Phosphorous acid (or phosphonic acid (singular)) 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 ...

H₃PO₃. This

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a se ...

diprotic In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

(readily ionizes two protons), not

triprotic In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

as might be suggested by this formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds. Organic derivatives of phosphorous acid, compounds with the formula RPO₃H₂, are called phosphonic acids.

Nomenclature and tautomerism

H₃PO₃ is more clearly described with the structural formula HPO(OH)₂. In the solid state, has tetrahedral geometry about the central phosphorus atom, with a P–H bond of 132 pm, one P=O double bond of 148 pm and two longer P–O(H) single bonds of 154 pm. This species exists in equilibrium with an extremely minor

tautomer Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hy ...

. IUPAC recommends 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. Other important

oxyacid An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produc ...

s of

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...

are

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, w ...

(H₃PO₄) and

hypophosphorous acid Hypophosphorous acid (HPA), or phosphinic acid, is a phosphorus oxyacid and a powerful reducing agent with molecular formula H3PO2. It is a colorless low-melting compound, which is soluble in water, dioxane and alcohols. The formula for this ...

(). The reduced phosphorus acids are subject to similar tautomeric shifts of hydrogen atoms between O and P atoms.

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, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...

phosphorus trichloride Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds. It is toxic ...

with water or steam:. : HPO(OH)₂ could be produced by the hydrolysis of

phosphorus trioxide Phosphorus trioxide is the chemical compound with the molecular formula P4O6. Although the molecular formula suggests the name tetraphosphorus hexaoxide, the name phosphorus trioxide preceded the knowledge of the compound's molecular structure, a ...

: :

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, HP(O)₂(OH)⁻ is a weak acid: : p''K''_a = 6.7 The conjugate base HP(O)₂(OH)⁻ is called hydrogen phosphite, and the second conjugate base, , is the

phosphite The general structure of a phosphite ester showing the lone pairs on the P In organic chemistry, a phosphite ester or organophosphite usually refers to an organophosphorous compound with the formula P(OR)3. They can be considered as esters of a ...

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

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 PH₃. 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 commonly reducing agents include the Earth met ...

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 Mercury(II) chloride (or mercury bichloride, mercury dichloride), historically also known as sulema or corrosive sublimate, is the inorganic chemical compound of mercury and chlorine with the formula HgCl2. It is white crystalline solid and is a ...

, 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

As a ligand

Upon treatment with metals of d⁶ configuration, phosphorous acid is known to coordinate as the otherwise rare P(OH)₃ tautomer. Examples include Mo(CO)₅(P(OH)₃) and u(NH₃)₄(H₂O)(P(OH)₃)sup>2+. WAJJOH

Heating a mixture of potassium tetrachloroplatinate and phosphorous acid gives the luminescent salt

potassium diplatinum(II) tetrakispyrophosphite Potassium diplatinum(II) tetrakispyrophosphite (abbreviated as t2(pop)4sup>4−) is the inorganic compound with the formula K4 t2(HO2POPO2H)4 It is a water-soluble yellow salt. The compound has a long-lived, strongly luminescent excited state ...

: :2 K₂PtCl₄ + 8 H₃PO₃ → K₄ t₂(HO₂POPO₂H)₄ + 8 HCl + 4 H₂O

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 phosphorous acid, synthetic fibres, organophosphorus pesticides, and the highly efficient water treatment agent

ATMP ATMP or aminotris(methylenephosphonic acid) is a phosphonic acid with chemical formula N(CH2PO3H2)3. It has chelating properties. It can be synthesized from the Mannich-type reaction of ammonia, formaldehyde, and phosphorous acid, in a manner ...

Ferrous In chemistry, the adjective Ferrous indicates a compound that contains iron(II), meaning iron in its +2 oxidation state, possibly as the divalent cation Fe2+. It is opposed to " ferric" or iron(III), meaning iron in its +3 oxidation state, suc ...

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) Passivation, in physical chemistry and engineering, refers to coating a material so 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 a ...

Organic derivatives

The IUPAC (mostly organic) name is phosphonic acid. This nomenclature is commonly reserved for

substituted A substitution reaction (also known as single displacement reaction or single substitution reaction) is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions ar ...

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 "methyl

phosphonate In organic chemistry, phosphonates or phosphonic acids are organophosphorus compounds containing groups (where R = alkyl, aryl, or just hydrogen). Phosphonic acids, typically handled as salts, are generally nonvolatile solids that are poorly ...

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...