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 90.0percent and not more than 115.0percent of the labeled amount of Mg(OH)2,the labeled amount being 80,160,and 240mg of Mg(OH)2per mL,respectively.It may contain not more than 0.05percent 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— Asolution of the equivalent of 1g of regular-strength Milk of Magnesia in 2mLof 3Nhydrochloric acid meets the requirements of the tests for Magnesium á191ñ.
Microbial limits á61ñ— Its total aerobic microbial count does not exceed 100cfu per mL,and it meets the requirements of the test for absence of Escherichia coli.
Acid-neutralizing capacity á301ñ— Not less than 5mEq 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:
0.8(0.0343M),
in which 0.0343is the theoretical acid-neutralizing capacity,in mEq,of Mg(OH)2;andMis the quantity,in mg,of Mg(OH)2in the specimen tested,based on the labeled quantity.
Soluble alkalies— Centrifuge about 50mLof Milk of Magnesia.Dilute 5.0mLof the clear supernatant with 40mLof water.Add 1drop of methyl red TS,and titrate the solution with 0.10Nsulfuric acid to the production of a persistent pink color:not more than 1.0mLof the acid is required.Where the specimen is Double-or Triple-Strength Milk of Magnesia,not more than 2.0or 3.0mLof the acid is required,respectively.
Carbonate and acid-insoluble matter— To the equivalent of 1g of regular-strength Milk of Magnesia add 2mLof 3Nhydrochloric 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 calciumunder Magnesium Carbonate.
Test preparation— Transfer the equivalent of 1.4g of regular-strength Milk of Magnesia to a beaker,add 60mLof Dilute hydrochloric acid,and stir until dissolved,heating if necessary.Transfer the solution so obtained to a 200-mLvolumetric flask containing 4mLof Lanthanum solution,dilute with water to volume,and mix.
Procedure— Proceed as directed in the test for Limit of calciumunder 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 preparationby 0.014:the limit is 0.07%.
Assay— Transfer an accurately measured quantity of Milk of Magnesia,previously shaken in its original container,equivalent to about 800mg of magnesium hydroxide,to a 250-mLvolumetric flask.Dissolve in 30mLof 3Nhydrochloric acid,dilute with water to volume,and mix.Filter,if necessary,and transfer 25.0mLof the filtrate to a beaker containing 75mLof water,and mix.Adjust the reaction of the solution with 1Nsodium hydroxide to a pHof 7(using pHindicator paper;see Indicator and Test Papersunder Reagents,in the section Reagents,Indicators,and Solutions),add 5mLof ammonia–ammonium chloride buffer TSand 0.15mLof eriochrome black TS,and titrate with 0.05Medetate disodium VSto a blue endpoint.Each mLof 0.05Medetate disodium is equivalent to 2.916mg of Mg(OH)2.
Auxiliary Information— Staff Liaison:Elena Gonikberg,Ph.D.,Scientist
Expert Committee:(PA4)Pharmaceutical Analysis 4
USP28–NF23Page 1166
Pharmacopeial Forum:Volume No.27(2)Page 2171
Phone Number:1-301-816-8251