Phosphorous acid

Phosphorous acid

Chembox new
ImageFile = Phosphonic-acid-3D-balls-A.png ImageSize = 150px
ImageName = Phosphonic acid
ImageFile1 = Phosphonic-acid-2D-dimensions.png ImageSize1 = 150px
ImageName1 = Phosphonic acid
IUPACName = phosphonic acid
OtherNames = phosphonic acid
Section1 = Chembox Identifiers
CASNo = 13598-36-2
RTECS = SZ6400000

Section2 = Chembox Properties
Formula = H3PO3
MolarMass = 82.00 g/mol
Appearance = colorless solid
Density = 1.65 g/cm3
Solubility = miscible
MeltingPt = 70.1 °C
BoilingPt = decomposes

Section3 = Chembox Structure
MolShape = tetrahedral
CrystalStruct =
Dipole =

Section7 = Chembox Hazards
ExternalMSDS =
MainHazards = skin irritant
NFPA-H = 2
NFPA-R = 1
NFPA-F = 0
RPhrases = 22-35
SPhrases = 26-36/37/39-45

Section8 = Chembox Related
Function = ?
OtherFunctn =
OtherCpds = H3PO4 (i.e., PO(OH)3)
H3PO2 (i.e., H2PO(OH))

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)2An analogous relationship connects H3PO4 and P4O10.


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.


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 + H3PO3In 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+ p"K"a = 1.3 [CRC Handbook of Chemistry and Physics, 87th Ed. 8-42] ::HP(O)2(OH) → HPO32− + H+ p"K"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.


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 H3PO4Since 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. [cite journal
title = Phosphite (Phosphorous acid ): Its Relevance in the Environment and Agriculture and Influence on Plant Phosphate Starvation Respose
author = Allison E. McDonald; Bruce R. Grant; William C. Plaxton
journal = Journal of Plant Nutrition
volume = 24
issue = 10
pages = 1505–1519
year = 2001
url =
doi = 10.1081/PLN-100106017
] 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. [ [ Organic Labs. Product label for 'Exel LG,' Retrieved April 9, 2007.] ] 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'. [ [ Yates, a Division of Orica Australia Pty Ltd. “MSDS ('Yates Anti Rot Phosacid Systemic Fungicide').” Version 1. SH&E Shared Services, Orica. Homebush, NSW (Australia): April 4, 2005 (retrieved from April 9, 2007).] ] []

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. [ [ “Phosphorous acid.” The American Heritage® Dictionary of the English Language, 4th ed. Boston: Houghton Mifflin, 2000 (retrieved from April 9, 2007).] ]


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.

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Look at other dictionaries:

  • phosphorous acid — n. a white or yellowish, crystalline acid, H3PO3, that absorbs oxygen readily: used as a chemical reducing agent and as an analytical reagent …   English World dictionary

  • phosphorous acid — Chem. a colorless, crystalline, water soluble acid of phosphorus, H3PO3, from which phosphites are derived. [1785 95] * * * ▪ chemical compound (H3PO3),also called  Orthophosphorous Acid,         one of several oxygen acids of phosphorus, used as …   Universalium

  • phosphorous acid — H3PO3; its salts are phosphites. * * * phosphorous acid n a deliquescent crystalline acid H3PO3 used esp. as a reducing agent and in making phosphites * * * phos·pho·rous ac·id (fos forґəs) a reducing inorganic acid, H3PO3, which readily… …   Medical dictionary

  • phosphorous acid — fosfito rūgštys statusas T sritis chemija apibrėžtis Fosfono rūgšties tautomeras. formulė P(OH)₃ atitikmenys: angl. phosphorous acid rus. фосфористая кислота …   Chemijos terminų aiškinamasis žodynas

  • phosphorous acid — fosfito rūgštys statusas T sritis chemija formulė H₂PHO₃ atitikmenys: angl. phosphorous acid rus. фосфористая кислота ryšiai: sinonimas – vandenilio hidridotrioksofosfatas (2–) …   Chemijos terminų aiškinamasis žodynas

  • phosphorous acid — noun a clear or yellow monobasic acid (H3PO2) • Syn: ↑hypophosphorous acid, ↑orthophosphorous acid • Hypernyms: ↑oxyacid, ↑oxygen acid …   Useful english dictionary

  • phosphorous acid — noun Date: 1788 a deliquescent crystalline acid H3PO3 used especially as a reducing agent and in making phosphites …   New Collegiate Dictionary

  • phosphorous acid — phospho′rous ac′id n. chem. a colorless, crystalline, water soluble acid of phosphorus, H3PO3, from which phosphites are derived • Etymology: 1785–95 …   From formal English to slang

  • phosphorous acid — /fɒsfərəs ˈæsəd/ (say fosfuhruhs asuhd) noun a colourless, water soluble, crystalline acid of phosphorus, H3PO3, from which phosphites are derived …   Australian English dictionary

  • phosphorous acid — noun a yellowish hygroscopic solid, HPO; a mild reducing agent See Also: phosphite …   Wiktionary

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