Arsenic acid

Arsenic acid

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Encyclopedia
Arsenic acid is the chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 H3AsO4. More descriptively written as AsO(OH)3, this colorless acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

 is the arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

 analogue of phosphoric acid
Phosphoric acid
Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...

. 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.

Properties


It is a tetrahedral species of idealized symmetry
Symmetry group
The symmetry group of an object is the group of all isometries under which it is invariant with composition as the operation...

 C3v with As-O bonds lengths ranging from 1.66 to 1.71 Å.

Being a triprotic acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

, its acidity is described by three equilibria:
H3AsO4 H2AsO + H+ (K1 = 10−2.19)
H2AsO HAsO + H+ (K2 = 10−6.94)
HAsO AsO + H+ (K3 = 10−11.5)


These Ka values are close to those for phosphoric acid. The highly basic arsenate ion
Arsenate
The arsenate ion is AsO43−.An arsenate 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]....

 (AsO) is the product of the third ionization. Unlike phosphoric acid, arsenic acid is oxidizing, illustrated by its ability to convert iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...

 to iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

.

Preparation


Arsenic acid is prepared by treating arsenic trioxide with concentrated nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...

, or by combination of arsenic pentoxide
Arsenic pentoxide
Arsenic pentoxide is the inorganic compound with the formula As2O5. This glassy, white solid is relatively unstable, consistent with the rarity of the As oxidation state. More common, and far more important commercially, is arsenic oxide ....

 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
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

.
2As + 3H2O + 5O3 → 2H3AsO4 + 5O2

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
Reagent
A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of...

 in the synthesis of some dyestuffs and organic arsenic compounds. The in rabbits is 6 mg/kg (0.006 g/kg).