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 C
6H
12N
4.