Arsenous acid

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Arsenous acid
Structural formula
Ball-and-stick model
IUPAC name
Arsorous acid
Other names
Arsenious acid
Arsenic oxide
13464-58-9 YesY
3D model (Jmol) Interactive image
ChEBI CHEBI:49900 YesY
ChemSpider 530 YesY
DrugBank DB04456 YesY
PubChem 545
Molar mass 125.94 g/mol
Appearance Only exists in aqueous solutions
Main hazards Toxic, corrosive
US health exposure limits (NIOSH):
PEL (Permissible)
[1910.1018] TWA 0.010 mg/m3[1]
REL (Recommended)
Ca C 0.002 mg/m3 [15-minute][1]
IDLH (Immediate danger)
Ca [5 mg/m3 (as As)][1]
Related compounds
Related compounds
Arsenic acid
Except where otherwise noted, 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.[2]


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.[3] 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.[4]


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.[4] 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. ^ a b c "NIOSH Pocket Guide to Chemical Hazards #0038". National Institute for Occupational Safety and Health (NIOSH). 
  2. ^ Munoz-Hernandez, M.-A. (1994). "Arsenic: Inorganic Chemistry". In King, R. B. Encyclopedia of Inorganic Chemistry. Chichester: John Wiley & Sons. 
  3. ^ 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. 
  4. ^ a b Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9. 

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