View source for Phosphorous acid

{{short description|Chemical compound (H3PO3)}}
{{redirect|Phosphonic acid|the compound class|phosphonate}}
{{Distinguish|Phosphoric acid|Phosphorus oxoacid}}
{{Chembox
| Verifiedfields = changed
| Watchedfields  = changed
| verifiedrevid  = 464205337
| Name           = 
| ImageFile      = 
| ImageFileL1    = Phosphonic-acid-2D-dimensions-vector.svg
| ImageNameL1    = Wireframe model of phosphorous acid
| ImageFileR1    = Phosphonic-acid-3D-balls-A.png
| ImageNameR1    = Ball and stick model of phosphorous acid
| ImageFile2    = Sample of H3PO3.jpg
| IUPACName      = Phosphonic acid
| OtherNames     = Dihydroxyphosphine oxide<br />
Dihydroxy(oxo)-''λ''<sup>5</sup>-phosphane<br />
Dihydroxy-''λ''<sup>5</sup>-phosphanone<br />
Orthophosphorous acid<br />
Oxo-''λ''<sup>5</sup>-phosphanediol<br />
Oxo-''λ''<sup>5</sup>-phosphonous acid<br />
Metaphosphoroic acid
| SystematicName = Phosphorous acid
| Section1       = {{Chembox Identifiers
| CASNo = 13598-36-2
| CASNo_Ref = {{cascite|correct|CAS}}
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 44976
| ChEMBL = 1235291
| ChemSpiderID = 10449259
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| EINECS =  237-066-7
| Gmelin = 1619
| KEGG_Ref = {{keggcite|correct|kegg}}
| KEGG = C06701
| PubChem = 107909
| RTECS = SZ6400000
| UNNumber = 2834
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 35V6A8JW8E
| InChI = 1/H3O3P/c1-4(2)3/h4H,(H2,1,2,3)
| InChIKey = ABLZXFCXXLZCGV-UHFFFAOYAF
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/H3O3P/c1-4(2)3/h4H,(H2,1,2,3)
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = ABLZXFCXXLZCGV-UHFFFAOYSA-N
| SMILES = OP(=O)O
| SMILES_Comment = {{chem2|HP(O)(OH)2}}
| SMILES1 = OP(O)O
| SMILES1_Comment = {{chem2|P(OH)3}}
 }}
| Section2       = {{Chembox Properties
| Formula = {{chem2|H3PO3}}
| MolarMass = 81.99 g/mol
| Appearance = white solid <br> [[deliquescent]]
| Density = 1.651 g/cm<sup>3</sup> (21 °C)
| Solubility = 310 g/100 mL
| SolubleOther = soluble in [[ethanol]]
| pKa = 1.3, 6.7
| MeltingPtC = 73.6
| BoilingPtC = 200
| BoilingPt_notes = (decomposes)
| MagSus = &minus;42.5·10<sup>−6</sup> cm<sup>3</sup>/mol
  }}
| Section3       = {{Chembox Structure
| MolShape = pseudo-tetrahedral
| CrystalStruct = 
| Dipole =
  }}
| Section4       = 
| Section5       = 
| Section6       = 
| Section7       = {{Chembox Hazards
| ExternalSDS = [https://www.sigmaaldrich.com/US/en/sds/sigald/215112 Sigma-Aldrich]
| MainHazards = skin irritant
| NFPA-H = 3
| NFPA-R = 1
| NFPA-F = 0
| GHS_ref=<ref>{{cite web |title=Phosphorous acid |url=https://pubchem.ncbi.nlm.nih.gov/compound/107909#section=Safety-and-Hazards |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref>
| GHSPictograms = {{GHS05}}{{GHS07}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|302|314}}
| PPhrases = {{P-phrases|260|264|270|280|301+312|301+330+331|303+361+353|304+340|305+351+338|310|321|330|363|405|501}}
  }}
| Section8       = {{Chembox Related
| OtherFunction_label = [[?]]
| OtherFunction = 
| OtherCompounds = [[phosphoric acid|{{chem2|H3PO4}}]] (i.e., {{chem2|PO(OH)3)}}<br />[[hypophosphorous acid|{{chem2|H3PO2}}]] (i.e., {{chem2|H2PO(OH))}}
  }}
}}

'''Phosphorous acid''' (or '''phosphonic acid''') is the [[Compound (chemistry)|compound]] described by the [[chemical formula|formula]] {{chem2|H3PO3}}. 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 {{chem2|RPO3H2}}, are called [[phosphonic acids]].

