Packaging and storage—
Preserve in well-closed containers.
Identification—
A:
Dissolve about 600 mg in 20 mL of 3 N hydrochloric acid, add 3 drops of
methyl red TS and about 30 mL of water, and heat to boiling. Add 6 N ammonium hydroxide until the color just changes to yellow, continue boiling for 2 minutes, and filter: the filtrate responds to the tests for
Magnesium 
191
.
B:
Wash the precipitate obtained in
Identification test
A with 50 mL of hot ammonium chloride solution (1 in 50), then dissolve the precipitate in 15 mL of 3 N hydrochloric acid: the solution responds to the tests for
Aluminum 191.
Loss on drying 731—
Dry it at 200
for 4 hours: it loses between 10.0% and 20.0% of its weight.
Soluble chloride—
Boil 1 g of it, accurately weighed, with 50.0 mL of water for 5 minutes, cool, add water to restore the original volume, mix, and filter. To 25.0 mL of the filtrate add 0.1 mL of
potassium chromate TS, and titrate with 0.10 N silver nitrate until a persistent pink color is obtained: not more than 5.0 mL of 0.10 N silver nitrate is required (3.5%).
Soluble sulfate 221—
A 2.5-mL portion of the filtrate obtained in the test for
Soluble chloride shows no more sulfate than corresponds to 1.0 mL of 0.020 N sulfuric acid (1.9%).
Sodium—
Transfer 2 g of it, accurately weighed, to a 100-mL volumetric flask, place in an ice bath, add 5 mL of nitric acid, and swirl to dissolve. Allow to warm to room temperature, dilute with water to volume, and mix. Filter, if necessary, to obtain a clear solution. Dilute 10.0 mL of the filtrate with water to 100.0 mL: the emission intensity of this solution, determined with a suitable flame photometer at 589 nm and corrected for background transmission at 580 nm, is not greater than that produced by a standard containing 2.2 µg of Na per mL, similarly measured (0.11%).
Heavy metals 231—
Dissolve 330 mg in 10 mL of 3 N hydrochloric acid, filter if necessary to obtain a clear solution, and dilute with water to 25 mL: the limit is 0.006%.
Organic volatile impurities, Method V 467:
meets the requirements.
Solvent—
Use dimethyl sulfoxide.
Magnesium hydroxide content—
Dissolve about 100 mg, accurately weighed, in 3 mL of dilute hydrochloric acid (1 in 10), and dilute with water to about 200 mL. Add, with stirring, 1 g of ammonium chloride, 20 mL of triethanolamine, 10 mL of ammonia–ammonium chloride buffer TS, and 0.1 mL of
eriochrome black TS, and titrate with 0.05 M edetate disodium VS to a blue color. Perform a blank determination, and make any necessary correction. Each mL of 0.05
M edetate disodium is equivalent to 2.916 mg of Mg(OH)
2: between 49.2% and 66.6% of Mg(OH)
2 is found, calculated on the dried basis.
Aluminum hydroxide content—
Edetate disodium titrant—
Prepare and standardize as directed in the
Assay under
Ammonium Alum.
Procedure—
Dissolve about 100 mg of Magaldrate, accurately weighed, in 3 mL of dilute hydrochloric acid (1 in 10), and dilute with water to about 30 mL. Add, with stirring, 25.0 mL of
Edetate disodium titrant, mix, and allow to stand for 5 minutes. Then add 20 mL of acetic acid-ammonium acetate buffer TS, 60 mL of alcohol, and 2 mL of
dithizone TS, and titrate with 0.05
M zinc sulfate to a bright rose-pink color. Perform a blank determination, and make any necessary correction. Each mL of 0.05 M
Edetate disodium titrant is equivalent to 3.900 mg of Al(OH)
3: between 32.1% and 45.9% of Al(OH)
3 is found, calculated on the dried basis.
Sulfate content—
Chromatographic column—
Transfer 15 mL of strongly acidic 50- to 100-mesh styrene-divinylbenzene cation-exchange resin to a 1-cm inside diameter glass column. Wash the resin with 30 mL of water.
Indicator solution—
Prepare a solution in water containing 2 mg of sodium alizarinsulfonate per mL.
Magnesium acetate solution—
Dissolve 26.8 g of magnesium acetate in 500 mL of water.
0.05 M Barium chloride—
Dissolve 12.2 g of barium chloride in about 900 mL of water, adjust with 1 N hydrochloric acid to a pH of 3.0, dilute with water to 1000 mL, and mix. Standardize this solution as follows: Transfer 10.0 mL of 0.1 N sulfuric acid VS to a 125-mL conical flask. Adjust by adding
Magnesium acetate solution to a pH of 3.0. Add 25 mL of methanol and 3 or 4 drops of
Indicator solution. Add from a buret an accurately measured volume of 8 to 9 mL of 0.05
M barium chloride. Add an additional 4 drops of
Indicator solution, and titrate slowly until the yellow color disappears and a pink tinge is visible. Calculate the molarity of the barium chloride titrant taken by the formula:
5(N / V),
in which
N is the normality of the sulfuric acid; and
V is the volume, in mL, of titrant consumed.
Test preparation—
Transfer about 875 mg of Magaldrate, accurately weighed, to a 25-mL volumetric flask. Dissolve in 10 mL of water and 5 mL of glacial acetic acid, dilute with water to volume, and mix. Transfer 5.0 mL of this solution to the chromatographic column and wash the column with 15 mL of water, collecting the eluate in a 125-mL conical flask (Test preparation).
Procedure—
Add to the Test preparation 5 mL of Magnesium acetate solution, 32 mL of methanol, and 3 or 4 drops of Indicator solution. Add from a buret an accurately measured volume of 5.0 to 5.5 mL of 0.05 M barium chloride. Add an additional 3 drops of Indicator solution, and titrate slowly until the yellow color disappears and a pink tinge is visible. Each mL of 0.05 M barium chloride is equivalent to 4.803 mg of sulfate (SO4): between 16.0% and 21.0% of SO4 is found, calculated on the dried basis.
Assay—
Transfer about 3 g of Magaldrate, accurately weighed, to a 250-mL beaker, add 100.0 mL of 1 N hydrochloric acid VS, and stir until the solution becomes clear. Titrate the excess acid with 1 N sodium hydroxide VS to a pH of 3.0, determined potentiometrically. Perform a blank determination (see
Residual Titrations under
Titrimetry 541 ). Each mL of 1 N hydrochloric acid is equivalent to 35.40 mg of Al
5Mg
10(OH)
31(SO
4)
2.
