|Jmol-3D images||Image 1|
|Molar mass||125.94 g/mol|
|Appearance||Only exists in aqueous solutions|
|Main hazards||Toxic, corrosive|
|Related compounds||Arsenic acid|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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.
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. 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.
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. Reactions attributed to aqueous arsenic trioxide are due to arsenous acid and its conjugate bases.
- Munoz-Hernandez, M.-A. (1994). "Arsenic: Inorganic Chemistry". In King, R. B. Encyclopedia of Inorganic Chemistry. Chichester: John Wiley & Sons.
- 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.
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419.
- "Arsenic trioxide". Retrieved January 29, 2006.