Magnesium Silicate
(mag nee' zee um sil' i kate).
» Magnesium Silicate is a compound of magnesium oxide and silicon dioxide. It contains not less than 15.0 percent of magnesium oxide (MgO) and not less than 67.0 percent of silicon dioxide (SiO2), calculated on the ignited basis.
Packaging and storage— Preserve in well-closed containers.
Identification—
A: Mix about 500 mg with 10 mL of 3 N hydrochloric acid, filter, and neutralize the filtrate to litmus paper with 6 N ammonium hydroxide: the neutralized filtrate responds to the tests for Magnesium 191.
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.
pH 791: between 7.0 and 10.8, determined in a well-mixed aqueous suspension (1 in 10).
Loss on drying 731 Dry it at 105 for 2 hours: it loses not more than 15.0% of its weight. (Retain the dried specimen for the test for Loss on ignition.)
Loss on ignition 733 Ignite the specimen retained from the test for Loss on drying at 900 to 1000 for 20 minutes: the previously dried specimen loses not more than 15% of its weight.
Soluble salts— Boil 10.0 g with 150 mL of water for 15 minutes. Cool to room temperature, allow the mixture to stand for 15 minutes, filter with the aid of suction, transfer the filtrate to a 200-mL volumetric flask, dilute with water to volume, and mix. Evaporate 50.0 mL of this solution, representing 2.5 g of the Silicate, in a tared platinum dish to dryness, and ignite gently to constant weight: the weight of the residue does not exceed 75.0 mg (3.0%).
Fluoride—
Indicator solution— Prepare a solution containing 100 mg of lanthanum alizarin complexan mixture per mL in 60% isopropyl alcohol. Filter the solution if it is not clear.
Test preparation— Prepare a slurry consisting of 5.0 g of Magnesium Silicate and 45 mL of 0.1 N hydrochloric acid, stir at room temperature for 15 minutes, and pass through a 0.45-µm filter into a 50-mL volumetric flask. Wash the filter with five 1-mL portions of 0.1 N hydrochloric acid, collecting the washings in the flask, dilute with 0.1 N hydrochloric acid to volume, and mix.
Procedure— Transfer 5.0 mL of the Test preparation to a 25-mL volumetric flask, add 5.0 mL of Indicator solution, dilute with water to volume, mix, and allow to stand for 1 hour 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 620 nm, against a blank consisting of 5.0 mL of 0.1 N hydrochloric acid, 5.0 mL of Indicator solution, and 15.0 mL of water. The absorbance is not greater than that produced by 5.0 mL of a solution containing 2.21 µg of sodium fluoride per mL of 0.1 N hydrochloric acid, when treated in the same manner as the Test preparation (10 ppm).
Free alkali— Add 2 drops of phenolphthalein TS to 20 mL of the diluted filtrate prepared in the test for Soluble salts, representing 1 g of Magnesium Silicate: if a pink color is produced, not more than 2.5 mL of 0.1 N hydrochloric acid is required to discharge it.
Lead 251 Dissolve 1.0 g in 20 mL of 3 N hydrochloric acid, evaporate on a steam bath to about 10 mL, dilute with water to about 20 mL, and cool: the limit is 0.001%.
Ratio of SiO2 to MgO— Divide the percentage of SiO2 obtained in the Assay for silicon dioxide by the percentage of MgO obtained in the Assay for magnesium oxide: the quotient obtained is between 2.50 and 4.50.
Heavy metals 231 Boil 4.0 g with a mixture of 50 mL of water and 10 mL of hydrochloric acid for 20 minutes, 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 15 to 20 mL of water, combining the washings with the original filtrate. Add 2 drops of phenolphthalein TS, then add a slight excess of 6 N ammonium hydroxide. Discharge the pink color with dilute hydrochloric acid (1 in 100), then add 8 mL of dilute hydrochloric acid (1 in 100). Dilute with water to 100 mL, and use 25 mL of the solution for the test: the limit is 20 µg per g.
Assay for magnesium oxide— Weigh accurately about 1.5 g, and transfer to a 250-mL conical flask. Add 50.0 mL of 1 N sulfuric acid VS, and digest on a steam bath for 1 hour. Cool to room temperature, add methyl orange TS, and titrate the excess acid with 1 N sodium hydroxide VS. Each mL of 1 N sulfuric acid is equivalent to 20.15 mg of MgO.
Assay for silicon dioxide— Transfer about 700 mg of Magnesium Silicate, accurately weighed, to a small platinum dish. Add 10 mL of 1 N sulfuric acid, and heat on a steam bath to dryness, leaving the dish uncovered. Treat the residue with 25 mL of water, and digest on a steam bath for 15 minutes. 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 30 minutes, cool, and weigh. Moisten the residue with water, and add 6 mL of hydrofluoric acid and 3 drops of sulfuric acid. Evaporate to dryness, ignite for 5 minutes, cool, and weigh: the loss in weight represents the weight of SiO2.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Robert H. Lafaver, M.S.
Scientific Liaison
1-301-816-8335
(EXC2010) Monographs - Excipients
USP35–NF30 Page 1847