==Nomenclature and tautomerism==
[[File:Tautomerism of H3PO3.png|220px|left]]

Solid {{chem2|HP(O)(OH)2}} has tetrahedral geometry about the central phosphorus atom, with a {{chem2|P\sH}} bond of 132&nbsp;[[picometer|pm]], one {{chem2|P\dO}} double bond of 148&nbsp;pm and two longer {{chem2|P\sOH}} single bonds of 154&nbsp;pm. In common with other phosphorus oxides with {{chem2|P\sH}} bonds (e.g.[[hypophosphorous acid]] and [[dialkyl phosphite]]s),<ref>{{cite journal | last1=Janesko | first1=Benjamin G. | last2=Fisher | first2=Henry C. | last3=Bridle | first3=Mark J. | last4=Montchamp | first4=Jean-Luc | title=P(═O)H to P–OH Tautomerism: A Theoretical and Experimental Study | journal=The Journal of Organic Chemistry | publisher=American Chemical Society (ACS) | volume=80 | issue=20 | date=2015-09-29 | issn=0022-3263 | doi=10.1021/acs.joc.5b01618 | pages=10025–10032| pmid=26372089 }}</ref> it exists in equilibrium with an extremely minor [[tautomer]] {{chem2|P(OH)3}}. (In contrast, [[arsenous acid]]'s major tautomer is the trihydroxy form.) IUPAC recommends that the trihydroxy form {{chem2|P(OH)3}} be called phosphorous acid, and the dihydroxy form {{chem2|HP(O)(OH)2}} phosphonic acid.<ref name = "IUPAC inorganic">{{RedBook2005}}.</ref> Only the reduced phosphorus compounds are spelled with an "''-ous''" ending. 

:{{chem2|P^{III}(OH)3 <-> HP^{V}(O)(OH)2}} {{spaces|5}} ''K'' = 10<sup>10.3</sup> (25°C, aqueous)<ref>{{cite journal |doi=10.1139/v79-039|title=Tautomerization Equilibria for Phosphorous Acid and its Ethyl Esters, Free Energies of Formation of Phosphorous and Phosphonic Acids and their Ethyl Esters, and p ''Ka'' Values for Ionization of the P—H Bond in Phosphonic Acid and Phosphonic Esters |year=1979 |last1=Guthrie |first1=J. Peter |journal=Canadian Journal of Chemistry |volume=57 |issue=2 |pages=236–239 |doi-access=free }}</ref>

==Preparation==
On an industrial scale, the acid is prepared by [[hydrolysis]] of [[phosphorus trichloride]] with water or steam:<ref name = betterman>{{Ullmann|doi= 10.1002/14356007.a19_527|title= Phosphorus Compounds, Inorganic|year= 2000|last1= Bettermann|first1= Gerhard|last2= Krause|first2= Werner|last3= Riess|first3= Gerhard|last4= Hofmann|first4= Thomas}}.</ref>
:{{chem2|PCl3 + 3 H2O -> HPO(OH)2 + 3 [[HCl]]}}

{{chem2|HPO(OH)2}} could be produced by the hydrolysis of [[phosphorus trioxide]]:
:{{chem2|P4O6 + 6 H2O -> 4 HPO(OH)2}}
<!--uncited:
Potassium phosphite is also a convenient precursor to phosphorous acid:
:{{chem2|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 has a p''K''<sub>a</sub> in the range 1.26–1.3.<ref>{{cite journal|first1=John W. |last1=Larson |first2=Margaret |last2=Pippin |title=Thermodynamics of ionization of hypophosphorous and phosphorous acids. Substituent effects on second row oxy acids |journal=Polyhedron |date=1989 |volume=8 |issue=4 |pages=527–530|doi=10.1016/S0277-5387(00)80751-2}}</ref><ref>{{cite book|title=CRC Handbook of Chemistry and Physics |edition=87th |pages=8–42}}</ref> 
:{{chem2|HP(O)(OH)2 -> HP(O)2(OH)-  +  H+}} {{quad}} p''K''<sub>a</sub>&nbsp;=&nbsp;1.3
It is a [[diprotic acid]], the hydrogenphosphite ion, {{chem2|HP(O)2(OH)-}} is a weak acid:
:{{chem2|HP(O)2(OH)- -> HPO3(2−) + H+}}  {{quad}}   p''K''<sub>a</sub>&nbsp;=&nbsp;6.7

The conjugate base {{chem2|HP(O)2(OH)-}} is called hydrogen phosphite, and the second conjugate base, {{chem2|HPO3(2−)}}, is the [[phosphite anion|phosphite]] ion.<ref name = "Novosad">{{cite book|first=Josef |last=Novosad |date=1994 |title=Encyclopedia of Inorganic Chemistry |publisher=John Wiley and Sons |isbn=0-471-93620-0}}</ref> (Note that the [[IUPAC nomenclature of inorganic chemistry 2005|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 {{chem2|[[H3PO4]]}}.

