Magnesium Hydroxide Paste
»Magnesium Hydroxide Paste is an aqueous paste of magnesium hydroxide,each 100g of which contains not less than 29.0g and not more than 33.0g of magnesium hydroxide [Mg(OH)2].
Packaging and storage— Preserve in tight containers.
Identification— One g of Paste dissolved in 10mLof 3Nhydrochloric acid responds to the tests for Magnesium á191ñ.
Microbial limits á61ñ— Its total aerobic microbial count does not exceed 400cfu per g,and it meets the requirements of the test for absence of Escherichia coli.
Soluble alkalies— Mix 25.0g of Paste with 75.0mLof water,transfer about 25mLof this diluted Paste to a filter,and reject the first 5mLof the filtrate.[NOTE—Retain the remaining diluted Paste for the tests for Carbonate and acid-insoluble matter,Arsenic,and Heavy metals.]Dilute 5mLof the clear filtrate 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.
Soluble salts— To 5.0mLof the clear filtrate obtained in the test for Soluble alkaliesadd 3drops of sulfuric acid,evaporate on a steam bath to dryness,and ignite gently to constant weight:the residue weighs not more than 12mg.
Carbonate and acid-insoluble matter— To 1mLof the diluted Paste obtained in the test for Soluble alkaliesadd 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—Prepare as directed in the test for Limit of calciumunder Magnesium Carbonate.
Test preparation— Transfer a portion of the Paste,equivalent to 250mg of Mg(OH)2,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— To 4.0mLof the diluted Paste obtained in the test for Soluble alkaliesadd 6mLof 3Nhydrochloric acid,and evaporate the solution on a steam bath to dryness,with frequent stirring.Dissolve the residue in 20mLof water,and evaporate to dryness in the same manner as before.Redissolve in 20mLof water,filter if necessary,and dilute with water to 25mL:the limit is 5ppm,based on the amount of diluted Paste taken.
Assay— Transfer an accurately weighed portion of Paste,equivalent to about 250mg of magnesium hydroxide,to a 100-mLvolumetric flask.Dissolve in 10mLof 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 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 magnesium hydroxide [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