Packaging and storage—
Preserve in tight containers.
Identification—
One g of Paste dissolved in 10 mL of 3 N hydrochloric acid responds to the tests for
Magnesium 
191
.
Microbial limits 61—
Its total aerobic microbial count does not exceed 400 cfu per g, and it meets the requirements of the test for absence of
Escherichia coli.
Soluble alkalies—
Mix 25.0 g of Paste with 75.0 mL of water, transfer about 25 mL of this diluted Paste to a filter, and reject the first 5 mL of the filtrate.
[NOTE—Retain the remaining diluted Paste for the tests for
Carbonate and acid-insoluble matter,
Arsenic, and
Heavy metals.
] Dilute 5 mL of the clear filtrate with 40 mL of water. Add 1 drop of
methyl red TS, and titrate the solution with 0.10 N sulfuric acid to the production of a persistent pink color: not more than 1.0 mL of the acid is required.
Soluble salts—
To 5.0 mL of the clear filtrate obtained in the test for
Soluble alkalies add 3 drops of sulfuric acid, evaporate on a steam bath to dryness, and ignite gently to constant weight: the residue weighs not more than 12 mg.
Carbonate and acid-insoluble matter—
To 1 mL of the diluted Paste obtained in the test for
Soluble alkalies add 2 mL of 3 N hydrochloric acid: not more than a slight effervescence occurs, and the solution is not more than slightly turbid.
Limit of calcium—
Test preparation—
Transfer a portion of the Paste, equivalent to 250 mg of Mg(OH)2, to a beaker, add 30 mL of Dilute hydrochloric acid, and stir until dissolved, heating if necessary. Transfer the solution so obtained to a 200-mL volumetric flask containing 4 mL of Lanthanum solution, dilute with water to volume, and mix.
Heavy metals, Method I 231—
To 4.0 mL of the diluted Paste obtained in the test for
Soluble alkalies add 6 mL of 3 N hydrochloric acid, and evaporate the solution on a steam bath to dryness, with frequent stirring. Dissolve the residue in 20 mL of water, and evaporate to dryness in the same manner as before. Redissolve in 20 mL of water, filter if necessary, and dilute with water to 25 mL: the limit is 5 ppm, based on the amount of diluted Paste taken.
Assay—
Transfer an accurately weighed portion of Paste, equivalent to about 250 mg of magnesium hydroxide, to a 100-mL volumetric flask. Dissolve in 10 mL of 3 N hydrochloric acid, dilute with water to volume, and mix. Filter, if necessary, and transfer 25.0 mL of the filtrate to a beaker containing 75 mL of water, and mix. Adjust the reaction of the solution to a pH of 7 (using pH indicator paper; see
Indicator and Test Papers under
Reagents in the section
Reagents, Indicators, and Solutions) with 1 N sodium hydroxide, add 5 mL of ammonia–ammonium chloride buffer TS and 0.15 mL of
eriochrome black TS, and titrate with 0.05
M edetate disodium VS to a blue endpoint. Each mL of 0.05
M edetate disodium is equivalent to 2.916 mg of magnesium hydroxide [Mg(OH)
2].
