Arsenous acid

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Arsenous acid
Structural formula
Ball-and-stick model
CAS number 13464-58-9 YesY=
PubChem 545
ChemSpider 530 YesY
DrugBank DB04456
ChEBI CHEBI:49900 YesY
Jmol-3D images Image 1
Molecular formula H3AsO3
Molar mass 125.94 g/mol
Appearance Only exists in aqueous solutions
Main hazards Toxic, corrosive
Related compounds
Related compounds Arsenic acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Arsenous acid (or arsenious acid) is the inorganic compound with the formula H3AsO3. It is known to occur in aqueous solutions, but it has not been isolated as a pure material, although this fact does not detract from the significance of As(OH)3.[1]


As(OH)3 is a pyramidal molecule consisting of three hydroxyl groups bonded to arsenic. The 1H NMR spectrum of arsenous acid solutions consists of a single signal consistent with the molecule's high symmetry.[2] In contrast, the nominally related phosphorus species H3PO3 mainly adopts the structure HPO(OH)2; P(OH)3 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.[3]


The preparation of As(OH)3 involves a slow hydrolysis of arsenic trioxide in water. Addition of base converts arsenous acid to the arsenite ions [AsO(OH)2], [AsO2(OH)]2–, and [AsO3]3–. The first pKa is 9.2, As(OH)3 is a weak acid.[3] Reactions attributed to aqueous arsenic trioxide are due to arsenous acid and its conjugate bases.


Arsenic-containing compounds are highly toxic and carcinogenic. The anhydride form of arsenous acid, arsenic trioxide, is used as a herbicide, pesticide, and rodenticide.


  1. ^ Munoz-Hernandez, M.-A. (1994). "Arsenic: Inorganic Chemistry". In King, R. B. Encyclopedia of Inorganic Chemistry. Chichester: John Wiley & Sons. 
  2. ^ Kolozsi, A.; Lakatos, A.; Galbács, G.; Madsen, A. Ø.; Larsen, E.; Gyurcsik, B. (2008). "A pH-Metric, UV, NMR, and X-ray Crystallographic Study on Arsenous Acid Reacting with Dithioerythritol" (pdf). Inorganic Chemistry 47: 3832–3840. doi:10.1021/ic7024439. PMID 18380458. 
  3. ^ a b Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419. 

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