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)
ECHA InfoCard 100.029.001
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
GHS pictograms The corrosion pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)The skull-and-crossbones pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)The exclamation-mark pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)The health hazard pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)The environment pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)
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
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
48 mg/kg (rat, 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

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·​12H2O) 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 bonds lengths ranging from 1.66 to 1.71 Å.[2]

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

H3AsO4 + H2O ⇌ H2AsO
+ H3O+ (K1 = 10−2.19)
+ H2O ⇌ HAsO2−
+ H3O+ (K2 = 10−6.94)
+ H2O ⇌ AsO3−
+ H3O+ (K3 = 10−11.5)

These Ka values are close to those for phosphoric acid. The highly basic arsenate ion (AsO3−
) is the product of the third ionization. Unlike phosphoric acid, arsenic acid is an oxidizer, 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·​12H2O, although the dihydrate H3AsO4·2H2O is produced when crystallisation occurs at lower temperatures.[3]

Other methods[edit]

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, 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]


  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. ^ a b G. Brauer, ed. (1963). "Arsenic Acid". Handbook of Preparative Inorganic Chemistry. 1 (2nd ed.). New York: Academic Press. p. 601. 
  4. ^ Grund, S. C.; Hanusch, K.; Wolf, H. U. (2005), "Arsenic and Arsenic Compounds", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a03_113.pub2