Sodium Selenite,
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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)
Na2SeO3 172.94 CAS RN®: 10102-18-8.-White, odorless, crystalline powder, usually partially hydrated. Freely soluble in water; insoluble in alcohol.
Assay: Accurately weigh about 180 mg, previously dried at 120 to constant weight, and dissolve it in 50 ml of water in a glass-stoppered flask. Add, successively, 3 g of potassium iodide and then 5 mL of hydrochloric acid, insert the stopper, and allow to stand for 10 minutes. Add 50 mL of water, 50.0 mL of 0.1 N sodium thiosulfate VS, and 3 mL of starch TS, and immediately titrate with 0.1 N iodine VS to a blue color. Perform a blank determination. The difference in volumes of 0.1 N iodine is equivalent to 4.323 mg of Na2SeO3. Between 98% and 101% is found.
Solubility: One g in 10 mL of water shows not more than a faint haze.
Carbonate: Το 500 mg add 1 mL of water and 2 mL of diluted hydrochloric acid: no effervescence is produced.
Chloride (Reagent test): A 500-mg portion shows not more than 0.05 mg of Cl (0.01%).
Nitrate (Reagent test): A 200-mg portion dissolved in 3 mL of water shows not more than 0.02 mg of NO3 (0.01%).
Selenate and Sulfate (as SO4): To 500 mg in a small evaporating dish add 20 mg of sodium carbonate and 10 mL of hydrochloric acid. Slowly evaporate the solution on a steam bath under a hood to dryness. Wash the sides of the dish with 5 mL of hydrochloric acid, and again evaporate to dryness. Dissolve the residue in a mixture of 15 mL of hot water and 1 mL of hydrochloric acid, Proceed as directed under Sulfate in Reagents (Reagent test, Method 1), beginning with "Filter the solution." The test specimen shows no more turbidity than that produced by 0.15 mg of SO4 (0.03%).
