» Methimazole Tablets contain not less than 94.0 percent and not more than 106.0 percent of the labeled amount of methimazole (C4H6N2S).
Packaging and storage Preserve in well-closed, light-resistant containers.
USP Reference standards 11
Identification Digest a quantity of powdered Tablets, equivalent to about 10 mg of methimazole, with 10 mL of warm chloroform for 20 minutes, filter, and evaporate the filtrate on a steam bath to dryness: the residue responds to the Identification tests under Methimazole.
Medium: water; 500 mL.
Apparatus 1: 100 rpm.
Time: 30 minutes.
Procedure Determine the amount of C4H6N2S dissolved from UV absorbances at the wavelength of maximum absorbance at about 252 nm of filtered portions of the solution under test, suitably diluted with Medium, if necessary, in comparison with a Standard solution having a known concentration of USP Methimazole RS in the same Medium.
Tolerances Not less than 80% (Q) of the labeled amount of C4H6N2S is dissolved in 30 minutes.
Uniformity of dosage units 905: meet the requirements.
Procedure for content uniformity Place 1 Tablet, previously crushed or finely powdered, in a 100-mL volumetric flask, add 50 mL of water, and shake by mechanical means for 30 minutes. Dilute with water to volume, mix, and filter, discarding the first 20 mL of filtrate. Dilute a portion of the subsequent filtrate quantitatively and stepwise with water so that the assumed concentration of methimazole is about 5 µg per mL. Dissolve an accurately weighed quantity of USP Methimazole RS in water, and dilute quantitatively and stepwise with water to obtain a Standard solution having a known concentration of about 5 µg per mL. Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 252 nm, with a suitable spectrophotometer, using water as the blank. Calculate the quantity, in mg, of methimazole (C4H6N2S) in the Tablet taken by the formula:
(T / 5)C(AU / AS)in which T is the labeled quantity, in mg, of methimazole in the Tablet; C is the concentration, in µg per mL, of USP Methimazole RS in the Standard solution; and AU and AS are the absorbances of the solution from the Tablet and the Standard solution, respectively.
Assay Weigh and finely powder not fewer than 20 Tablets. Weigh accurately a portion of the powder, equivalent to about 120 mg of methimazole, and place in a 100-mL volumetric flask. Add about 80 mL of water, insert the stopper, and shake by mechanical means or occasionally by hand during 30 minutes. Dilute with water to volume, and mix. Filter, and transfer 50.0 mL of the filtrate to a 125-mL conical flask. Add from a buret 3.5 mL of 0.1 N sodium hydroxide VS, mix, and add, with agitation, about 7 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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USP35NF30 Page 3850