Packaging and storage—
Preserve in tight containers.
Labeling—
Label it to indicate whether it is Light Magnesium Oxide or Heavy Magnesium Oxide.
Identification—
A solution in diluted hydrochloric acid meets the requirements of the tests for
Magnesium 
191
.
Loss on ignition 733—
Transfer to a tared platinum crucible about 500 mg, weigh accurately, and ignite at 800 ± 25
to constant weight: it loses not more than 10.0% of its weight.
Free alkali and soluble salts—
Boil 2.0 g with 100 mL of water for 5 minutes in a covered beaker, and filter while hot. To 50 mL of the cooled filtrate add
methyl red TS, and titrate with 0.10 N sulfuric acid: not more than 2.0 mL of the acid is consumed. Evaporate 25 mL of the remaining filtrate to dryness, and dry at 105
for 1 hour: not more than 10 mg of residue remains (2.0%).
Acid-insoluble substances—
Mix 2 g with 75 mL of water, add hydrochloric acid in small portions, with agitation, until no more dissolves, and boil for 5 minutes. If an insoluble residue remains, filter, wash well with water until the last washing is free from chloride, and ignite: the weight of the ignited residue does not exceed 2 mg (0.1%).
Limit of calcium—
Dilute hydrochloric acid, Lanthanum solution, Standard preparations, and Blank solution—
Prepare as directed in the test for
Limit of calcium under
Magnesium Carbonate.
Test preparation—
Transfer 250 mg of Magnesium Oxide, freshly ignited, 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.
Procedure—
Proceed as directed in the test for
Limit of calcium under
Magnesium Carbonate: the limit is 1.1%.
Heavy metals 231—
Dissolve 2.0 g in 35 mL of 3 N hydrochloric acid, and evaporate the solution on a steam bath to dryness. Toward the end of the evaporation, stir frequently to disintegrate the residue so that finally a dry powder is obtained. Dissolve the residue in 20 mL of water, and evaporate to dryness in the same manner as before. Redissolve the residue in 20 mL of water, filter if necessary, and dilute with water to 40 mL. To 20 mL add water to make 25 mL: the limit is 20 µg per g.
Iron 241—
Boil 40 mg with 5 mL of 2 N nitric acid for 1 minute. Cool, dilute with water to 50 mL, and mix. Dilute 25 mL of this solution with water to 45 mL, and add 2 mL of hydrochloric acid: the limit is 0.05%.
Assay—
Ignite about 500 mg of Magnesium Oxide to constant weight in a tared platinum crucible, weigh the residue accurately, dissolve it in 30.0 mL of 1 N sulfuric acid VS, add
methyl orange TS, and titrate the excess acid with 1 N sodium hydroxide VS. From the volume of 1 N sulfuric acid consumed, deduct the volume of 1 N sulfuric acid corresponding to the content of calcium in the Magnesium Oxide taken for the assay. The difference is the volume of 1 N sulfuric acid equivalent to the MgO in the portion of Magnesium Oxide taken. Each mL of 1 N sulfuric acid is equivalent to 20.15 mg of MgO and to 20.04 mg of Ca.
