Packaging and storage—
Preserve in well-closed containers.
Labeling—
Label it to indicate whether it is anhydrous or hydrous.
Water, Method III 
921
—
Dry about 2 g, accurately weighed, at 105
for 4 hours: the anhydrous form loses not more than 0.5%, and the hydrous form between 12.0% and 15.0%, of its weight.
Heavy metals 231—
Dissolve 2.0 g in 10 mL of water, add 1 drop of phenolphthalein TS, and neutralize the solution with hydrochloric acid, added dropwise. Heat the solution to boiling, and again neutralize by the dropwise addition of hydrochloric acid. Cool, and dilute with water to 25 mL: the limit is 0.001%.
Assay—
Transfer the anhydrous sodium carbonate obtained in the test for
Water to a flask with the aid of 50 mL of water, add methyl red TS, and titrate with 1 N sulfuric acid VS. Add the acid slowly, with constant stirring, until the solution becomes faintly pink. Heat the solution to boiling, cool, and continue the titration. Heat again to boiling, and titrate further as necessary until the faint pink color is no longer affected by continued boiling. Each mL of 1 N sulfuric acid is equivalent to 52.99 mg of Na
2CO
3.
