Selenious Acid
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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)
(Selenous Acid), H2SeO3 128.97 CAS RN: 7783-00-8.-Colorless or white crystals, efflorescent in dry air and hygroscopic in moist air. Soluble in water and in alcohol.
1 Assay
Accurately weigh about 100 mg, transfer to a glass-stoppered flask, and dissolve in 50 mL of water. Add 10 mL of potassium iodide solution (3 in 10) and 5 mL of hydrochloric acid, mix, insert the stopper in the flask, and allow to stand for 10 minutes. Dilute with 50 mL of water, add 3 mL of starch TS, and titrate with 0.1 N sodium thiosulfate VS until the color is no longer diminished, then titrate with 0.1 N iodine VS to a blue color. Subtract the volume of 0.1 N iodine solution from the volume of 0.1 N sodium thiosulfate to give the volume of 0.1 N thiosulfate equivalent to selenious acid. Each mL of 0.1 N sodium thiosulfate is equivalent to 3.225 mg of H2SeO3: not less than 93% is found.
2 Insoluble Matter
Dissolve 1 g in 5 mL of water, the solution is clear and complete.
3 Residue on Ignition (Reagent test)
not more than 1.0 mg (0.01%), from 10 g.
4 Selenate and Sulfate
Dissolve 500 mg in 10 mL of water, and add 0.1 mL of hydrochloric acid and 1 mL of barium chloride TS: no turbidity or precipitate is formed within 10 minutes.
