Sodium Arsenite

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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NaAsO₂ 129.91 CAS RN®: 7784-46-5.—White, crystalline powder. Soluble in water; slightly soluble in alcohol.

 Assay: Transfer about 5.5 g, accurately weighed, to a 500-mL volumetric ￾ask, dissolve in and dilute with water to volume, and mix. Pipet 25
mL of this solution into a suitable container, add 50 mL of water and 5 g of dibasic sodium phosphate, swirl to dissolve, and titrate with 0.1 N
iodine VS, adding 3 mL of starch TS as the endpoint is approached. Each mL of 0.1 N iodine is equivalent to 3.746 mg of As. Between 57.0%
and 60.5% is found (equivalent to 98.8% to 104.9% of NaAsO₂ ).

Chloride (Reagent test): One g shows not more than 0.10 mg of Cl (0.01%).

Heavy Metals: Dissolve 200 mg in 8 mL of dilute hydrochloric acid (3 in 8), and evaporate on a steam bath to dryness. Dissolve the residue in 5
mL of dilute hydrochloric acid (2 in 5), and again evaporate to dryness. Dissolve the residue in 10 mL of water, and add 2 mL of diluted acetic
acid and 10 mL of hydrogen sul￾de TS. Any brown color produced is not darker than that of a control containing 0.01 mg of added Pb (0.005%).

Iron: Dissolve 1 g in 20 mL of dilute hydrochloric acid (1 in 5), and add, dropwise, a slight excess of bromine TS. Boil the solution to remove
the excess bromine, cool, dilute with water to 40 mL, and add 10 mL of ammonium thiocyanate solution (3 in 10). Any red color produced is not
darker than that of a control containing 0.02 mg of added Fe (0.02%).

Sulfide: Dissolve 1 g in 20 mL of water, and add 5 drops of lead acetate TS: no brown color is produced (about 0.0005%).

Sulfate (Reagent test, Method II): Dissolve 5 g in 100 mL of water, add methyl orange TS, neutralize with 1 N hydrochloric acid, add 3 mL
of the acid in excess, and filter: the filtrate yields not more than 3 mg of residue (0.02%),