Arsenic acid

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.

It is a tetrahedral species of idealized symmetry C3v with As-O bonds lengths ranging from 1.66-1.71Å.

Being a triprotic acid, its acidity is described by three equilibria:
 * H3AsO4$$\overrightarrow{\leftarrow}$$ H2AsO4-  +  H+,    K1 = 10-2.19
 * H2AsO4-$$\overrightarrow{\leftarrow}$$ HAsO42-  +  H+,    K2 = 10-6.94
 * HAsO42-$$\overrightarrow{\leftarrow}$$  AsO43-  +  H+,    K3 = 10-11.5

These Ka's are close to those for phosphoric acid. The highly basic arsenate ion, AsO43-, is the product of the third ionization. Unlike phosphoric acid, arsenic acid is oxidizing, illustrated by its ability to convert iodide to iodine.

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.

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.