Arsenic acid or trihydrogen arsenate is the

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...

. More descriptively written as , this colorless

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

is the

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...

analogue of phosphoric acid. Arsenate and

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...

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 In chemistry, a hemihydrate (or semihydrate) is a hydrate whose solid contains one molecule of water of crystallization per two other molecules, or per two unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a ...

form () does form stable crystals. Crystalline samples dehydrate with condensation at 100 °C.

Properties

It is a tetrahedral species of idealized

symmetry Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definit ...

''C''_3v with As–O bond lengths ranging from 1.66 to 1.71 Å. Being a triprotic acid, its acidity is described by three equilibria: :, p''K''_a1 = 2.19 :, p''K''_a2 = 6.94 :, p''K''_a3 = 11.5 These p''K''_a values are close to those for phosphoric acid. The highly basic arsenate ion () 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 Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...

Preparation

Arsenic acid is prepared by treating arsenic trioxide with concentrated

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...

. Dinitrogen trioxide is produced as a by-product. : The resulting solution is cooled to give colourless crystals of the

, although the dihydrate is produced when crystallisation occurs at lower temperatures.

Other methods

Arsenic acid is slowly formed when

arsenic pentoxide Arsenic pentoxide is the inorganic compound with the formula As2O5. This glassy, white, deliquescent solid is relatively unstable, consistent with the rarity of the As(V) oxidation state. More common, and far more important commercially, is ars ...

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 Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

. :

Applications

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 biocide is defined in the European legislation as a chemical substance or microorganism intended to destroy, deter, render harmless, or exert a controlling effect on any harmful organism. The US Environmental Protection Agency (EPA) uses a slig ...

, a finishing agent for glass and metal, and a

reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

in the synthesis of some

dyestuff A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...

s and organic arsenic compounds.

Safety

Arsenic acid is extremely toxic and carcinogenic, like all arsenic compounds. It is also corrosive. The in rabbits is 6 mg/kg (0.006 g/kg).

References

{{Authority control Arsenates Arsenic(V) compounds Oxoacids Oxidizing acids