Identification—
A:
Dissolve 0.5 g in 5 mL of water, acidify with hydrochloric acid, and filter the resulting precipitate. Wash the precipitate with water, and dry it over silica gel for 5 hours: the IR absorption spectrum of a mineral oil dispersion of it exhibits maxima only at the same wavelengths as that of a similar preparation of
USP Methylparaben RS.
B:
Ignite about 0.3 g, cool, and dissolve the residue in about 3 mL of 3 N hydrochloric acid. A platinum wire dipped in this solution imparts an intense, persistent yellow color to a nonluminous flame.
Assay—
Gently reflux about 100 mg of Methylparaben Sodium, accurately weighed, with 30 mL of 1 N sodium hydroxide for 30 minutes. Cool, add 25.0 mL of potassium bromate solution (2.78 in 500), 5 mL of potassium bromide solution (1 in 8), and 10 mL of hydrochloric acid, and immediately insert the stopper into the flask. Cool, shake for 15 minutes, and allow to stand for 15 minutes. Quickly add 15 mL of
potassium iodide TS, taking care to avoid the escape of bromine vapor, at once replace the stopper in the flask, and shake vigorously. Rinse the stopper and the neck of the flask with a small quantity of water, and titrate the liberated iodine with 0.1 N sodium thiosulfate VS, adding 3 mL of
starch TS as the endpoint is approached.
[NOTE—About 15 mL is needed.
] Perform a blank determination (see
Residual Titrations under
Titrimetry 
541
), and note the difference in volumes required. Each mL of the difference in volume of 0.1 N sodium thiosulfate is equivalent to 2.902 mg of C
8H
7NaO
3.
