2MgO·3SiO2·xH2O(anhydrous) 260.86

Silicic acid (H4Si3O8),magnesium salt (1:2),hydrate.
Magnesium silicate hydrate (Mg2Si3O8·xH2O) [39365-87-2].

Anhydrous [14987-04-3].

Packaging and storage— Preserve in well-closed containers.

Identification—

Water,Method IIIá921ñ— Weigh accurately about 1g in a tared platinum crucible provided with a cover.Gradually apply heat to the crucible at first,then strongly ignite to constant weight:it loses between 17.0%and 34.0%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 Trisilicate,in a tared platinum dish to dryness,and ignite gently to constant weight:the weight of the residue does not exceed 38.0mg (1.5%).

Chloride á221ñ— A20-mLportion of the diluted filtrate prepared in the test for Soluble salts,representing 1g of Magnesium Trisilicate,shows no more chloride than corresponds to 0.75mLof 0.020Nhydrochloric acid (0.055%).

Sulfate— Treat the residue obtained in the test for Soluble saltswith 2mLof hydrofluoric acid,and evaporate on a steam bath to dryness.Mix the residue with water,transfer to a filter,and wash,using approximately 50mLof water for the complete procedure.Heat the filtrate to boiling,and add 0.1mLof hydrochloric acid and 5mLof barium chloride TS.Maintain the mixture near its boiling point for 1hour,filter,wash the precipitate thoroughly with water,dry,and ignite to constant weight:the weight of the residue does not exceed 30mg (0.5%).

Free alkali— Add 2drops of phenolphthalein TSto 20mLof the diluted filtrate prepared in the test for Soluble salts,representing 1g of the Trisilicate:if a pink color is produced,not more than 1.0mLof 0.10Nhydrochloric acid is required to discharge it.

Arsenic,Method Iá211ñ: 8ppm.

Heavy metals á231ñ— Boil 2.67g with a mixture of 50mLof water and 5mLof 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 washing 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 0.003%.

Acid-consuming capacity— Weigh accurately about 200mg into a glass-stoppered,125-mLconical flask.Add 30.0mLof 0.1Nhydrochloric acid VSand 20.0mLof water.Place the flask in a bath maintained at 37,and shake the mixture occasionally during a period of 4hours but leave the mixture undisturbed during the last 15minutes of the heating period.Cool to room temperature.To 25.0mLof the supernatant add methyl red TS,and titrate the excess acid with 0.1Nsodium hydroxide VS.One g of Magnesium Trisilicate,calculated on the anhydrous basis,consumes not less than 140mLand not more than 160mLof 0.10Nhydrochloric acid.

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 Trisilicate,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.

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 2.10and 2.37.

Expert Committee:(PA4)Pharmaceutical Analysis 4

USP28–NF23Page 1176

Phone Number:1-301-816-8251