(mag nee' zee um hye drox' ide).
Magnesium hydroxide [1309-42-8].
» Magnesium Hydroxide, dried at 105 for 2 hours, contains not less than 95.0 percent and not more than 100.5 percent of Mg(OH)2.
Packaging and storage Preserve in tight containers.
Identification A 1 in 20 solution in 3 N hydrochloric acid responds to the tests for Magnesium 191.
Microbial enumeration tests 61 and Tests for specified microorganisms 62 It meets the requirements of the test for absence of Escherichia coli.
Loss on drying 731 Dry it at 105 for 2 hours: it loses not more than 2.0% of its weight.
Loss on ignition 733 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 2.0 g with 100 mL of water for 5 minutes in a covered beaker, filter while hot, cool, and dilute the filtrate with water to 100 mL. Titrate 50 mL of the diluted filtrate with 0.10 N sulfuric acid, using methyl red TS as the indicator: not more than 2.0 mL of the acid is consumed. Evaporate 25 mL of the diluted filtrate to dryness, and dry at 105 for 3 hours: not more than 10 mg of residue remains.
Carbonate Boil a mixture of 0.10 g with 5 mL of water, cool, and add 5 mL of 6 N acetic acid: not more than a slight effervescence is observed.
Limit of calcium
Dilute hydrochloric acid , Lanthanum solution, Standard preparations, and Blank solutionPrepare as directed in the test for Limit of calcium under Magnesium Carbonate.
Test preparation Transfer 250 mg of Magnesium Hydroxide, previously dried, to a beaker, add 30 mL of Dilute hydrochloric acid, and stir until dissolved, heating if necessary. Transfer the solution so obtained to a 200-mL volumetric flask containing 4 mL of Lanthanum solution, dilute with water to volume, and mix.
Heavy metals, Method I 231 Dissolve 1.0 g in 15 mL of 3 N hydrochloric 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 20 mL of water, and filter. To the filtrate, which should be neutral to litmus, add 2 mL of 1 N acetic acid, and dilute with water to 25 mL: the limit is 20 µg per g.
Limit of lead [noteWhen water is specified as a diluent, use deionized ultra-filtered water. ]
Blank solution Transfer 3.0 mL of nitric acid to a 50-mL volumetric flask, and dilute with water to volume.
Thallium internal standard 20 ppb [noteUse this solution only if an ICPMS instrument is used. This internal standard is added in-line via a mixing block between the sample probe and the spray chamber. ] Dilute 20.0 mL of a commercially prepared thallium ICP standard solution (1000 ppb) with water to 1 L.
Dilute nitric acid Dilute 2.0 mL of nitric acid with water to 100 mL.
Standard stock solution 100 ppb Prepare this solution fresh every two months. Quantitatively dilute an accurately measured volume of a commercially prepared lead ICP standard (1000 ppm) with Dilute nitric acid to obtain a solution containing 10 ppm of lead. Further dilute this solution with Dilute nitric acid to obtain a solution containing 1000 ppb of lead. Transfer 10.0 mL of this solution to a separate 100-mL volumetric flask, add 2.0 mL of nitric acid, and dilute with water to volume.
Standard solutions Prepare these solutions fresh weekly. [noteThe concentrations specified below are recommended if an ICPMS instrument is used. If an ICPAES instrument is used, the concentrations of the Standard solutions may be modified to adapt to the working range of the instrument. ] Transfer 5.0 mL of the Standard stock solution 100 ppb to a 50-mL volumetric flask, add 3.0 mL of nitric acid, and dilute with water to volume (Standard lead solution 10 ppb). Transfer 5.0 mL of Standard lead solution 10 ppb to a 50-mL volumetric flask, add 3.0 mL of nitric acid, and dilute with water to volume (Standard lead solution 1 ppb).
Test solution [noteThe concentration specified below is recommended if an ICPMS instrument is used. If an ICPAES instrument is used, the concentration of the Test solution may be modified to adapt to the working range of the instrument. ] Accurately weigh about 0.25 g of Magnesium Hydroxide. Cautiously add 3.0 mL of nitric acid, and mix until the sample is dissolved. Accurately transfer this solution to a 50-mL volumetric flask, and dilute with water to volume.
Procedure (see Plasma Spectrochemistry 730) The inductively coupled plasmamass spectrometer (ICPMS) is equipped with a quadrupole mass spectrometer and an ion detector maintained under vacuum. The instrument should read all isotopes for lead (206, 207, and 208 amu) and the thallium internal standard (205 amu), and should report the total lead content using the most naturally abundant isotope at 208 amu. Alternatively, lead could be determined using an inductively coupled plasmaatomic emission spectrometer (ICPAES) by measuring the emission at 220.353 nm, with the settings optimized as directed by the manufacturer. [noteTo minimize matrix interference when using an ICPAES instrument, it is recommended that the method of standard additions be used. ]
Instrument performance must be verified to conform to the manufacturer's specifications for resolution and sensitivity. Before analyzing samples, the instrument must pass a suitable performance check. Generate the calibration curve using the Blank solution, Standard lead solution 1 ppb, and Standard lead solution 10 ppb: a linear regression coefficient is not less than 0.999.
Aspirate the Test solution, at least in duplicate, and calculate the amount of lead using the calibration curve. Report the average reading as the lead content of the sample. Calculate the content of lead in the portion of Magnesium Hydroxide taken: not more than 0.00015% (1.5 ppm) is found.
Assay Transfer about 75 mg of Magnesium Hydroxide, previously dried and accurately weighed, to a conical flask. Add 2 mL of 3 N hydrochloric acid, and swirl to dissolve. Add 100 mL of water, adjust the reaction of the solution to a pH of 7 (using pH indicator paper; see Indicator and Test Papers under Reagents in the section Reagents, Indicators, and Solutions) with 1 N sodium hydroxide, add 5 mL of ammonia-ammonium chloride buffer TS and 0.15 mL of eriochrome black TS, and titrate with 0.05 M edetate disodium VS to a blue endpoint. Each mL of 0.05 M edetate disodium is equivalent to 2.916 mg of Mg(OH)2.
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USP35NF30 Page 3749Pharmacopeial Forum: Volume No. 32(4) Page 1087