» Methenamine Tablets contain not less than 95.0 percent and not more than 105.0 percent of the labeled amount of methenamine (C6H12N4).
Packaging and storage Preserve in well-closed containers.
USP Reference standards 11
Identification Dissolve about 500 mg of powdered Tablets in 10 mL of water, add 10 mL of 2 N sulfuric acid, and heat: formaldehyde is liberated, recognizable by its odor and by its darkening of paper moistened with silver ammonium nitrate TS. On the subsequent addition of an excess of 1 N sodium hydroxide to the solution, ammonia is evolved.
Dissolution, Procedure for a Pooled Sample 711
Medium: water; 900 mL.
Apparatus 1: 100 rpm.
Time: 45 minutes.
Procedure Determine the amount of C6H12N4 dissolved, employing the procedure set forth in the Assay, making any necessary modifications.
Tolerances Not less than 75% (Q) of the labeled amount of C6H12N4 is dissolved in 45 minutes.
Uniformity of dosage units 905: meet the requirements.
Chromotropic acid solution and Standard preparation Prepare as directed in the Assay under Methenamine Oral Solution.
Assay preparation Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 500 mg of methenamine, to a 250-mL volumetric flask, dilute with water to volume, mix, and filter, discarding the first 20 mL of the filtrate. Transfer 25.0 mL of the subsequent filtrate to a 1000-mL volumetric flask, dilute with water to volume, and mix. Proceed as directed for Assay preparation in the Assay under Methenamine Oral Solution, beginning with Transfer a 2.0-mL portion of this stock solution.
Procedure Proceed as directed for Procedure in the Assay under Methenamine Oral Solution. Calculate the quantity, in mg, of methenamine (C6H12N4) in the portion of Tablets taken by the formula:
500C[(AU BU)/(AS BS)]in which C is the concentration, in µg per mL, of USP Methenamine RS in the Standard preparation; and AU, AS, BU, and BS are as defined therein.
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USP35NF30 Page 3847Pharmacopeial Forum: Volume No. 30(1) Page 130