Phosphorous acid is the compound described by the formula H₃PO₃. It is one of the oxoacids of phosphorus, other important members being phosphoric acid (H₃PO₄) and hypophosphorous acid (H₃PO₂). Note that only the reduced phosphorus compounds are spelled with an "ous" ending. Other names for this acid are orthophosphorous acid and dihydroxyphosphine oxide.

HP(O)(OH)₂ is the product of the hydrolysis of its acid anhydride, P₄O₆:

P₄O₆ + 6 H₂O → 4 HP(O)(OH)₂

An analogous relationship connects H₃PO₄ and P₄O₁₀.

Tautomerization

H₃PO₃ is better described with the structural formula HP(O)(OH)₂. This species exists in equilibrium with a minor tautomer P(OH)₃. The latter is called phosphorous acid, whereas the dihydroxy form is called phosphonic acid. Many of the reduced phosphorus acids are subject to similarly complicated equilibria involving shifts of H between O and P. In the solid state, HP(O)(OH)₂ is tetrahedral with one shorter P=O bond of 148 pm and two longer P-O(H) bonds of 154 pm.

Preparation

Although commercially available, the acid is most commonly prepared by hydrolysis of phosphorus trichloride with water or steam:

PCl₃ + 3 H₂O → HP(O)(OH)₂ + 3 HCl

Potassium phosphite is a convenient precursor to phosphorous acid:

K₂HPO₃ + 2 HCl → 2 KCl + H₃PO₃

In practice aqueous potassium phosphite is treated with excess hydrochloric acid. By concentrating the solution and precipitations with alcohols, the pure acid can be separated from the salt.

Acid-base properties

Phosphorous acid is a diprotic acid, since the hydrogen bonded directly to the central phosphorus atom is not readily ionizable. Chemistry examinations often test students' appreciation of the fact that all three hydrogen atoms are not acidic under aqueous conditions, in contrast with phosphoric acid. HP(O)₂(OH)⁻ is a moderately strong acid.

HP(O)(OH)₂ → HP(O)₂(OH)⁻ + H⁺ pK_a = 1.3

HP(O)₂(OH)⁻ → HPO₃²⁻ + H⁺ pK_a = 6.7

The monodeprotonated species, HP(O)₂(OH)⁻. is called the phosphite ion.

The IUPAC (mostly organic) name is phosphonic acid. This nomenclature is commonly reserved for substituted derivatives, that is, organic group bonded to phosphorus, not simply an ester. For example, (CH₃)PO(OH)₂ is "methylphosphonic acid", which may of course form "methylphosphonate" esters.

Both phosphorous acid and its deprotonated forms are good reducing agents, although not necessarily quick to react. They are oxidized to phosphoric acid or its salts. It reduces solutions of noble metal cations to the metals.

Uses

Conversion to phosphine

Phosphine, being a flammable and toxic gas, is inconvenient to store. Fortunately this useful species is readily prepared by thermal decomposition of phosphorous acid, which degrades at about 180°C:

4 HP(O)(OH)₂ → PH₃ + 3 H₃PO₄

Since phosphoric acid is a syrupy non-volatile liquid, the gaseous PH₃ is readily separated.

In agriculture

A large quantity of phosphorous acid is used as phosphatic fertilizer. Pure phosphorous acid is also used for preparing phosphite salts, such as monopotassium phosphite or aluminum phosphonite. These salts, as well as aqueous solutions of pure phosphorous acid, have shown effectiveness in controlling a variety of microbial plant diseases—in particular, treatment using either trunk injection or foliar containing phosphorous acid salts is indicated in response to infections by phytophthora and pythium-type plant pathogens (both within class oomycetes, known as water molds), such as dieback/root rot and downy mildew. Anti-microbial products containing salts of phosphorous acid are marketed in Australia as 'Yates Anti-Rot'; and in the United States of America, for example, aluminum salts of phosphorous acid (known generically as 'Fosetyl-Al') are sold under the trade name 'Aliette'.

As a chemical reagent

Phosphorous acid is used in chemical reactions as a reducing agent that is somewhat less vigorous than the related hypophosphorous acid.

References

Other references

• Holleman, A. F.; Wiberg, E. “Inorganic Chemistry.” Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
• D. E. C. Corbridge. “Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology.” 5th ed. Elsevier: Amsterdam. ISBN 0-444-89307-5.