(meth en' a meen).
» Methenamine, dried over phosphorus pentoxide for 4 hours, contains not less than 99.0 percent and not more than 100.5 percent of C6H12N4.
Packaging and storage Preserve in well-closed containers.
USP Reference standards 11
B: Heat a solution (1 in 10) with 2 N sulfuric acid: 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.
Loss on drying 731 Dry it over phosphorus pentoxide for 4 hours: it loses not more than 2.0% of its weight.
Residue on ignition 281: not more than 0.1%.
Chloride 221 A 1.0-g portion shows no more chloride than corresponds to 0.20 mL of 0.020 N hydrochloric acid (0.014%).
Sulfate To 10 mL of a solution (1 in 50), acidified with 5 drops of hydrochloric acid, add 5 drops of barium chloride TS: no turbidity is produced within 1 minute.
Ammonium salts To 10 mL of a solution (1 in 20) add 1 mL of alkaline mercuric-potassium iodide TS: the mixture is not darker in color than a mixture of 1 mL of the reagent and 10 mL of water.
Heavy metals, Method I 231 Dissolve 2 g in 10 mL of water, add 2 mL of 3 N hydrochloric acid, and dilute with water to 25 mL. Proceed as directed, except to use glacial acetic acid to adjust the pH: the limit is 0.001%.
Chromotropic acid spot test solution Suspend 100 mg of chromotropic acid in 2 mL of water, and cautiously add 3 mL of sulfuric acid. Allow to cool. Add 25 mL of sulfuric acid, and mix. [noteIf excessive heat generated during mixing causes a violet color to appear in the solution, discard the solution and prepare another, taking precautions to avoid excessive heat. ]
Procedure Transfer about 1 g of Methenamine, previously dried and accurately weighed, to a beaker. Add 40.0 mL of 1 N sulfuric acid VS, and heat to a gentle boil, adding water from time to time if necessary, until the formaldehyde has been expelled. Test for the absence of formaldehyde by adding a drop of the assay solution to a glass fiber filter disk, on a watch glass, on which has previously been placed 3 or 4 drops of Chromotropic acid spot test solution. Formaldehyde produces a violet color with this reagent; repeat the test until no violet color is obtained on the warmed test filter disk upon comparison with a blank filter disk to which no assay specimen is added. Cool, add 20 mL of water, then add methyl red TS, and titrate the excess acid with 1 N sodium hydroxide VS. Perform a blank determination (see Residual Titrations under Titrimetry 541). Each mL of 1 N sulfuric acid is equivalent to 35.05 mg of C6H12N4.
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USP35NF30 Page 3846