Sodium Sulfate
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This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition
Issued and maintained by the United States Pharmacopeial Convention (USP)
Na2SO4 · 10H2O 322.19
Sulfuric acid disodium salt, decahydrate.
Disodium sulfate decahydrate CAS RN®: 7727-73-3; UNII: 0YPR65R21J.
Anhydrous 142.04 CAS RN®: 7757-82-6; UNII: 36KCS0R750.
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%).
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 retentive, ashless filter paper, wash until free from chloride when tested with silver nitrate TS, and place the filter into a suitable tared crucible. Carefully burn away the paper, and ignite at 800 ± 25° to constant weight. The weight of the barium sulfate so obtained, multiplied by 0.6086, represents its equivalent of Na2SO4.
