Arsenic acid
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Arsenic acid, arsoric acid |
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Other names
Arsenic acid |
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| Identifiers | |
| CAS number | 7778-39-4  |
| ChemSpider | 229 |
| UNII | N7CIZ75ZPN |
| EC number | 231-901-9 |
| KEGG | C01478 |
| ChEBI | CHEBI:18231 |
| RTECS number | CG0700000 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | H3AsO4 |
| Molar mass | 141.94 g/mol |
| Appearance | White translucent crystals, hygroscopic. |
| Density | 2.5 g/cm3 |
| Melting point |
35.5 °C, 309 K, 96 °F |
| Boiling point |
120 °C, 393 K, 248 °F (decomp) |
| Solubility in water | 16.7 g/100 mL |
| Solubility | soluble in alcohol |
| Vapor pressure | 55 hPa (50 °C) |
| Acidity (pKa) | 2.19, 6.94, 11.5 |
| Structure | |
| Molecular shape | Tetrahedral |
| Hazards | |
| EU classification | Toxic (T) Dangerous for the environment (N) |
| R-phrases | R23/25, R45, R50/53 |
| S-phrases | S53, S45, S60, S61 |
| NFPA 704 |
 0
3
0
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| Flash point | Non-flammable |
| LD50 | 48 mg/kg (rat, oral) |
| Related compounds | |
| Other anions | Phosphoric acid |
| Other cations | Sodium arsenate |
| Related compounds | Arsenous acid Arsenic pentoxide |
 (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 | |
Arsenic acid is the chemical compound with the formula H3AsO4. More descriptively written as AsO(OH)3, this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as such has not been isolated, but only found in solution where it is largely ionized. Its hemihydrate form (H3AsO4·½H2O) does form stable crystals. Crystalline samples dehydrate with condensation at 100 °C.[1]
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Properties[edit]
It is a tetrahedral species of idealized symmetry C3v with As-O bonds lengths ranging from 1.66 to 1.71 Å.[2]
Being a triprotic acid, its acidity is described by three equilibria:
- H3AsO4
H2AsO−
4 + H+ (K1 = 10−2.19) - H2AsO−
4 HAsO2−
4 + H+ (K2 = 10−6.94) - HAsO2−
4 AsO3−
4 + H+ (K3 = 10−11.5)
H2AsOThese Ka values are close to those for phosphoric acid. The highly basic arsenate ion (AsO3−
4) is the product of the third ionization. Unlike phosphoric acid, arsenic acid is oxidizing, illustrated by its ability to convert iodide to iodine.
Preparation[edit]
Arsenic acid is prepared by treating arsenic trioxide with concentrated nitric acid.[3]
- As2O3 + 2 HNO3 + 3 H2O → 2 H3AsO4(H2O)0.5 + N2O3
The resulting solution is cooled to give colourless crystals of H3AsO4(H2O)0.5. When the crystallisation is conducted at lower temperatures, the dihydrate crystallises.[3]
Other methods[edit]
It also forms upon dissolving arsenic pentoxide with water, which is slow. It is also formed when meta- or pyroarsenic acid is treated with cold water. Arsenic acid can be prepared by reacting moist elemental arsenic with ozone.
- 2 As + 3 H2O + 5 O3 → 2 H3AsO4 + 5 O2
Applications[edit]
Commercial applications of arsenic acid are limited by its toxicity. It has found occasional use as a wood preservative, broad-spectrum biocide, a finishing agent for glass and metal, and a reagent in the synthesis of some dyestuffs and organic arsenic compounds. The LD50 in rabbits is 6 mg/kg (0.006 g/kg).[4]
References[edit]
- ^ Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5.
- ^ Lee, C.; Harrison, W. T. A. (2007). "Tetraethylammonium dihydrogenarsenate bis(arsenic acid) and 1,4-diazoniabicyclo[2.2.2]octane bis(dihydrogenarsenate) arsenic acid: hydrogen-bonded networks containing dihydrogenarsenate anions and neutral arsenic acid molecules". Acta Crystallographica C 63 (Pt 7): m308–m311. doi:10.1107/S0108270107023967. PMID 17609552.
- ^ a b G. Brauer, ed. (1963). "Arsenic Acid". Handbook of Preparative Inorganic Chemistry 1 (2nd ed.). New York: Academic Press. p. 601.
- ^ Joachimoglu, G. (1915). Biochemische Zeitschrift 70: 144.
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