Phosphorous acid

"Phosphonic acid" redirects here. For Phosphonic acids, see Phosphonate.

Not to be confused with Phosphoric acid.

Phosphorous acid

Names
IUPAC name phosphonic acid
Other names Dihydroxyphosphine oxide Dihydroxy(oxo)-λ5-phosphane Dihydroxy-λ5-phosphanone Orthophosphorous acid Oxo-λ5-phosphanediol Oxo-λ5-phosphonous acid
Identifiers
CAS Number	13598-36-2 Y
ChEBI	CHEBI:44976 Y
ChemSpider	10449259 Y 10459438 (17O3) Y
Jmol 3D model	Interactive image
KEGG	C06701 Y
RTECS number	SZ6400000
UNII	35V6A8JW8E N
InChI InChI=1S/H3O3P/c1-4(2)3/h4H,(H2,1,2,3) Y Key: ABLZXFCXXLZCGV-UHFFFAOYSA-N Y InChI=1/H3O3P/c1-4(2)3/h4H,(H2,1,2,3) Key: ABLZXFCXXLZCGV-UHFFFAOYAF
SMILES OP(=O)O
Properties
Chemical formula	H3PO3
Molar mass	82.00 g/mol
Appearance	white solid deliquescent
Density	1.651 g/cm3 (21 °C)
Melting point	73.6 °C (164.5 °F; 346.8 K)
Boiling point	200 °C (392 °F; 473 K) (decomposes)
Solubility in water	310 g/100 mL
Solubility	soluble in alcohol
Acidity (pKa)	1.3, 6.7
Structure
Molecular shape	tetrahedral
Vapor pressure	{{{value}}}
Related compounds
Related compounds	H3PO4 (i.e., PO(OH)3) H3PO2 (i.e., H2PO(OH))
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Phosphorous acid is the compound described by the formula H₃PO₃. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds.

Nomenclature and tautomerism

H₃PO₃ is more clearly described with the structural formula HPO(OH)₂. This species exists in equilibrium with a minor tautomer P(OH)₃. IUPAC recommendations, 2005, are that the latter be called phosphorous acid, whereas the dihydroxy form is called phosphonic acid. Only the reduced phosphorus compounds are spelled with an "ous" ending.

The P(OH)₃ tautomer has been observed as a ligand bonded to molybdenum. Other important oxyacids of phosphorus are phosphoric acid (H₃PO₄) and hypophosphorous acid (H₃PO₂). The reduced phosphorus acids are subject to similar tautomerism involving shifts of H between O and P.

Structure and oxidation state

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. The central phosphorus atom is assigned an oxidation state of +3.

Preparation

HPO(OH)₂ is the product of the hydrolysis of its acid anhydride:

P₄O₆ + 6 H₂O → 4 HPO(OH)₂

(An analogous relationship connects H₃PO₄ and P₄O₁₀).

On an industrial scale, the acid is prepared by hydrolysis of phosphorus trichloride with water or steam:

PCl₃ + 3 H₂O → HPO(OH)₂ + 3 HCl

Potassium phosphite is also 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.

Reactions

Acid–base properties

Phosphorous acid is a strong acid with a pK_a in the range 1.26–1.3.

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

It is a diprotic acid, the hydrogenphosphite ion, HP(O)₂(OH)⁻ is a moderately strong acid:

HP(O)₂(OH)⁻ → HPO₃²⁻ + H⁺            pK_a = 6.7

The conjugate base HP(O)₂(OH)⁻ is called hydrogen phosphite, and the second conjugate base, HPO²⁻
₃, is the phosphite ion. (Note that the IUPAC recommendations are hydrogen phosphonate and phosphonate respectively).

The hydrogen bonded directly to the 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 H₃PO₄.

Redox

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. When phosphorous acid is treated with a cold solution of mercuric chloride, a white precipitate of mercurous chloride forms:

H₃PO₃ + 2 HgCl₂ + H₂O → Hg₂Cl₂ + H₃PO₄ + 2 HCl

Mercurous chloride is reduced further by phosphorous acid to mercury on heating or on standing:

H₃PO₃ + Hg₂Cl₂ + H₂O → 2 Hg + H₃PO₄ + 2 HCl

Phosphorous acid on heating at 200 °C converts to phosphoric acid and phosphine:

4 H₃PO₃ → 3 H₃PO₄ + PH₃

Uses

The most important use of phosphorous acid (phosphonic acid) is the production of phosphites (phosphonates) which are used in water treatment. Phosphorous acid is also used for preparing phosphite salts, such as potassium phosphite. These salts, as well as aqueous solutions of pure phosphorous acid, are fungicides. Phosphites have shown effectiveness in controlling a variety of 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 the monoethyl ester of phosphorous acid (known generically as 'Fosetyl-Al') are sold under the trade name 'Aliette'. Phosphorous acid and its salts, unlike phosphoric acid, are somewhat toxic and should be handled carefully.

Organic derivatives

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.