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{{Chembox {{Chembox
| verifiedrevid = 455187134 | verifiedrevid = 455188109
| Name = Arsenous acid | Name = Arsenous acid
| ImageFile = Arsenous-acid-2D.png | ImageFile = Arsenous-acid-2D.svg
| ImageSize = 200px | ImageSize = 170px
| ImageAlt = | ImageAlt = Structural formula
| ImageCaption = | ImageCaption =
| ImageFile1 = Arsenous-acid-3D-vdW.png | ImageFile1 = Arsenous-acid-3D-balls.png
| ImageSize1 = 200px | ImageSize1 = 170px
| ImageAlt1 = | ImageAlt1 = Ball-and-stick model
| ImageCaption1 = | ImageCaption1 =
| IUPACName = Arsorous acid | IUPACName = Arsorous acid
| PIN = | PIN =
| SystematicName = | SystematicName =
| OtherNames = Arsenious acid | OtherNames = Arsenious acid<br>Arsenic oxide
| Section1 = {{Chembox Identifiers |Section1={{Chembox Identifiers
| Abbreviations = | Abbreviations =
| CASNo = 13464-58-9 | CASNo = 13464-58-9
| CASNo_Comment = | CASNo_Comment =
| CASNo_Ref = | CASNo_Ref = {{cascite|correct|??}}
| UNII_Ref = {{fdacite|correct|FDA}}
| CASNos =
| PubChem = 545 | UNII = 935XD1L5K2
| PubChem = 545
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 530 | ChemSpiderID = 530
| SMILES = O(O)O | SMILES = O(O)O
| InChI = 1/AsH3O3/c2-1(3)4/h2-4H | InChI = 1/AsH3O3/c2-1(3)4/h2-4H
| InChIKey = GCPXMJHSNVMWNM-UHFFFAOYAA | InChIKey = GCPXMJHSNVMWNM-UHFFFAOYAA
| StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/AsH3O3/c2-1(3)4/h2-4H | StdInChI = 1S/AsH3O3/c2-1(3)4/h2-4H
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = GCPXMJHSNVMWNM-UHFFFAOYSA-N | StdInChIKey = GCPXMJHSNVMWNM-UHFFFAOYSA-N
| EINECS = | EINECS =
| EC-number = | EC_number =
| EINECSCASNO = | UNNumber =
⚫ | DrugBank_Ref = {{drugbankcite|correct|drugbank}}
| UNNumber =
⚫ | DrugBank_Ref = {{drugbankcite|correct|drugbank}}
| DrugBank = DB04456 | DrugBank = DB04456
| KEGG_Ref = {{keggcite|correct|kegg}} | KEGG_Ref = {{keggcite|correct|kegg}}
| KEGG = | KEGG =
| MeSHName = | MeSHName =
| ChEBI_Ref = {{ebicite|correct|EBI}} | ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 49900 | ChEBI = 49900
| RTECS = | RTECS =
| ATCvet = | Beilstein =
⚫ | Gmelin =
| ATCCode_prefix =
⚫ | 3DMet =
| ATCCode_suffix =
| ATC_Supplemental =
| Beilstein =
⚫ | Gmelin =
⚫ | 3DMet =
}} }}
| Section2 = {{Chembox Properties |Section2={{Chembox Properties
| Formula = H<sub>3</sub>AsO<sub>3</sub> | Formula = H<sub>3</sub>AsO<sub>3</sub>
| MolarMass = 125.94 g/mol | MolarMass = 125.94 g/mol
| Appearance = Only exists in aqueous solutions | Appearance = Only exists in aqueous solutions
| Density = | Density =
| MeltingPt = | MeltingPt =
| Melting_notes = | MeltingPt_notes =
| BoilingPt = | BoilingPt =
| Boiling_notes = | BoilingPt_notes =
| LogP = | LogP =
| VaporPressure = | VaporPressure =
| HenryConstant = | HenryConstant =
| AtmosphericOHRateConstant = | AtmosphericOHRateConstant =
| ConjugateBase = ]
⚫ | pKa =
| pKb = | pKa =
⚫ | pKb =
| MagSus = -51.2·10<sup>−6</sup> cm<sup>3</sup>/mol
}} }}
| Section3 = {{Chembox Structure |Section3={{Chembox Structure
| CrystalStruct = | CrystalStruct =
| Coordination = | Coordination =
| MolShape = | MolShape =
}} }}
| Section4 = {{Chembox Thermochemistry |Section4={{Chembox Thermochemistry
| Solubility =
| SolubleOther =
| Solvent =
}} }}
| Section7 = {{Chembox Hazards |Section7={{Chembox Hazards
| ExternalMSDS = | ExternalSDS =
⚫ | MainHazards = Toxic, corrosive
| EUClass =
| EUIndex = | NFPA-H =
⚫ | NFPA-F =
⚫ | MainHazards = Toxic, corrosive
| NFPA-H = | NFPA-R =
| NFPA-F = | NFPA-S =
| NFPA-R = | LD50 =
| IDLH = Ca <ref name=PGCH>{{PGCH|0038}}</ref>
⚫ | NFPA-O =
| REL = Ca C 0.002 mg/m<sup>3</sup> <ref name=PGCH/>
| RPhrases =
| PEL = TWA 0.010 mg/m<sup>3</sup><ref name=PGCH/>
| SPhrases =
| RSPhrases =
| LD50 =
}} }}
| Section8 = {{Chembox Related |Section8={{Chembox Related
| OtherAnions = | OtherAnions =
| OtherCations = | OtherCations =
| OtherFunctn = | OtherFunction =
| Function = | OtherFunction_label =
| OtherCpds = ] }} | OtherCompounds = ] }}
}} }}


