| IUPAC name
| Other names
|Molar mass||66.00 g/mol|
|Appearance||Colorless, deliquescent crystals or oily liquid|
|Density|| 1.493 g/cm3|
1.22 g/cm3 (50 wt% aq. solution)
|Melting point||26.5 °C (79.7 °F; 299.6 K)|
|Boiling point||130 °C (266 °F; 403 K) (decomposes)|
|Solubility||Very soluble in alcohols, diethyl ether|
|Vapor pressure||~0 mmHg|
|Safety data sheet||Sigma-Aldrich (50%)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Hypophosphorous acid (HPA) or phosphinic acid, is a phosphorus oxyacid and a powerful reducing agent with molecular formula H3PO2. The formula for this acid is generally written H3PO2, but a more descriptive presentation is HOP(O)H2, which highlights its monoprotic character. HPA sometimes exists in equilibrium with the minor tautomer HP(OH)2.
Hypophosphorous acid can reduce chromium(III) oxide to chromium(II) oxide:
- H3PO2 + 2 Cr2O3 → 4 CrO + H3PO4
Hypophosphorous acid is a colorless, deliquescent solid at standard conditions, that readily melts above 26.5 °C and due to its strong hygroscopicity, it cannot be solidified again without decomposition in open air.
It is sold by chemical suppliers, as 50% aq. solution.
Because hypophosphorous acid can reduce elemental iodine to form hydroiodic acid, which is a reagent effective for reducing ephedrine or pseudoephedrine to methamphetamine, the United States Drug Enforcement Administration designated hypophosphorous acid (and its salts) as a List I precursor chemical and thus cannot be acquired by the hobby chemist.
White phosphorus is carefully added to hot aqueous solution of an alkaline hydroxide, like NaOH or even Ca(OH)2. This produces hypophosphite salts.
- P4 + 4 OH− + 4 H2O → 4 H
2 + 2 H2
The hypophosphite salt is then treated with a strong, non-oxidizing acid to give the free hypophosphorous acid:
2 + H+ → HOP(O)H2
Anhydrous acid cannot be obtained by simple evaporation of the water, as the acid ready oxidizes to phosphorous acid and phosphoric acid, as well as disproportionates to phosphorous acid and phosphine.
Pure anhydrous hypophosphorous acid however, can be obtained by the continuous extraction of its aqueous solution with diethyl ether.
- Reducing agent
- Reduction of arenediazonium salts to arenes
- Make chromium(II) oxide
- Make metal complexes
- Electroless nickel plating
HPA is a mid-strength acid, irritant and corrosive. Reduction may generate phosphine, which is very toxic and flammable.
In air-tight plastic bottles, away from light and oxidizers.
Should be oxidized to phosphoric acid by careful oxidation in aq. solution. Diluted hydrogen peroxide can be used for this task.