Hypophosphorous acid

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Hypophosphorous acid
Names
IUPAC name
Phosphinic acid
Other names
Hydroxy(oxo)-λ5-phosphane
Hydroxy-λ5-phosphanone
Oxo-λ5-phosphanol
Oxo-λ5-phosphinous acid
Properties
H3PO2
HOP(O)H2
Molar mass 66.00 g/mol
Appearance Colorless, deliquescent crystals or oily liquid
Odor Odorless
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)
Miscible
Solubility Very soluble in alcohols, diethyl ether
Vapor pressure ~0 mmHg
Acidity (pKa) 1.2
Hazards
Safety data sheet Sigma-Aldrich (50%)
Flash point Non-flammable
Related compounds
Related compounds
Phosphine
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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.

Properties

Chemical

Hypophosphorous acid can reduce chromium(III) oxide to chromium(II) oxide:

H3PO2 + 2 Cr2O3 → 4 CrO + H3PO4

Physical

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.

Availability

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.

Preparation

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
PO
2
+ 2 H2

The hypophosphite salt is then treated with a strong, non-oxidizing acid to give the free hypophosphorous acid:

H
2
PO
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.

Projects

  • Reducing agent
  • Reduction of arenediazonium salts to arenes
  • Make chromium(II) oxide
  • Make metal complexes
  • Electroless nickel plating

Handling

Safety

HPA is a mid-strength acid, irritant and corrosive. Reduction may generate phosphine, which is very toxic and flammable.

Storage

In air-tight plastic bottles, away from light and oxidizers.

Disposal

Should be oxidized to phosphoric acid by careful oxidation in aq. solution. Diluted hydrogen peroxide can be used for this task.

References

Relevant Sciencemadness threads