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'{{chembox | verifiedrevid = 443400721 | ImageFile = Arsenate.svg | ImageSize = 150px | ImageName = Arsenic acid | ImageFile1 = Arsenic-acid-3D-vdW.png | ImageSize1 = 150px | ImageName1 = Arsenic acid | IUPACName = Arsenic acid, arsoric acid | OtherNames = Arsenic acid<br />Orthoarsenic acid<br />Desiccant L-10<br />Zotox | Section1 = {{Chembox Identifiers | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | ChemSpiderID = 229 | UNII_Ref = {{fdacite|correct|FDA}} | UNII = N7CIZ75ZPN | KEGG_Ref = {{keggcite|correct|kegg}} | KEGG = C01478 | InChI = 1/AsH3O4/c2-1(3,4)5/h(H3,2,3,4,5) | ChEBI_Ref = {{ebicite|correct|EBI}} | ChEBI = 18231 | SMILES = O=[As](O)(O)O | InChIKey = DJHGAFSJWGLOIV-UHFFFAOYAY | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChI = 1S/AsH3O4/c2-1(3,4)5/h(H3,2,3,4,5) | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = DJHGAFSJWGLOIV-UHFFFAOYSA-N | CASNo = 7778-39-4 | CASNo_Ref = {{cascite|correct|CAS}} | RTECS = CG0700000 }} | Section2 = {{Chembox Properties | Formula = H₃AsO₄ | MolarMass = 141.94 g/mol | Appearance = White translucent crystals,<br />[[hygroscopic]]. | Density = 2.5 g/cm³ | Solubility = 16.7 g/100 mL | SolubleOther = soluble in [[alcohol]] | MeltingPtC = 35.5 | BoilingPt = decom ≥ 100 °C | pKa = 2.19, 6.94, 11.5 }} | Section3 = {{Chembox Structure | MolShape = Tetrahedral | CrystalStruct = }} | Section7 = {{Chembox Hazards | ExternalMSDS = | EUClass = Toxic ('''T''')<br/>Dangerous for the environment ('''N''') <!-- Index No. 033-005-00-1 --> | FlashPt = Non-flammable | RPhrases = {{R23/25}}, {{R45}}, {{R50/53}} | SPhrases = {{S53}}, {{S45}}, {{S60}}, {{S61}} | NFPA-H = 3 | NFPA-F = 0 | NFPA-R = 0 }} | Section8 = {{Chembox Related | OtherAnions = [[Phosphoric acid]] | OtherCations = [[Sodium arsenate]] | OtherCpds = [[Arsenous acid]]<br />[[Arsenic pentoxide]] }} }} '''Arsenic acid''' is the [[chemical compound]] with the [[chemical formula|formula]] H₃AsO₄. More descriptively written as AsO(OH)₃, this colorless [[acid]] is the [[arsenic]] analogue of [[phosphoric acid]]. Arsenate and phosphate salts behave very similarly. Arsenic acid as such has not been isolated, but only found in solution where it is largely ionized. Its hemihydrate form (H₃AsO₄·½H₂O) does form stable crystals. Crystalline samples dehydrate with condensation at 100 °C.<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref> ==Properties== It is a tetrahedral species of idealized [[symmetry group|symmetry]] C_3v with As-O bonds lengths ranging from 1.66 to 1.71&nbsp;Å.<!--the terminal As=O is not so clear due to H-bonding--><ref>{{cite journal | author = Lee, C.; Harrison, W. T. A. | title = Tetraethylammonium dihydrogenarsenate bis(arsenic acid) and 1,4-diazoniabicyclo[2.2.2]octane bis(dihydrogenarsenate) arsenic acid: hydrogen-bonded networks containing dihydrogenarsenate anions and neutral arsenic acid molecules | journal = [[Acta Crystallographica]] | year = 2007 | volume = C63 | pages = m308-m311 | doi=10.1107/S0108270107023967 | pmid = 17609552 | issue = Pt 7}}</ref> Being a triprotic [[acid]], its acidity is described by three equilibria: :H₃AsO₄ {{eqm}} H₂AsO{{su|b=4|p=−}} + H⁺ (K₁ = 10^−2.19) :H₂AsO{{su|b=4|p=−}} {{eqm}} HAsO{{su|b=4|p=2−}} + H⁺ (K₂ = 10^−6.94) :HAsO{{su|b=4|p=2−}} {{eqm}} AsO{{su|b=4|p=3−}} + H⁺ (K₃ = 10^−11.5) These K_a values are close to those for phosphoric acid. The highly basic [[arsenate|arsenate ion]] (AsO{{su|b=4|p=3−}}) is the product of the third ionization. Unlike phosphoric acid, arsenic acid is oxidizing, illustrated by its ability to convert [[iodide]] to [[iodine]]. ==Preparation== Arsenic acid is prepared by treating arsenic trioxide with concentrated [[nitric acid]], or by combination of [[arsenic pentoxide]] with water. The latter reaction is very slow. It is also formed when meta- or [[pyroarsenic acid]] is treated with cold water. ==Other methods== Arsenic acid can be prepared by reacting moist elemental arsenic with [[ozone]]. : 2As + 3H₂O + 5O₃ → 2H₃AsO₄ + 5O₂ ==Applications== Commercial applications of arsenic acid are limited by its toxicity. It has found occasional use as a [[wood preservative]], broad-spectrum biocide, a finishing agent for glass and metal, and a [[reagent]] in the synthesis of some [[dyestuff]]s and organic arsenic compounds. The {{LD50}} in rabbits is 6 mg/kg (0.006 g/kg).<ref>Joachimoglu. ''Biochem. Z.'' '''70''', 144 (1915)</ref> ==References== {{reflist}} {{Hydrogen compounds}} [[Category:Arsenates]] [[Category:Hydrogen compounds]] [[Category:Oxidizing agents]] [[Category:Oxidizing acids]] [[ar:حمض الزرنيخيك]] [[cs:Kyselina trihydrogenarseničná]] [[de:Arsensäure]] [[es:Ácido arsénico]] [[fr:Acide arsénique]] [[hr:Arsenatna kiselina]] [[it:Acido arsenico]] [[hu:Arzénsav]] [[nl:Arseenzuur]] [[ja:ヒ酸]] [[pl:Kwas arsenowy(V)]] [[pt:Ácido arsênico]] [[ru:Мышьяковая кислота]] [[sh:Arsenatna kiselina]] [[fi:Arseenihappo]] [[sv:Arseniksyra]] [[tr:Arsenik asit]] [[zh:砷酸]]'