Arsenic acid

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Arsenic acid


IUPAC name	Arsenic acid, arsoric acid
Other names	Arsenic acid orthoarsenic acid desiccant L-10 zotox
Identifiers
CAS number	[7778-39-4]
RTECS number	CG0700000
Properties
Molecular formula	H₃AsO₄ (Solution Only) H₃AsO₄·1⁄2H₂O (Stable Form)
Molar mass	141.9429 g/mol
Appearance	White translucent crystals, hygroscopic.
Density	? g/cm³, solid
Melting point	35.5 °C (308.65 K)
Boiling point	decom ≥ 100 °C
Solubility in water	16.7 g/100 mL
Acidity (pK_a)	2.19, 6.94, 11.5
Structure
Molecular shape	Tetrahedral
Hazards
Main hazards	Toxic.
R-phrases	R26, R27, R28, R45.
S-phrases	S53, S45, S60, S61.
Flash point	Non-flammable.
Related compounds
Other anions	Phosphoric acid
Other cations	Sodium arsenate
Related compounds	Arsenous acid Arsenic pentoxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and 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 (H₃AsO₄·1⁄2H₂O) does form stable crystals. Crystalline samples dehydrate with condensation at 100 °C.^[1]

It is a tetrahedral species of idealized symmetry C_3v with As-O bonds lengths ranging from 1.66-1.71Å.^[2]

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

H₃AsO₄ $\overrightarrow{\leftarrow}$ H₂AsO₄^- + H⁺, K₁ = 10^-2.19

H₂AsO₄^- $\overrightarrow{\leftarrow}$ HAsO₄^2- + H⁺, K₂ = 10^-6.94

HAsO₄^2- $\overrightarrow{\leftarrow}$ AsO₄^3- + H⁺, K₃ = 10^-11.5

These K_a's are close to those for phosphoric acid. The highly basic arsenate ion, AsO₄^3-, is the product of the third ionization. Unlike phosphoric acid, arsenic acid is oxidizing, illustrated by its ability to convert iodide to iodine.

[edit] Applications

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 retard of some dyestuffs and organic arsenic compounds.

[edit] Historic Use

Atoxyl was first synthesized in 1859 by Antoine Béchamp by chemically reacting arsenic acid with aniline. Béchamp optimistically chose the name Atoxyl, referring to the reduced toxicity of the resulting compound (compared to arsenic). Atoxyl was initially used in medicine to treat simple skin diseases. In 1905, two British physicians, H. W. Thomas and A. Breinl, discovered that Atoxyl was active against the trypanosomes of sleeping sickness. However, the effect of atoxyl was not very pronounced, and the effective dose was so high that its toxic side effects far outweighed the benefits. In 1909, Paul Ehrlich and Sahachiro Hata used Atoxyl as the basis to create Salvarsan, another arsenical used as an anti-syphiltic drug.

[edit] References

^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
^ Lee, C.; Harrison, W. T. A. "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 (2007). C63, m308-m311. doi=10.1107/S0108270107023967.