Hypophosphorous Acid
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This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition
Issued and maintained by the United States Pharmacopeial Convention (USP)
H₃PO₂ 66.00
Phosphinic acid;
Hypophosphorous acid CAS RN®: 6303-21-5.
1 DEFINITION
Hypophosphorous Acid contains NLT 30.0% and NMT 32.0% of H₃PO₂.
2 IDENTIFICATION
A. Hypophosphorous Acid yields a white precipitate with mercuric chloride TS. This precipitate becomes gray when an excess of hypophosphite is present.
B. Hypophosphorous Acid acidified with sulfuric acid and warmed with cupric sulfate TS yields a red precipitate.
3 ASSAY
3.1 Procedure
Sample solution: Pour 7 mL of Hypophosphorous Acid into a tared, glass-stoppered flask, and weigh. Dilute with about 25 mL of water, and add phenolphthalein TS.
Analysis: Titrate with 1 N sodium hydroxide VS. Each mL of 1 N sodium hydroxide is equivalent to 66.00 mg of H₃PO₂.
Acceptance criteria: 30.0%–32.0%
4 SPECIFIC TESTS
4.1 Limit of Barium and Oxalate
Sample solution: Hypophosphorous Acid and water (1:3)
Analysis 1: Neutralize 30 mL of the Sample solution with 6 N ammonium hydroxide: the mixture exhibits little or no precipitation. Filter, acidify 10 mL of the filtrate with hydrochloric acid, and add 2 mL of potassium sulfate TS.
Acceptance criteria 1: No turbidity is produced (barium).
Analysis 2: To a 10-mL portion of the filtrate obtained in Analysis 1, add 1 mL of calcium chloride TS.
Acceptance criteria 2: The filtrate shows no turbidity upon the addition of the calcium chloride TS (oxalate).
5 ADDITIONAL REQUIREMENTS
Packaging and Storage: Preserve in tight containers.
