Phosphorous acid (or phosphonic acid) is the compound described by the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

. It is diprotic (readily ionizes two protons), not triprotic as might be suggested by its formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds. Organic derivatives of phosphorous acid, compounds with the formula , are called phosphonic acids.

Nomenclature and tautomerism

Solid has tetrahedral geometry about the central phosphorus atom, with a bond of 132 pm, one double bond of 148 pm and two longer single bonds of 154 pm. In common with other phosphorus oxides with bonds (e.g. hypophosphorous acid and dialkyl phosphites), it exists in equilibrium with an extremely minor tautomer . (In contrast, arsenous acid's major tautomer is the trihydroxy form.) IUPAC recommends that the trihydroxy form be called phosphorous acid, and the dihydroxy form phosphonic acid.. Only the reduced phosphorus compounds are spelled with an "''-ous''" ending. : ''K'' = 10^10.3 (25°C, aqueous)

Preparation

On an industrial scale, the acid is prepared by

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

of phosphorus trichloride with water or steam:. : could be produced by the hydrolysis of phosphorus trioxide: :

Reactions

Acid–base properties

Phosphorous acid has a p''K''_a in the range 1.26–1.3. : p''K''_a = 1.3 It is a diprotic acid, the hydrogenphosphite ion, is a weak acid: : p''K''_a = 6.7 The conjugate base is called hydrogen phosphite, and the second conjugate base, , is the phosphite ion. (Note that the IUPAC recommendations are hydrogen phosphonate and phosphonate respectively). The hydrogen atom bonded directly to the phosphorus atom is not readily ionizable. Chemistry examinations often test students' appreciation of the fact that ''not'' all three hydrogen atoms are acidic under aqueous conditions, in contrast with .

Redox properties

On heating at 200 °C, phosphorous acid disproportionates to

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, ...

and

phosphine Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...

: : This reaction is used for laboratory-scale preparations of . Phosphorous acid slowly oxidizes in air to phosphoric acid. Both phosphorous acid and its deprotonated forms are good

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include hydrogen, carbon ...

s, although not necessarily quick to react. They are oxidized to

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

As a ligand

Upon treatment with metals of d⁶ configuration, phosphorous acid is known to coordinate as the otherwise rare tautomer. Examples include and . Heating a mixture of

potassium tetrachloroplatinate Potassium tetrachloroplatinate(II) is the chemical compound with the chemical formula, formula K2PtCl4. This reddish orange salt (chemistry), salt is an important reagent for the preparation of other Complex (chemistry), coordination complexes of ...

and phosphorous acid gives the luminescent salt potassium diplatinum(II) tetrakispyrophosphite: :

Uses

The most important use of phosphorous acid (phosphonic acid) is the production of basic lead phosphite, which is a stabilizer in PVC and related chlorinated polymers. It is used in the production of basic lead phosphonate PVC stabilizer, aminomethylene phosphonic acid and hydroxyethane diphosphonic acid. It is also used as a strong reducing agent and in the production of synthetic fibres, organophosphorus pesticides, and the highly efficient water treatment agent ATMP.

Ferrous In chemistry, iron(II) refers to the chemical element, element iron in its +2 oxidation number, oxidation state. The adjective ''ferrous'' or the prefix ''ferro-'' is often used to specify such compounds, as in ''ferrous chloride'' for iron(II ...

materials, including steel, may be somewhat protected by promoting oxidation ("rust") and then converting the oxidation to a metalophosphate by using

and further protected by surface coating. (See:

Passivation (chemistry) In physical chemistry and engineering, passivation is coating a material so that it becomes "passive", that is, less readily affected or corroded by the environment. Passivation involves creation of an outer layer of shield material that is app ...

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, is " methylphosphonic acid", which may of course form "methyl phosphonate"

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...