Magnesium Hydroxide
Mg(OH)2 58.32

Magnesium hydroxide.
Magnesium hydroxide [1309-42-8].
»Magnesium Hydroxide,dried at 105for 2hours,contains not less than 95.0percent and not more than 100.5percent of Mg(OH)2.
Packaging and storage— Preserve in tight containers.
Identification— A1in 20solution in 3Nhydrochloric acid responds to the tests for Magnesium á191ñ.
Microbial limits á61ñ— It meets the requirements of the test for absence of Escherichia coli.
Loss on drying á731ñ— Dry it at 105for 2hours:it loses not more than 2.0%of its weight.
Loss on ignition á733ñ— Ignite it at 800,increasing the heat gradually,to constant weight:it loses between 30.0%and 33.0%of its weight.
Soluble salts— Boil 2.0g with 100mLof water for 5minutes in a covered beaker,filter while hot,cool,and dilute the filtrate with water to 100mL.Titrate 50mLof the diluted filtrate with 0.10Nsulfuric acid,using methyl red TSas the indicator:not more than 2.0mLof the acid is consumed.Evaporate 25mLof the diluted filtrate to dryness,and dry at 105for 3hours:not more than 10mg of residue remains.
Carbonate— Boil a mixture of 0.10g with 5mLof water,cool,and add 5mLof 6Nacetic acid:not more than a slight effervescence is observed.
Limit of calcium—
Dilute hydrochloric acid ,Lanthanum solution,Standard preparations,and Blank solution—Prepare as directed in the test for Limit of calciumunder Magnesium Carbonate.
Test preparation— Transfer 250mg of Magnesium Hydroxide,previously dried,to a beaker,add 30mLof 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:the limit is 1.5%.
Heavy metals,Method Iá231ñ— Dissolve 1.0g in 15mLof 3Nhydrochloric acid,and evaporate the solution on a steam bath to dryness.Toward the end of the evaporation,stir the residue frequently,disintegrate it so that finally a dry powder is obtained,dissolve the residue in 20mLof water,and filter.To the filtrate,which should be neutral to litmus,add 2mLof 1Nacetic acid,and dilute with water to 25mL:the limit is 20µg per g.
Lead á251ñ— Prepare a Test Preparationby dissolving 1g of Magnesium Hydroxide in 20mLof 3Nhydrochloric acid.Use 10mLof Diluted Standard Lead Solution(10µg of Pb)for the test:the limit is 0.001%.
Assay— Transfer about 75mg of Magnesium Hydroxide,previously dried and accurately weighed,to a conical flask.Add 2mLof 3Nhydrochloric acid,and swirl to dissolve.Add 100mLof water,adjust the reaction of the solution to a pHof 7(using pHindicator paper;see Indicator and Test Papersunder Reagentsin the section Reagents,Indicators,and Solutions)with 1Nsodium hydroxide,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 1171
Phone Number:1-301-816-8251