Carbonic acid, magnesium salt, basic; or, Carbonic acid, magnesium salt (1:1), hydrate
Magnesium carbonate, basic; or, Magnesium carbonate (1:1) hydrate
[
23389-33-5].
Anhydrous
84.31
[
546-93-0].
Packaging and storage—
Preserve in well-closed containers.
Identification—
When treated with 3 N hydrochloric acid, it dissolves with effervescence, and the resulting solution responds to the tests for
Magnesium 
191
.
Soluble salts—
Mix 2.0 g with 100 mL of a mixture of equal volumes of
n-propyl alcohol and water. Heat the mixture to the boiling point with constant stirring, cool to room temperature, dilute with water to 100 mL, and filter. Evaporate 50 mL of the filtrate on a steam bath to dryness, and dry at 105
for 1 hour: the weight of the residue does not exceed 10 mg (1.0%).
Acid-insoluble substances—
Mix 5.0 g with 75 mL of water, add hydrochloric acid in small portions, with agitation, until no more of the magnesium carbonate 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 2.5 mg (0.05%).
Arsenic, Method I 211—
Prepare the
Test Preparation by dissolving 750 mg in 25 mL of 3 N hydrochloric acid. The limit is 4 ppm.
Limit of calcium—
Dilute hydrochloric acid—
Dilute 100 mL of hydrochloric acid with water to 1000 mL.
Lanthanum solution—
To 58.65 g of lanthanum oxide add 400 mL of water, and add, gradually with stirring, 250 mL of hydrochloric acid. Stir until dissolved, dilute with water to 1000 mL, and mix.
Standard preparations—
Transfer 249.7 mg of calcium carbonate, previously dried at 300
for 3 hours and cooled in a desiccator for 2 hours, to a 100-mL volumetric flask, dissolve in a minimum amount of hydrochloric acid, dilute with water to volume, and mix. Transfer 5.0, 10.0, and 15.0 mL of this stock solution to separate 1000-mL volumetric flasks, each containing 20 mL of
Lanthanum solution and 40 mL of
Dilute hydrochloric acid, add water to volume, and mix. These
Standard preparations contain 5.0, 10.0, and 15.0 µg of calcium in each mL, respectively.
Blank solution—
Transfer 4 mL of
Lanthanum solution and 10 mL of
Dilute hydrochloric acid to a 200-mL volumetric flask, dilute with water to volume, and mix.
Test preparation—
Transfer 250 mg of Magnesium Carbonate 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—
Concomitantly determine the absorbances of the
Standard preparations and the
Test preparation at the calcium emission line at 422.7 nm with a suitable atomic absorption spectrophotometer (see
Spectrophotometry and Light-Scattering 851) equipped with a calcium hollow-cathode lamp and an air–acetylene flame, using the
Blank solution as the blank. Plot the absorbances of the
Standard preparations versus their concentrations of calcium, in µg per mL, and draw a straight line through the three points. From the line so obtained and the absorbance of the
Test preparation, determine the concentration, in µg per mL, of calcium in the
Test preparation. Calculate the percentage of calcium in the specimen by multiplying this value by 0.08: the limit is 0.45%.
Heavy metals, Method I 231—
Dissolve 0.67 g in 10 mL of 3 N hydrochloric acid in a suitable crucible, and evaporate the solution on a steam bath to dryness. Ignite at 550 ± 25
until all carbonaceous material is consumed. Dissolve the residue in 15 mL of water and 5 mL of hydrochloric acid, and evaporate 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 in the same manner as before to dryness. Redissolve the residue in 20 mL of water, filter, if necessary, and add to the filtrate 2 mL of 1 N acetic acid and water to make 25 mL: the limit is 0.003%.
Iron 241—
Boil 50 mg with 5 mL of 2 N nitric acid for 1 minute. Cool, dilute with water to 45 mL, add 2 mL of hydrochloric acid, and mix: the limit is 0.02%.
Assay—
Dissolve about 1 g of Magnesium Carbonate, accurately weighed, 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 weight of Magnesium Carbonate taken for the assay. The difference is the volume of 1 N sulfuric acid equivalent to the magnesium oxide present. Each mL of 1 N sulfuric acid is equivalent to 20.15 mg of MgO and to 20.04 mg of Ca.