'''Arsenous acid''', also known as '''arsenious acid''', is the ] with the formula H<sub>3</sub>AsO<sub>3</sub>. It is known to occur in aqueous ]s, but it has not been isolated as a pure material, although this fact does not detract from the significance of As(OH)<sub>3</sub>.<ref>{{cite book '''Arsenous acid''' (or '''arsenious acid''') is the ] with the formula H<sub>3</sub>AsO<sub>3</sub>. It is known to occur in ]s, but it has not been isolated as a pure material, although this fact does not detract from the significance of As(OH)<sub>3</sub>.<ref>{{cite encyclopedia | editor = King, R. B. | author = Munoz-Hernandez, M.-A. | title = Arsenic: Inorganic Chemistry | year = 1994 | encyclopedia = Encyclopedia of Inorganic Chemistry | publisher = John Wiley & Sons | location = Chichester }}</ref>
| first = R. Bruce "(ed.)"
| last = King
| year = 1994
| title = ]
| publisher = John Wiley & Sons
| location = Chichester
}}</ref>


==Properties== ==Properties==
] exists in as the dihydroxy tautomer in contrast to arsenous acid.]]
As(OH)<sub>3</sub> is a pyramidal molecule consisting of three ] groups bonded to arsenic. The <sup>1</sup>H ] spectrum of arsenous acid solutions consists of a single signal consistent with the molecule's high symmetry.{{Citation needed|date=October 2010}} In contrast, the nominally related ] species H<sub>3</sub>PO<sub>3</sub> mainly adopts the structure HPO(OH)<sub>2</sub>; P(OH)<sub>3</sub> is a very minor equilibrium component of such solutions. The differing behaviors of the As and P compounds reflect a trend whereby high oxidation states are more stable for lighter members of main group elements than their heavier congeners.<ref>{{cite book As(OH)<sub>3</sub> is a pyramidal molecule consisting of three ] groups bonded to arsenic. The <sup>1</sup>H ] spectrum of arsenous acid solutions consists of a single signal consistent with the molecule's high symmetry.<ref>{{cite journal | title = A pH-Metric, UV, NMR, and X-ray Crystallographic Study on Arsenous Acid Reacting with Dithioerythritol | author1 = Kolozsi, A. | author2 = Lakatos, A. | author3 = Galbács, G. | author4 = Madsen, A. Ø. | author5 = Larsen, E. | author6 = Gyurcsik, B. | journal = Inorganic Chemistry | year = 2008 | volume = 47 | issue = 9 | pages = 3832–3840 | url = http://www.staff.u-szeged.hu/~galbx/Arsenous%20acid%20study%20%28Inorg%20Chem%202008%29.pdf | doi = 10.1021/ic7024439 | pmid = 18380458 | access-date = 2011-12-18 | archive-url = https://web.archive.org/web/20120425201802/http://www.staff.u-szeged.hu/~galbx/Arsenous%20acid%20study%20(Inorg%20Chem%202008).pdf | archive-date = 2012-04-25 | url-status = dead }}</ref> In contrast, the nominally related ] H<sub>3</sub>PO<sub>3</sub> adopts the structure HPO(OH)<sub>2</sub>. The structural analogue of arsenous acid (P(OH)<sub>3</sub>) is a very minor equilibrium component of such solutions. The differing behaviors of the As and P compounds reflect a trend whereby high oxidation states are more stable for lighter members of main group elements than their heavier congeners.<ref name = "Greenwood">{{Greenwood&Earnshaw}}</ref>
| first = N.N.

