Packaging and storage—
Preserve in well-closed containers.
Identification—
A:
Mix 25 g with 200 mL of hot water, add 60 mL of 2 N sulfuric acid, and boil until the fatty acids separate as a transparent layer. Cool the mixture, and remove the solidified layer of fatty acids: a portion of the water layer responds to the tests for
Zinc 
191
.
B:
Place the separated fatty acids obtained in
Identification test
A in a filter wetted with water, and wash with boiling water until free from sulfate. Collect the fatty acids in a small beaker, allow to cool, pour off the separated water, then melt the acids, filter into a dry beaker while hot, and dry at 105
for 20 minutes: the fatty acids congeal (see
Congealing Temperature 651) at a temperature not below 54
.
Arsenic, Method I 211—
Prepare the
Test Preparation as follows. Mix 5.0 g with 50 mL of water, cautiously add 5 mL of sulfuric acid, and boil gently until the fatty acids layer is clear and the volume is reduced to about 25 mL. Filter while hot, cool the filtrate, and dilute with water to 50 mL. Transfer a 20-mL aliquot to the arsine generator flask, and dilute with water to 35 mL. The limit is 1.5 ppm.
Lead—
Ignite 0.50 g in a platinum crucible for 15 to 20 minutes in a muffle furnace at 475
to 500
. Cool, add 3 drops of nitric acid, evaporate over a low flame to dryness, and ignite again at 475
to 500
for 30 minutes. Dissolve the residue in 1 mL of 8 N nitric acid, and proceed as directed in the test for
Lead under
Magnesium Stearate. The limit is 0.001%.
Alkalies and alkaline earths—
Mix 2.0 g with 50 mL of water, add 10 mL of hydrochloric acid, boil until the solution is clear, filter while hot, and wash the separated fatty acids with about 50 mL of hot water. Render the combined filtrate and washings alkaline with 6 N ammonium hydroxide, add
ammonium sulfide TS to precipitate the zinc completely, dilute with water to 200 mL, mix, and filter. To 100 mL of the clear filtrate add 0.5 mL of sulfuric acid, evaporate to dryness, and ignite to constant weight: the weight of the residue does not exceed 10 mg (1.0%).
Organic volatile impurities, Method IV 467:
meets the requirements.
Assay—
Boil about 1 g of Zinc Stearate, accurately weighed, with 50 mL of 0.1 N sulfuric acid for at least 10 minutes, or until the fatty acids layer is clear, adding more water as necessary to maintain the original volume, cool, and filter. Wash the filter and the flask thoroughly with water until the last washing is not acid to litmus paper. Add to the combined filtrate and washings 15 mL of ammonia–ammonium chloride buffer TS and 0.2 mL of
eriochrome black TS, heat the solution to about 40
, and titrate with 0.05
M edetate disodium VS until the solution is deep blue in color. Each mL of 0.05
M edetate disodium is equivalent to 4.069 mg of ZnO.
