Selenious Acid

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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1 DEFINITION

Selenious Acid contains NLT 93.0% and NMT 101.0% of H₂SeO₃.

2 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.

3 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%

4 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.

5 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers.