more than 0.5% of its weight.
Residue on ignition 281: not more than 0.1%.
Selenium 291: 0.003%, a 200-mg specimen being used.
Ordinary impurities 466
Test solution: ethyl acetate.
Standard solution: ethyl acetate.
Eluant: a mixture of toluene, isopropyl alcohol, and ammonium hydroxide (70:29:1), in a nonequilibrated chamber.
Visualization: 2.
Assay— Dissolve about 250 mg of Methimazole, accurately weighed, in 75 mL of water. Add from a buret 15 mL of 0.1 N sodium hydroxide VS, mix, and add, with agitation, about 30 mL of 0.1 N silver nitrate. Add 1 mL of bromothymol blue TS, and continue the titration with the 0.1 N sodium hydroxide VS until a permanent, blue-green color is produced. Each mL of 0.1 N sodium hydroxide is equivalent to 11.42 mg of C4H6N2S.
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USP35–NF30 Page 3850