Hypophosphorous Acid
(hye'' poe fos' for us as' id).

H3PO2 66.00
Phosphinic acid;    
Hypophosphorous acid     [6303-21-5].
Hypophosphorous Acid contains NLT 30.0% and NMT 32.0% of H3PO2.
•  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.
•  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%
Inorganic Impurities 
•  Heavy Metals, Method II 231
Analysis:  Place 0.90 mL (1 g) of Hypophosphorous Acid in a small beaker, and add 3 mL of water. Add 1 mL of nitric acid, and evaporate on a steam bath to about 1 mL. Again add 1 mL of nitric acid, and evaporate on a steam bath. Dissolve the residue in 3 mL of water, add 6 N ammonium hydroxide until the solution is distinctly alkaline to litmus, then boil gently until the odor of ammonia disappears. Add 2 mL of 1 N acetic acid and 15 mL of warm water, filter, and dilute the filtrate with water to 25 mL.
Acceptance criteria:  NMT 20 ppm
•  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).
•  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 Robert H. Lafaver, M.S.
Scientific Liaison
(EXC2010) Monographs - Excipients
USP35–NF30 Page 1824
Pharmacopeial Forum: Volume No. 36(4) Page 947