Magnesium Silicate
»Magnesium Silicate is a compound of magnesium oxide and silicon dioxide.It contains not less than 15.0percent of magnesium oxide (MgO)and not less than 67.0percent of silicon dioxide (SiO2),calculated on the ignited basis.
Packaging and storage— Preserve in well-closed containers.
Identification—
A: Mix about 500mg with 10mLof 3Nhydrochloric acid,filter,and neutralize the filtrate to litmus paper with 6Nammonium hydroxide:the neutralized filtrate responds to the tests for Magnesium á191ñ.
B: Prepare a bead by fusing a few crystals of sodium ammonium phosphate on a platinum loop in the flame of a Bunsen burner.Place the hot,transparent bead in contact with Magnesium Silicate,and again fuse:silica floats about in the bead,producing,upon cooling,an opaque bead with a web-like structure.
pHá791ñ: between 7.0and 10.8,determined in a well-mixed aqueous suspension (1in 10).
Loss on drying á731ñ— Dry it at 105for 2hours:it loses not more than 15.0%of its weight.(Retain the dried specimen for the test for Loss on ignition.)
Loss on ignition á733ñ— Ignite the specimen retained from the test for Loss on dryingat 900to 1000for 20minutes:the previously dried specimen loses not more than 15%of its weight.
Soluble salts— Boil 10.0g with 150mLof water for 15minutes.Cool to room temperature,allow the mixture to stand for 15minutes,filter with the aid of suction,transfer the filtrate to a 200-mLvolumetric flask,dilute with water to volume,and mix.Evaporate 50.0mLof this solution,representing 2.5g of the Silicate,in a tared platinum dish to dryness,and ignite gently to constant weight:the weight of the residue does not exceed 75.0mg (3.0%).
Fluoride—
Indicator solution— Prepare a solution containing 100mg of lanthanum alizarin complexan mixture per mLin 60%isopropyl alcohol.Filter the solution if it is not clear.
Test preparation— Prepare a slurry consisting of 5.0g of Magnesium Silicate and 45mLof 0.1Nhydrochloric acid,stir at room temperature for 15minutes,and pass through a 0.45-µm filter into a 50-mLvolumetric flask.Wash the filter with five 1-mLportions of 0.1Nhydrochloric acid,collecting the washings in the flask,dilute with 0.1Nhydrochloric acid to volume,and mix.
Procedure— Transfer 5.0mLof the Test preparationto a 25-mLvolumetric flask,add 5.0mLof Indicator solution,dilute with water to volume,mix,and allow to stand for 1hour in diffuse light at ambient temperature.Determine the absorbance of this solution in a 1-cm cell with a suitable spectrophotometer,at the wavelength of maximum absorbance at about 620nm,against a blank consisting of 5.0mLof 0.1Nhydrochloric acid,5.0mLof Indicator solution,and 15.0mLof water.The absorbance is not greater than that produced by 5.0mLof a solution containing 2.21µg of sodium fluoride per mLof 0.1Nhydrochloric acid,when treated in the same manner as the Test preparation(10ppm).
Free alkali— Add 2drops of phenolphthalein TSto 20mLof the diluted filtrate prepared in the test for Soluble salts,representing 1g of Magnesium Silicate:if a pink color is produced,not more than 2.5mLof 0.1Nhydrochloric acid is required to discharge it.
Lead á251ñ— Dissolve 1.0g in 20mLof 3Nhydrochloric acid,evaporate on a steam bath to about 10mL,dilute with water to about 20mL,and cool:the limit is 0.001%.
Ratio of SiO2to MgO— Divide the percentage of SiO2obtained in the Assay for silicon dioxideby the percentage of MgOobtained in the Assay for magnesium oxide:the quotient obtained is between 2.50and 4.50.
Heavy metals á231ñ— Boil 4.0g with a mixture of 50mLof water and 10mLof hydrochloric acid for 20minutes,adding water to maintain the volume during the boiling.Add ammonium hydroxide until the mixture is only slightly acid to litmus paper.Filter with the aid of suction,and wash with 15to 20mLof water,combining the washings with the original filtrate.Add 2drops of phenolphthalein TS,then add a slight excess of 6Nammonium hydroxide.Discharge the pink color with dilute hydrochloric acid (1in 100),then add 8mLof dilute hydrochloric acid (1in 100).Dilute with water to 100mL,and use 25mLof the solution for the test:the limit is 20µg per g.
Assay for magnesium oxide— Weigh accurately about 1.5g,and transfer to a 250-mLconical flask.Add 50.0mLof 1Nsulfuric acid VS,and digest on a steam bath for 1hour.Cool to room temperature,add methyl orange TS,and titrate the excess acid with 1Nsodium hydroxide VS.Each mLof 1Nsulfuric acid is equivalent to 20.15mg of MgO.
Assay for silicon dioxide— Transfer about 700mg of Magnesium Silicate,accurately weighed,to a small platinum dish.Add 10mLof 1Nsulfuric acid,and heat on a steam bath to dryness,leaving the dish uncovered.Treat the residue with 25mLof water,and digest on a steam bath for 15minutes.Decant the supernatant through an ashless filter paper,with the aid of suction,and wash the residue,by decantation,three times with hot water,passing the washings through the filter paper.Finally transfer the residue to the filter,and wash thoroughly with hot water.Transfer the filter paper and its contents to the platinum dish previously used.Heat to dryness,incinerate,ignite strongly for 30minutes,cool,and weigh.Moisten the residue with water,and add 6mLof hydrofluoric acid and 3drops of sulfuric acid.Evaporate to dryness,ignite for 5minutes,cool,and weigh:the loss in weight represents the weight of SiO2.
Auxiliary Information— Staff Liaison:Catherine Sheehan,B.Sc.,Scientist
Expert Committee:(EMC)Excipients:Monograph Content
USP28–NF23Page 3029
Phone Number:1-301-816-8262