Arsenic acid

Arsenic acid
Arsenic acid
CAS number 7778-39-4 YesY
ChemSpider 229 YesY
KEGG C01478 YesY
ChEBI CHEBI:18231 YesY
RTECS number CG0700000
Jmol-3D images Image 1
Molecular formula H3AsO4
Molar mass 141.94 g/mol
Appearance White translucent crystals,
Density 2.5 g/cm3
Melting point

35.5 °C, 309 K, 96 °F

Boiling point

decom ≥ 100 °C

Solubility in water 16.7 g/100 mL
Solubility soluble in alcohol
Acidity (pKa) 2.19, 6.94, 11.5
Molecular shape Tetrahedral
EU classification Toxic (T)
Dangerous for the environment (N)
R-phrases R23/25, R45, R50/53
S-phrases S53, S45, S60, S61
NFPA 704
NFPA 704.svg
Flash point Non-flammable
Related compounds
Other anions Phosphoric acid
Other cations Sodium arsenate
Related compounds Arsenous acid
Arsenic pentoxide
 YesY acid (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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]



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 is in equilibrium with H2AsO
+ H+ (K1 = 10−2.19)
is in equilibrium with HAsO2−
+ H+ (K2 = 10−6.94)
is in equilibrium with AsO3−
+ H+ (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 oxidizing, illustrated by its ability to convert iodide to iodine.


Arsenic acid is prepared by treating arsenic trioxide with concentrated nitric acid, or by combination of arsenic pentoxide with water. The latter reaction is very slow. It is also formed when meta- or pyroarsenic acid is treated with cold water.

Other methods

Arsenic acid can be prepared by reacting moist elemental arsenic with ozone.

2As + 3H2O + 5O3 → 2H3AsO4 + 5O2


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).[3]


  1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. 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 C63 (Pt 7): m308-m311. doi:10.1107/S0108270107023967. PMID 17609552. 
  3. ^ Joachimoglu. Biochem. Z. 70, 144 (1915)

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Look at other dictionaries:

  • arsenic acid — n. a white, poisonous, crystalline compound, H3AsO4, used to make insecticides, medicines, etc …   English World dictionary

  • arsenic acid — ar·sen·ic acid är .sen ik n any of three arsenic containing acids that are analogous to the phosphoric acids * * * ar·sen·ic ac·id (ahr senґik) the hydrate, H3AsO4, of arsenic pentoxide, which is itself also referred to as arsenic acid …   Medical dictionary

  • arsenic acid anhydride — arseno(V) oksidas statusas T sritis chemija formulė As₂O₅ atitikmenys: angl. arsenic acid anhydride; arsenic pentoxide; arsenic(V) oxide rus. мышьяка(V) оксид; мышьяковый ангидрид ryšiai: sinonimas – diarseno pentoksidas …   Chemijos terminų aiškinamasis žodynas

  • arsenic acid — noun an acid formed from arsenic pentoxide • Hypernyms: ↑acid …   Useful english dictionary

  • arsenic acid — noun A poisonous, white crystalline compound, HAsO, formed from arsenic pentoxide and water; it has the same structure as phosphoric acid, and is used in the manufacture of arsenates …   Wiktionary

  • arsenic acid — Chem. a white, crystalline, water soluble powder, H3AsO4·1/2H2O, used chiefly in the manufacture of arsenates. [1795 1805] * * * …   Universalium

  • arsenic acid — crystalline compound (Chemistry) …   English contemporary dictionary

  • arsenic acid — arsen′ic ac′id n. chem. a white, crystalline, water soluble powder, H3AsO4∙½H2O, used esp. in the manufacture of arsenates • Etymology: 1795 …   From formal English to slang

  • arsenic acid — /aˌsɛnɪk ˈæsəd/ (say ah.senik asuhd) noun a water soluble, crystalline compound, H3AsO4.½H2O, used in the manufacture of arsenates …  

  • Arsenic — (pronEng|ˈɑrsənɪk) is a chemical element that has the symbol As and atomic number of 33. Arsenic was first documented by Albertus Magnus in 1250cite book |last=Emsley |first=John |title=Nature s Building Blocks: An A Z Guide to the Elements |year …   Wikipedia

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