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 is only found in solution, where it is largely ionized. Its hemihydrate form (H3AsO4·1/H2O) does form stable crystals. Crystalline samples dehydrate with condensation at 100 °C.
Arsenic acid, arsoric acid
3D model (JSmol)
|UN number||1553, 1554|
CompTox Dashboard (EPA)
|Molar mass||141.94 g/mol|
|Appearance||White translucent crystals,|
|Melting point||35.5 °C (95.9 °F; 308.6 K)|
|Boiling point||120 °C (248 °F; 393 K) decomposes|
|16.7 g/100 mL|
|Solubility||soluble in alcohol|
|Vapor pressure||55 hPa (50 °C)|
|Acidity (pKa)||2.19, 6.94, 11.5|
|Main hazards||Extremely toxic, carcinogenic, corrosive|
|GHS Signal word||Danger|
|H301, H312, H314, H318, H331, H350, H361, H400, H410|
|P201, P202, P260, P261, P264, P270, P271, P273, P280, P281, P301+310, P301+330+331, P302+352, P303+361+353, P304+340, P305+351+338, P308+313, P310, P311, P312, P321, P322, P330, P363, P391|
|NFPA 704 (fire diamond)|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|48 mg/kg (rat, oral)
6 mg/kg (rabbit, oral)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Being a triprotic acid, its acidity is described by three equilibria:
- H3AsO4 + H2O ⇌ H
4 + H3O+ (pKa1 = 2.19)
4 + H2O ⇌ HAsO2−
4 + H3O+ (pKa2 = 6.94)
4 + H2O ⇌ AsO3−
4 + H3O+ (pKa3 = 11.5)
These pKa 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 an oxidizer, as illustrated by its ability to convert iodide to iodine.
- As2O3 + 2 HNO3 + 2 H2O → 2 H3AsO4 + N2O3
Arsenic acid is slowly formed when arsenic pentoxide is dissolved in water, and when meta- or pyroarsenic acid is treated with cold water. Arsenic acid can also be prepared directly from elemental arsenic by moistening it and treating with ozone.
- 2 As + 3 H2O + 5 O3 → 2 H3AsO4 + 5 O2
Commercial applications of arsenic acid are limited by its toxicity. It is a precursor to a variety of pesticides. It has found occasional use as a wood preservative, a broad-spectrum biocide, a finishing agent for glass and metal, and a reagent in the synthesis of some dyestuffs and organic arsenic compounds.
- 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.
- G. Brauer, ed. (1963). "Arsenic Acid". Handbook of Preparative Inorganic Chemistry. 1 (2nd ed.). New York: Academic Press. p. 601.
- Minerals Yearbook, 2008, V. 1, Metals and Minerals. Government Printing Office. 2010. pp. 6–. ISBN 978-1-4113-3015-3.
- Grund, Sabina C.; Hanusch, Kunibert; Wolf, Hans Uwe (2008). "Arsenic and Arsenic Compounds". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a03_113.pub2. ISBN 978-3527306732.