Sodium Sulfate
Na2SO4·10H2O 322.20

Sulfuric acid disodium salt, decahydrate.
Disodium sulfate decahydrate [7727-73-3].

Anhydrous 142.04 [7757-82-6].
» Sodium Sulfate contains ten molecules of water of hydration, or is anhydrous. It contains not less than 99.0 percent of Na2SO4, calculated on the dried basis.
Packaging and storage— Preserve in tight containers, preferably at a temperature not exceeding 30.
Labeling— Label it to indicate whether it is the decahydrate or is anhydrous.
Identification— A solution (1 in 20) responds to the tests for Sodium 191 and for Sulfate 191.
Acidity or alkalinity— To 10 mL of a solution, containing the equivalent of 1.0 g of Na2SO4·10H2O in 20 mL of water, add 1 drop of bromothymol blue TS: not more than 0.50 mL of either 0.010 N hydrochloric acid or 0.010 N sodium hydroxide is required to change the color of the solution.
Loss on drying 731 Dry at 105 for 4 hours: the decahydrate loses between 51.0% and 57.0% of its weight, and the anhydrous form loses not more than 0.5% of its weight.
Chloride 221 A portion equivalent to 1.0 g of Na2SO4·10H2O shows no more chloride than corresponds to 0.30 mL of 0.020 N hydrochloric acid (0.02%).
Heavy metals 231 Dissolve a portion containing the equivalent of 2.0 g of Na2SO4·10H2O in 10 mL of water, add 2 mL of 0.1 N hydrochloric acid, then add water to make 25 mL: the limit is 0.001%.
Assay— Weigh accurately a portion of Sodium Sulfate, equivalent to about 400 mg of anhydrous sodium sulfate, dissolve in 200 mL of water, and add 1 mL of hydrochloric acid. Heat to boiling, and gradually add, in small portions and while constantly stirring, an excess of hot barium chloride TS (about 8 mL). Heat the mixture on a steam bath for 1 hour, collect the precipitate of barium sulfate on a tared filtering crucible, wash until free from chloride, dry, ignite, and weigh. The weight of the barium sulfate so obtained, multiplied by 0.6086, represents its equivalent of Na2SO4.
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Monograph Elena Gonikberg, Ph.D.
Senior Scientist
(MDGRE05) Monograph Development-Gastrointestinal Renal and Endocrine
USP32–NF27 Page 3585
Pharmacopeial Forum: Volume No. 34(5) Page 1192