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 . More descriptively written as , this colorless acid 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, ...

analogue of

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...

Arsenate The arsenate ion is . An arsenate (compound) is any compound that contains this ion. Arsenates are salts or esters of arsenic acid. The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As(V). Arsenate res ...

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 phosph ...

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

Properties

It is a tetrahedral species of idealized symmetry ''C''_3v with As–O bond lengths ranging from 1.66 to 1.71 Å. Being a

triprotic 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 ...

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

. 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 An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine de ...

to iodine.

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 Dinitrogen trioxide is the chemical compound with the formula N2O3. It is one of the simple nitrogen oxides. It forms upon mixing equal parts of nitric oxide and nitrogen dioxide and cooling the mixture below −21 °C (−6 °F): :NO + ...

is produced as a by-product. : The resulting solution is cooled to give colourless crystals of the hemihydrate , 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 arse ...

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 Wood easily degrades without sufficient preservation. Apart from structural wood preservation measures, there are a number of different chemical preservatives and processes (also known as "timber treatment", "lumber treatment" or "pressure treat ...

, a broad-spectrum biocide, a finishing agent for glass and metal, and a reagent 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 A corrosive substance is one that will damage or destroy other substances with which it comes into contact by means of a chemical reaction. Etymology The word ''corrosive'' is derived from the Latin verb ''corrodere'', which means ''to gnaw'', ...

. The in rabbits is 6 mg/kg (0.006 g/kg).

References

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