Chemistry:Arsenic acid

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Arsenic acid
Structural formula
Ball-and-stick model
IUPAC name
Arsenic acid, arsoric acid
Other names
Arsenic acid
Orthoarsenic acid
Desiccant L-10
3D model (JSmol)
EC Number
  • 231-901-9
RTECS number
  • CG0700000
UN number 1553, 1554
Molar mass 141.94 g/mol
Appearance White translucent crystals,
Density 2.5 g/cm3
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
Conjugate base Arsenate
Main hazards Extremely toxic, carcinogenic, corrosive
GHS pictograms GHS05: CorrosiveGHS06: ToxicGHS07: HarmfulGHS08: Health hazardGHS09: Environmental hazard
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)
Flammability code 0: Will not burn. E.g. waterHealth code 4: Very short exposure could cause death or major residual injury. E.g. VX gasReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no codeNFPA 704 four-colored diamond
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
48 mg/kg (rat, oral)

6 mg/kg (rabbit, oral)

Related compounds
Other anions
Phosphoric acid
Other cations
Sodium arsenate
Related compounds
Arsenous acid
Arsenic pentoxide
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
Tracking categories (test):
Three bottles of arsenic acid from the Great Exhibition: impure, pure and distilled.

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 (H3AsO4·1/2H2O) does form stable crystals. Crystalline samples dehydrate with condensation at 100 °C.[1]


It is a tetrahedral species of idealized symmetry C3v with As–O bond lengths ranging from 1.66 to 1.71 Å.[2]

Being a triprotic acid, its acidity is described by three equilibria:

H3AsO4 + H2O ⇌ H2AsO4 + H3O+  (pKa1 = 2.19)
H2AsO4 + H2O ⇌ HAsO2−4 + H3O+  (pKa2 = 6.94)
HAsO2−4 + 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.


Arsenic acid is prepared by treating arsenic trioxide with concentrated nitric acid. Dinitrogen trioxide is produced as a by-product.[3]

As2O3 + 2 HNO3 + 2 H2O → 2 H3AsO4 + N2O3

The resulting solution is cooled to give colourless crystals of the hemihydrate H3AsO4·1/2H2O, although the dihydrate H3AsO4·2H2O is produced when crystallisation occurs at lower temperatures.[3]

Other methods

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.[4]


Arsenic acid is extremely toxic and carcinogenic, like all arsenic compounds. It is also corrosive. The -1">50 in rabbits is 6 mg/kg (0.006 g/kg).[5]


  1. Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5. 
  2. 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. 
  3. 3.0 3.1 G. Brauer, ed (1963). "Arsenic Acid". Handbook of Preparative Inorganic Chemistry. 1 (2nd ed.). New York: Academic Press. p. 601. 
  4. Minerals Yearbook, 2008, V. 1, Metals and Minerals. Government Printing Office. 2010. pp. 6–. ISBN 978-1-4113-3015-3. 
  5. 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.