Selenious Acid
(se lee' nee us as' id).

H2SeO3 128.97
Selenium dioxide, monohydrated;    
Selenious acid     [7783-00-8].
DEFINITION
Selenious Acid contains NLT 93.0% and NMT 101.0% of H2SeO3.
IDENTIFICATION
•  A.
Sample:  50 mg
Analysis:  Dissolve the Sample in 5 mL of water. Add 100 mg of sodium bicarbonate, and mix.
Acceptance criteria:  Gas bubbles develop.
•  B.
Sample:  50 mg
Analysis:  Dissolve the Sample in 5 mL of 0.1 N hydrochloric acid. Add 50 mg of stannous chloride.
Acceptance criteria:  A curdy tan-orange precipitate is formed.
ASSAY
•  Procedure
Sample solution:  Transfer 100 mg to a suitable glass-stoppered flask, and dissolve in 50 mL of water.
Analysis:  Add 10 mL of a 300-mg/mL potassium iodide solution and 5 mL of hydrochloric acid to the Sample solution, and mix. Insert the stopper in the flask, and allow to stand for 10 min. Add 50 mL of water and 3 mL of starch TS. 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.
Acceptance criteria:  93.0%–101.0%
IMPURITIES
•  Residue on Ignition 281
Sample:  10 g
Acceptance criteria:  NMT 1.0 mg (0.01%)
•  Insoluble Matter
Sample:  1 g
Analysis:  Dissolve the Sample in 5 mL of water.
Acceptance criteria:  It dissolves completely, and the solution is clear.
•  Selenate and Sulfate
Sample solution:  Dissolve 0.5 g in 10 mL of water.
Analysis:  Add 0.1 mL of hydrochloric acid and 1 mL of barium chloride TS to the Sample solution, and mix.
Acceptance criteria:  No turbidity or precipitate is formed in 10 min.
ADDITIONAL REQUIREMENTS
•  Packaging and Storage: Preserve in tight containers.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Elena Gonikberg, Ph.D.
Principal Scientific Liaison
1-301-816-8251
(SM32010) Monographs - Small Molecules 3
USP35–NF30 Page 4623
Pharmacopeial Forum: Volume No. 29(5) Page 1571