Arsenous acid
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| Ball-and-stick model | |
| Names | |
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| IUPAC name
Arsorous acid
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| Other names
Arsenious acid
Arsenic oxide |
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| Identifiers | |
13464-58-9  |
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| ChEBI | CHEBI:49900 |
| ChemSpider | 530 |
| DrugBank | DB04456 |
| Jmol 3D model | Interactive image |
| PubChem | 545 |
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| Properties | |
| H3AsO3 | |
| Molar mass | 125.94 g/mol |
| Appearance | Only exists in aqueous solutions |
| Vapor pressure | {{{value}}} |
| Related compounds | |
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Related compounds
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Arsenic acid |
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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.[1]
Properties
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]
Reactions
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.
Toxicology
Arsenic-containing compounds are highly toxic and carcinogenic. The anhydride form of arsenous acid, arsenic trioxide, is used as a herbicide, pesticide, and rodenticide.
References
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