Packaging and storage
Preserve in well-closed containers, and avoid prolonged exposure to temperatures exceeding 30
The residue obtained in the Assay
is yellow when hot and white when cool.
Calcium, magnesium, and other foreign substances
Heat about 2 g gently until melted, and continue the heating, gradually raising the temperature until the mass is thoroughly charred. Ignite the mass until the residue is uniformly yellow. To the residue add 6 mL of 3 N hydrochloric acid: no effervescence occurs. Heat the mixture on a steam bath for 10 to 15 minutes: not more than a trace of insoluble residue remains. Filter the solution, dilute with water to 10 mL, add 6 N ammonium hydroxide until the precipitate first formed redissolves, then add 2 mL each of ammonium oxalate TS
and dibasic sodium phosphate TS: not more than a slight turbidity is produced in 5 minutes.
Weigh accurately in a porcelain crucible about 700 mg of Ointment, heat gently until melted, and continue the heating, gradually raising the temperature until the mass is thoroughly charred. Ignite the mass until the residue is uniformly yellow, and cool. Dissolve the residue in 10 mL of 2 N sulfuric acid, warming if necessary to effect complete solution, transfer the solution to a beaker, and rinse the crucible with small portions of water until the combined solution and rinsings measure 50 mL. Add 15 mL of ammoniaammonium chloride buffer TS and 1 mL of eriochrome black TS
, and titrate with 0.05 M
edetate disodium VS until the solution is blue in color. Each mL of 0.05 M
edetate disodium is equivalent to 4.069 mg of ZnO.