Magnesium Carbonate
〔546-93-0〕
Content
Magnesium Carbonate contains the equivalent of 40.0 − 44.0% of
magnesium oxide (MgO=40.30).
Description
Magnesium Carbonate occurs as white, bulky powder or brittle
lumps.
Identification
To 0.2 g of Magnesium Carbonate, add gradually 3 ml of diluted
hydrochloric acid (1 → 4). It dissolves while effervescence occurs. Add ammonia TS to
make the solution alkaline. The solution responds to the test for Magnesium Salt as
described in the Qualitative Tests.
Purity
(1)
Clarity of solution
Very slightly turbid.
Test Solution Weigh 1.0 g of Magnesium Carbonate, dissolve in 10 ml of diluted
hydrochloric acid (2 → 3), and add 10 ml of water.
(2)
Water-soluble substances Not more than 1.0%.
Weigh 2.0 g of Magnesium Carbonate, add 100 ml of freshly boiled and cooled water,
boil for 5 minutes while stirring, cool, filter, combine the filtrate and the washings, add
water to make 100 ml, measure 50 ml of this solution, evaporate to dryness in a water
bath, and dry the residue at 105℃ for 1 hour. Weigh the residue.
(3)
Heavy metals Not more than 30 µg/g as Pb.
Test Solution Weigh 1.0 g of Magnesium Carbonate, dissolve in 10 ml of diluted
hydrochloric acid (1 → 4), evaporate to dryness in a water bath, and dissolve in about
40 ml of water. Filter if necessary, and add 2 ml of diluted acetic acid (1 → 20) and
water to make 50 ml.
Control Solution To 3.0 ml of Lead Standard Solution, add 2 ml of diluted acetic
acid (1 → 20) and water to make 50 ml.
(4)
Calcium oxide
Not more than 0.60% as CaO.
Weigh exactly 0.600g of Magnesium Carbonate, dissolve in 35 ml of water and 6 ml
of diluted hydrochloric acid (1 → 4), add 250 ml of water and 5 ml of tartaric acid
solution (1 → 5), then add 10 ml of triethanolamine solution (3 → 10) and 10 ml of
potassium hydroxide solution (1 → 2), allow to stand for 5 minutes. Titrate with 0.01
mol/l EDTA (indicator: 0.1 g of NN indicator) and calculate the content of calcium oxide.
The end point is the time when the color of the solution changes from red-purple to blue.
Perform a blank test in the same manner, and make any necessary correction.
1 ml of 0.01 mol/l EDTA = 0.5608 mg of CaO
(5)
Arsenic
Not more than 4.0 µg/g as As2O3.
Test Solution Weigh 0.50 g of Magnesium Carbonate, moisten with 1.5 ml of
water,
and dissolve in 3.5 ml of diluted hydrochloric acid (1 → 4).
Apparatus Apparatus B.
Assay
Weigh exactly 0.400g of Magnesium Carbonate, dissolve in 10 ml of water
and 3.5 ml of diluted hydrochloric acid (1 → 4), and add water to make exactly 500 ml.
Measure exactly 25 ml of this solution, add 50 ml of water and 5 ml of ammonia−
ammonium chloride buffer (pH 10.7), and titrate with 0.01 mol/l EDTA (indicator: 0.04 g
of eriochrome black T−sodium chloride indicator). Perform a blank test in the same
manner, make any necessary correction, and calculate the consumed volume as a ml.
Take the consumed volume of 0.01 mol/l EDTA obtained in Purity (4) as b ml, and
calculate the content by the formula
Content of magnesium oxide ( MgO) =
(a − 0.033b) × 0.8061
(%) .
Weight (g ) of the sample