| last = Greenwood
One tautomer of arsenous acid is HAsO(OH)<sub>2</sub>, which is called ]. It has not been isolated or well-characterized.
| coauthors= A. Earnshaw

| year = 1997
==Synthesis==
| title = Chemistry of the Elements
The preparation of As(OH)<sub>3</sub> involves a slow ] of ] in water. Addition of ] converts arsenous acid to the ] <sup>−</sup>, <sup>2−</sup>, and <sup>3−</sup>.
| publisher = Butterworth-Heinemann
| location = Oxford
}}</ref>


==Reactions== ==Reactions==
The preparation of As(OH)<sub>3</sub> involves a slow ] of ] in water. Addition of ] converts arsenous acid to the ] <sup>-</sup>, <sup>2-</sup>, and <sup>3-</sup>. The first pK<sub>a</sub> is 9.2. Reactions attributed to aqueous ] are due to arsenous acid and its conjugate bases. With its first p''K''<sub>a</sub> being 9.2, As(OH)<sub>3</sub> is a weak acid.<ref name = "Greenwood"/> Reactions attributed to aqueous ] are due to arsenous acid and its conjugate bases.

Like arsenic trioxide, arsenous acid is sometimes ]. For example, it reacts with hydrochloric, hydrobromic, and hydroiodic acids to produce arsenic trichloride, tribromide, and triiodide.

:As(OH)<sub>3</sub> + 3 HCl ⇌ AsCl<sub>3</sub> + 3 H<sub>2</sub>O
:As(OH)<sub>3</sub> + 3 HBr ⇌ AsBr<sub>3</sub> + 3 H<sub>2</sub>O
:As(OH)<sub>3</sub> + 3 HI ⇌ AsI<sub>3</sub> + 3 H<sub>2</sub>O

Reaction of arsenous acid with ] gives ]. This historically significant conversion is the ]:<ref>{{cite journal|title=Ueber einige anomale Reaktionen|author=G. Meyer|journal=Berichte der Deutschen Chemischen Gesellschaft|pages=1439–1443|volume=13|year=1883|doi=10.1002/cber.188301601316}}</ref>
:As(OH)<sub>3</sub> + CH<sub>3</sub>I + NaOH ⇌ CH<sub>3</sub>AsO(OH)<sub>2</sub> + NaI + H<sub>2</sub>O
Alkylation occurs at arsenic, and the ] of arsenic increases from +3 to +5.


==Toxicology== ==Toxicology==
Arsenic containing compounds are highly toxic and ]. The ] form of arsenous acid, ], is used as a ], ], and ]. Arsenic-containing compounds are highly toxic and ]. The ] form of arsenous acid, ], is used as a ], ], and ].


==References== ==References==
{{reflist}}
<references/>

* {{cite web | title=Arsenic trioxide| work=| url=http://www.pesticideinfo.org/Detail_Chemical.jsp?Rec_Id=PC33821| accessdate=January 29, 2006 }}


==External links== ==External links==
* *


{{Hydrogen compounds}} {{Hydrogen compounds}}


{{DEFAULTSORT:Arsenous Acid}} {{DEFAULTSORT:Arsenous Acid}}
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