Arsenic acid

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Arsenic acid

Arsenic acid is the chemical compound

Chemical compound

A chemical compound is a Chemical substance consisting of two or more different chemical element Chemical bond together in a fixed mass ratio that can be split into simpler substances....
with the formula

Chemical formula

A chemical formula is a way of expressing information about the atoms that constitute a particular chemical compound, and how the relationship between those atoms changes in chemical reactions....
H₃AsO₄. More descriptively written as AsO(OH)₃, this colorless acid

Acid

An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion Activity greater than in pure water, i.e....
is the arsenic analogue of phosphoric acid

Phosphoric acid

Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula Hydrogen3PhosphorusOxygen4....
. 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.

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Encyclopedia

Arsenic acid is the chemical compound

Chemical compound

Chemical formula

Acid

Phosphoric acid

Symmetry group

The symmetry group of an object is the group of all isometries under which it is invariant with Function composition as the operation. It is a subgroup of the isometry group of the space concerned....
C_3v with As-O bonds lengths ranging from 1.66-1.71Å.

Being a triprotic acid

Acid

An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion Activity greater than in pure water, i.e....
, its acidity is described by three equilibria:

H₃AsO₄ H₂AsO₄^- + H⁺, K₁ = 10^-2.19

H₂AsO₄^- HAsO₄^2- + H⁺, K₂ = 10^-6.94

HAsO₄^2- AsO₄^3- + H⁺, K₃ = 10^-11.5

These K_a's are close to those for phosphoric acid. The highly basic arsenate ion

Arsenate

The arsenate ion is ArsenicOxygen43−.An arsenate is any chemical compound that contains this ion.The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As[V]....
, AsO₄^3-, 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 an iodine with a −1 electric charge. Compounds with iodine in formal oxidation state −1 are called iodides. This can include ionic compounds such as caesium iodide or covalent compounds such as phosphorus triiodide....
to iodine

Iodine

Iodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
.

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 or reactant is a substance or compound consumed during a chemical reaction. Solvents and catalysts, although they are involved in the reaction, are usually not referred to as reactants....
in the retard of some dyestuffs and organic arsenic compounds.

Historic Use

Atoxyl was first synthesized in 1859 by Antoine Béchamp

Antoine Béchamp

Pierre Jacques Antoine B?champ was a French biology. He studied silkworm parasites, and was the first to synthesise Atoxyl....
by chemically reacting arsenic acid with aniline

Aniline

Aniline, phenylamine or aminobenzene is an organic compound with the Chemical formula C6H7N. It is the simplest and one of the most important aromatic amines, being used as a precursor to more complex chemicals....
. 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

Sleeping sickness

Sleeping sickness or human African trypanosomiasis is a parasitic disease of people and animals, caused by protozoa of species Trypanosoma brucei and transmitted by the tsetse fly....
. However, the effect of atoxyl was not very pronounced, and the effective dose

Effective dose

An effective dose in pharmacology is the amount of medication that produces a therapeutic response in 50% of the people taking it, sometimes also called ED-50....
was so high that its toxic

Toxicity

Toxicity is the degree to which a substance is able to damage an exposed organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver ....
side effects far outweighed the benefits. In 1909, Paul Ehrlich

Paul Ehrlich

Paul Ehrlich was a German scientist in the fields of hematology, immunology, and chemotherapy, and Nobel Prize in Physiology or Medicine. He is noted for his research in autoimmunity, calling it "horror autotoxicus"....
and Sahachiro Hata

Sahachiro Hata

; was a Japan bacteriologist who developed the Arsphenamine drug in 1909 in the laboratory of Paul Ehrlich.Hata was born in Shimane Prefecture, and completed his medical education in Kyoto....
used Atoxyl as the basis to create Salvarsan, another arsenical used as an anti-syphiltic drug.