| Arsenous acid | |
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Arsorous acid |
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Other names
Arsenious acid |
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| Identifiers | |
| CAS number | 13464-58-9 = |
| PubChem | 545 |
| ChemSpider | 530 |
| DrugBank | DB04456 |
| ChEBI | CHEBI:49900 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | H3AsO3 |
| Molar mass | 125.94 g/mol |
| Appearance | Only exists in aqueous solutions |
| Hazards | |
| Main hazards | Toxic, corrosive |
| Related compounds | |
| Related compounds | Arsenic acid |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Arsenous acid, also known as 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]
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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. 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.
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