A: Place about 1g of finely powdered Tablets in a flask equipped with a stopper and glass tubing,the tip of which is immersed in calcium hydroxide TSin a test tube.Add 5mLof 3Nhydrochloric acid to the flask,and immediately insert the stopper:gas evolves in the flask and a precipitate is formed in the test tube.

B: To a 7-g portion of finely powdered Tablets add 10mLof 3Nhydrochloric acid and 5drops of methyl red TS,heat to boiling,and add 6Nammonium hydroxide until the color of the solution changes to deep yellow.Continue boiling for 2minutes,and filter:the filtrate responds to the tests for Magnesium á191ñ.

C: Wash the precipitate obtained in Identificationtest Bwith a hot solution of ammonium chloride (1in 50),and dissolve the precipitate in hydrochloric acid:the solution responds to the tests for Aluminum á191ñ.

Potassium chloride solution— Prepare a solution containing 38.1g of potassium chloride in each 1000mL.

Digestion fluid— Mix 5mLof hydrochloric acid,10mLof nitric acid,and 10mLof water,and use promptly.

Aluminum stock solution— Transfer 1.000g of aluminum metal to a 1000-mLvolumetric flask,and add 50mLof 6Nhydrochloric acid.Swirl to ensure contact of the aluminum and the acid,and allow the reaction to proceed until all of the aluminum has dissolved.Dilute with water to volume,and mix.

Standard preparations— To separate 100-mLvolumetric flasks transfer 3.0mL,4.0mL,and 5.0mLof Aluminum stock solution,respectively.To each flask add 10mLof Potassium chloride solutionand 7.5mLof Digestion fluid,dilute with water to volume,and mix.These Standard preparationscontain 30,40,and 50µg of aluminum per mL,respectively.

Assay preparation— Weigh and finely powder not fewer than 20Tablets.Transfer an accurately weighed portion of the powder,equivalent to about 30mg of aluminum hydroxide,to a 100-mLvolumetric flask,add 25-mLof Digestion fluid,and heat on a steam bath for 30minutes or on a hot plate until the volume is reduced by about one-half.Cool,dilute with water to volume,and mix.Filter,discarding the first 20mLof the filtrate.Transfer 15.0mLof the filtrate to a 50-mLvolumetric flask,add 5.0mLof Potassium chloride solution,dilute with water to volume,and mix.[NOTE—Reserve a portion of the filtrate for use in the Assay for magnesium carbonate.]

Procedure— Concomitantly determine the absorbances of the Standard preparationsand the Assay preparationat the aluminum emission line at 309.3nm,with a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-scattering á851ñ)equipped with an aluminum hollow-cathode lamp and a nitrous oxide–acetylene flame,using water as the blank.Plot the absorbances of the Standard preparationsversus concentration,in µg per mL,of aluminum,and draw the straight line best fitting the three plotted points.From the graph so obtained,determine the concentration,C,in µg per mL,of aluminum in each mLof the Assay preparation.Calculate the quantity,in mg,of aluminum hydroxide [Al(OH)3]in the portion of Tablets taken by the formula: in which 78.00is the molecular weight of aluminum hydroxide,and 26.98is the atomic weight of aluminum.

Lanthanum chloride solution— Transfer 17.6g of lanthanum chloride to a 1000-mLvolumetric flask,add 500mLof water,and carefully add 50mLof hydrochloric acid.Mix,and allow to cool.Dilute with water to volume,and mix.

Digestion fluid— Mix 5mLof hydrochloric acid,10mLof nitric acid,and 10mLof water,and use promptly.

Magnesium stock solution— Transfer 1.000g of magnesium metal to a 1000-mLvolumetric flask containing 50mLof water,and slowly add 10mLof hydrochloric acid.Dilute with water to volume,and mix.Transfer 5.0mLof this solution to a 500-mLvolumetric flask,dilute with water to volume,and mix.

Standard preparations— To separate 100-mLvolumetric flasks transfer 4.0,6.0,and 8.0mLof Magnesium stock solution,respectively.To each flask add 0.5mLof Digestion fluidand 10mLof Lanthanum chloride solution,dilute with water to volume,and mix.These Standard preparationscontain 0.40,0.60,and 0.80µg of magnesium per mL,respectively.

Assay preparation— Transfer an accurately measured volume of the filtrate used to prepare the Assay preparationin the Assay for aluminum hydroxide,equivalent to about 0.4mg of magnesium carbonate,to a 200-mLvolumetric flask,add 20mLof Lanthanum chloride solution,dilute with water to volume,and mix.

Procedure— Concomitantly determine the absorbances of the Standard preparationsand the Assay preparationat the magnesium emission line at 285.2nm,with a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-scattering á851ñ)equipped with a magnesium hollow-cathode lamp and an air–acetylene flame,using water as the blank.Plot the absorbances of the Standard preparationsversus concentration,in µg per mL,of magnesium,and draw the straight line best fitting the three plotted points.From the graph so obtained,determine the concentration,C,in µg per mL,of magnesium in each mLof the Assay preparation.Calculate the quantity,in mg,of magnesium carbonate (MgCO3)in the portion of Tablets taken by the formula: in which 84.31is the molecular weight of magnesium carbonate;24.31is the atomic weight of magnesium;and Vis the volume taken,in mL,of the Assay preparationprepared as directed in the Assay for aluminum hydroxide.