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.
Identification
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.
Tests
Residue on ignition: 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, 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, and titrate with 0.1 N sodium thiosulfate 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.
