Magnesium Carbonate
(mag nee' zee um kar' bo nate).

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].
DEFINITION
Magnesium Carbonate is a basic hydrated magnesium carbonate or a normal hydrated magnesium carbonate. It contains the equivalent of NLT 40.0% and NMT 43.5% of magnesium oxide (MgO).
IDENTIFICATION
When treated with 3 N hydrochloric acid, it dissolves with effervescence, and the resulting solution meets the requirements.
ASSAY
•  Procedure
Sample:  1 g
Analysis:  Dissolve the Sample 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. Perform the blank determination. Calculate the volume, VS, of 1 N sulfuric acid, in mL, consumed by the Sample:
Result = (VB VA) × NNaOH
 VB = = volume of 1 N sodium hydroxide consumed by the blank determination (mL) VA = = volume of 1 N sodium hydroxide consumed by the Sample (mL) NNaOH = = exact normality of the sodium hydroxide solution
Calculate the volume of 1 N sulfuric acid, VCa, in mL, consumed by calcium, which is present in the portion of Magnesium Carbonate taken for the Assay:
Result = (W × LCa)/(FCa × 100)
 W = = weight of Magnesium Carbonate taken (mg) LCa = = content of calcium as determined in the test for Limit of Calcium (%) FCa = = weight of Ca that is equivalent to each mL of 1 N sulfuric acid, 20.04 mg
Calculate the percentage of magnesium oxide (MgO) in the portion of Magnesium Carbonate taken:
Result = (VS VCa) × FMgO/W × 100
 VS = = volume of 1 N sulfuric acid consumed by the Sample, as calculated above (mL) VCa = = volume of 1 N sulfuric acid consumed by calcium, as calculated above (mL) FMgO = = weight of MgO that is equivalent to each mL of 1 N sulfuric acid, 20.15 mg W = = weight of Magnesium Carbonate taken (mg)
Acceptance criteria:  40.0%–43.5% of MgO
IMPURITIES
•  Soluble Salts
Sample:  2.0 g
Analysis:  Mix the Sample 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 h.
Acceptance criteria:  The weight of the residue does not exceed 10 mg (NMT 1.0%).
•  Acid-Insoluble Substances
Sample:  5.0 g
Analysis:  Mix the Sample 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 min. If an insoluble residue remains, filter, wash well with water until the last washing is free from chloride, and ignite.
Acceptance criteria:  The weight of the ignited residue does not exceed 2.5 mg (NMT 0.05%).
Test preparation:  750 mg in 25 mL of 3 N hydrochloric acid
Acceptance criteria:  NMT 4 ppm
•  Limit of Calcium
[Note—A commercially available atomic absorption standard solution for calcium may be used where preparation of a calcium standard stock solution is described below. Concentrations of the Standard solutions and the Sample solution may be modified to fit the linear or working range of the instrument. ]
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, and dilute with water to 1000 mL.
Standard solutions:  Transfer 249.7 mg of calcium carbonate, previously dried at 300 for 3 h and cooled in a desiccator for 2 h, to a 100-mL volumetric flask. Dissolve in a minimum amount of hydrochloric acid, and dilute with water to volume. Transfer 1.0, 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. Dilute with water to volume. These Standard solutions contain 1.0, 5.0, 10.0, and 15.0 µg/mL of calcium, respectively.
Blank solution:  Transfer 4 mL of Lanthanum solution and 10 mL of Dilute hydrochloric acid to a 200-mL volumetric flask, and dilute with water to volume.
Sample solution:  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, and dilute with water to volume.
Instrumental conditions
Mode:  Atomic absorption spectrophotometry
Lamp:  Calcium hollow-cathode
Flame:  Nitrous oxide–acetylene
Analytical wavelength:  Calcium emission line at 422.7 nm
Analysis
Samples:  Standard solutions, Blank solution, and Sample solution
Using the Blank solution as blank, determine the concentration, C, in µg/mL, of calcium in the Sample solution using the calibration graph.
Calculate the percentage of calcium in the portion of Magnesium Carbonate taken:
Result = (V/W × C × F) × 100
 V = = volume of the Sample solution (mL) W = = weight of Magnesium Carbonate taken (mg) C = = as defined above F = = conversion factor from µg/mL to mg/mL, 0.001
Acceptance criteria:  NMT 0.45%
Test preparation:  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.
Acceptance criteria:  NMT 30 ppm
Test preparation:  Boil 50 mg with 5 mL of 2 N nitric acid for 1 min. Cool, dilute with water to 45 mL, add 2 mL of hydrochloric acid, and mix.
Acceptance criteria:  NMT 200 ppm
SPECIFIC TESTS
It meets the requirements of the test for absence of Escherichia coli.
•  Packaging and Storage: Preserve in well-closed containers.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Elena Gonikberg, Ph.D.
Principal Scientific Liaison
1-301-816-8251
(SM32010) Monographs - Small Molecules 3