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Milk of Magnesia
Mg(OH)2 58.32

Magnesium hydroxide.
Magnesium hydroxide [1309-42-8].
» Milk of Magnesia is a suspension of Magnesium Hydroxide. Milk of Magnesia, Double-Strength Milk of Magnesia, and Triple-Strength Milk of Magnesia contain not less than 90.0 percent and not more than 115.0 percent of the labeled amount of Mg(OH)2, the labeled amount being 80, 160, and 240 mg of Mg(OH)2 per mL, respectively. It may contain not more than 0.05 percent of a volatile oil or a blend of volatile oils, suitable for flavoring purposes.
Packaging and storage— Preserve in tight containers, preferably at a temperature not exceeding 35. Avoid freezing.
Labeling— Double- or Triple-Strength Milk of Magnesia is so labeled, or may be labeled as 2× or 3× Concentrated Milk of Magnesia, respectively.
Identification— A solution of the equivalent of 1 g of regular-strength Milk of Magnesia in 2 mL of 3 N hydrochloric acid meets the requirements of the tests for Magnesium 191.
Microbial limits 61 Its total aerobic microbial count does not exceed 100 cfu per mL, and it meets the requirements of the test for absence of Escherichia coli.
Acid-neutralizing capacity 301 Not less than 5 mEq of acid is consumed by the minimum single dose recommended in the labeling, and not less than the number of mEq calculated by the formula:
in which 0.0343 is the theoretical acid-neutralizing capacity, in mEq, of Mg(OH)2; and M is the quantity, in mg, of Mg(OH)2 in the specimen tested, based on the labeled quantity.
Soluble alkalies— Centrifuge about 50 mL of Milk of Magnesia. Dilute 5.0 mL of the clear supernatant with 40 mL of water. Add 1 drop of methyl red TS, and titrate the solution with 0.10 N sulfuric acid to the production of a persistent pink color: not more than 1.0 mL of the acid is required. Where the specimen is Double- or Triple-Strength Milk of Magnesia, not more than 2.0 or 3.0 mL of the acid is required, respectively.
Carbonate and acid-insoluble matter— To the equivalent of 1 g of regular-strength Milk of Magnesia add 2 mL of 3 N hydrochloric acid: not more than a slight effervescence occurs, and the solution is not more than slightly turbid.
Limit of calcium—
Dilute hydrochloric acid, Lanthanum solution, Standard preparations, and Blank solution —Proceed as directed in the test for Limit of calcium under Magnesium Carbonate.
Test preparation— Transfer the equivalent of 1.4 g of regular-strength Milk of Magnesia to a beaker, add 60 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. Calculate the percentage of calcium in the Milk of Magnesia taken by multiplying the concentration, in µg per mL, of calcium found in the Test preparation by 0.014: the limit is 0.07%.
Residual solvents 467: meets the requirements.
(Official January 1, 2007)
Assay— Transfer an accurately measured quantity of Milk of Magnesia, previously shaken in its original container, equivalent to about 800 mg of magnesium hydroxide, to a 250-mL volumetric flask. Dissolve in 30 mL of 3 N hydrochloric acid, dilute with water to volume, and mix. Filter, if necessary, and transfer 25.0 mL of the filtrate to a beaker containing 75 mL of water, and mix. Adjust the reaction of the solution with 1 N sodium hydroxide to a pH of 7 (using pH indicator paper; see Indicator and Test Papers under Reagents, in the section Reagents, Indicators, and Solutions), add 5 mL of ammonia–ammonium chloride buffer TS and 0.15 mL of eriochrome black TS, and titrate with 0.05 M edetate disodium VS to a blue endpoint. Each mL of 0.05 M edetate disodium is equivalent to 2.916 mg of Mg(OH)2.
Auxiliary Information— Staff Liaison : Elena Gonikberg, Ph.D., Scientist
Expert Committee : (MDGRE05) Monograph Development-Gastrointestinal Renal and Endocrine
USP29–NF24 Page 1291
Pharmacopeial Forum : Volume No. 27(2) Page 2171
Phone Number : 1-301-816-8251