Selenious Acid
H2SeO3 128.97

Selenium dioxide, monohydrated.
Selenious acid [7783-00-8].
» Selenious Acid contains not less than 93.0 percent and not more than 101.0 percent of H2SeO3.
Packaging and storage— Preserve in tight containers.
A: Dissolve about 50 mg in 5 mL of water, add about 100 mg of sodium bicarbonate, and mix: gas bubbles develop.
B: Dissolve about 50 mg in 5 mL of 0.1 N hydrochloric acid, and add about 50 mg of stannous chloride: a curdy tan-orange precipitate is formed.
Residue on ignition 281: not more than 1.0 mg, from 10.0 g (0.01%).
Insoluble matter— Dissolve 1 g in 5 mL of water: it dissolves completely and the solution is clear.
Selenate and sulfate— Dissolve about 0.5 g in 10 mL of water, add 0.1 mL of hydrochloric acid and 1 mL of barium chloride TS, and mix: no turbidity or precipitate is formed in 10 minutes.
Assay— Transfer about 100 mg of Selenious Acid, accurately weighed, to a suitable 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. Add 50 mL of water and 3 mL of starch TS, and titrate with 0.1 N sodium thiosulfate VS until the solution is colorless, then titrate with 0.1 N iodine VS to a blue endpoint. Subtract the volume of 0.1 N iodine from the volume of 0.1 N sodium thiosulfate to obtain the volume of 0.1 N sodium thiosulfate equivalent to selenious acid. Each mL of 0.1 N sodium thiosulfate is equivalent to 3.225 mg of H2SeO3.
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Topic/Question Contact Expert Committee
Monograph Elena Gonikberg, Ph.D.
Senior Scientist
(MDGRE05) Monograph Development-Gastrointestinal Renal and Endocrine
USP32–NF27 Page 3547
Pharmacopeial Forum: Volume No. 29(5) Page 1571
Chromatographic Column—
Chromatographic columns text is not derived from, and not part of, USP 32 or NF 27.