===Redox properties===
On heating at 200&nbsp;°C, phosphorous acid disproportionates to [[phosphoric acid]] and [[phosphine]]:<ref>{{cite book|last1=Gokhale|first1=S. D.|last2=Jolly|first2=W. L.|title=Inorganic Syntheses|chapter=Phosphine|date=1967 |volume=9 |pages= 56–58 |doi=10.1002/9780470132401.ch17|isbn=9780470132401}}</ref> 
:{{chem2|4 H3PO3 -> 3 H3PO4 + PH3}}
This reaction is used for laboratory-scale preparations of {{chem2|PH3}}.

Phosphorous acid slowly oxidizes in air to phosphoric acid.<ref name = betterman/>

Both phosphorous acid and its deprotonated forms are good [[reducing agent]]s, 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:
:{{chem2|H3PO3 + 2 HgCl2 + H2O -> Hg2Cl2 + H3PO4 + 2 HCl}}
Mercurous chloride is reduced further by phosphorous acid to [[mercury (element)|mercury]] on heating or on standing:
:{{chem2|H3PO3 + Hg2Cl2 + H2O -> 2 Hg + H3PO4 + 2 HCl}}

===As a ligand===
[[File:WAJJOH.svg|thumb|left|upright|Structure of {{chem2|Mo(CO)5P(OH)3}}.<ref name=Mo/>]]
Upon treatment with metals of d<sup>6</sup> configuration, phosphorous acid is known to coordinate as the otherwise rare {{chem2|P(OH)3}} tautomer. Examples include {{chem2|Mo(CO)5(P(OH)3)}} and {{chem2|[Ru(NH3)4(H2O)(P(OH)3)](2+)}}.<ref name=Mo>{{cite journal |title=Synthesis of molybdenum complex with novel P(OH)<sub>3</sub> ligand based on the one-pot reaction of Mo(CO)<sub>6</sub> with HP(O)(OEt)<sub>2</sub> and water |first1=Chanjuan |last1=Xi |first2=Yuzhou |last2=Liu |first3=Chunbo |last3=Lai |first4=Lishan |last4=Zhou |journal=Inorganic Chemistry Communications| doi=10.1016/j.inoche.2004.09.012 |year=2004 |volume=7 |pages=1202–1204 |issue=11}}</ref><ref>{{cite journal |title=The ruthenium(II) center and the phosphite-phosphonate tautomeric equilibrium|author1=Sernaglia, R. L. |author2=Franco, D. W. |doi=10.1021/ic00317a018 |year=2005 |journal=Inorg. Chem. |volume=28 |issue=18| pages=3485–3489}}</ref>

{{clear left}}
Heating a mixture of [[potassium tetrachloroplatinate]] and phosphorous acid gives the luminescent salt [[potassium diplatinum(II) tetrakispyrophosphite]]:<ref>{{cite book |doi=10.1002/9780470132555.ch61 |title=Inorganic Syntheses |year=2007 |last1=Alexander |first1=K. A. |last2=Bryan |first2=S. A. |last3=Dickson |first3=M. K. |last4=Hedden |first4=D. |last5=Roundhill |chapter=Potassium Tetrakis&#91;Dihydrogen Diphosphito(2–)&#93;Diplatinate(II) |volume=24 |pages=211–213 |isbn=9780470132555}}</ref>
:{{chem2|2 K2PtCl4 +  8  H3PO3  ->  K4[Pt2(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.<ref name = betterman/>  

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]] materials, including steel, may be somewhat protected by promoting oxidation ("rust") and then converting the oxidation to a metalophosphate by using [[phosphoric acid]] and further protected by surface coating. (See: [[Passivation (chemistry)]]).

==Organic derivatives==
The [[IUPAC nomenclature of organic chemistry|IUPAC (mostly organic)]] name is '''phosphonic acid'''. This nomenclature is commonly reserved for [[Substituent|substituted]] derivatives, that is, organic group bonded to phosphorus, not simply an ester. For example, {{chem2|(CH3)PO(OH)2}} is "[[methylphosphonic acid]]", which may of course form "methyl[[phosphonate]]" [[ester]]s.

==References==
{{reflist|30em}}

==Further reading==
*{{cite book|last1=Holleman |first1=A. F. |last2=Wiberg |first2=E. |title=Inorganic Chemistry |publisher=Academic Press |location=San Diego |date=2001 |isbn=0-12-352651-5}}
*{{cite book|first=D. E. C. |last=Corbridge. |title=Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology |year=1995 |edition=5th |publisher=Elsevier |location=Amsterdam |isbn=0-444-89307-5}}
*{{cite book|title=Concise Inorganic Chemistry |first=J.D.|last=Lee |date=3 January 2008|publisher=Oxford University Press |isbn=978-81-265-1554-7}}

{{Hydrogen compounds}}

{{Authority control}}
[[Category:Pnictogen oxoacids]]
[[Category:Phosphorus oxoacids]]
[[Category:Phosphonates]]
[[Category:Phosphonic acids]]
[[Category:Phosphorus(III) compounds]]

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