In chemistry, an arsenite is a chemical compound containing an
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, ...
oxyanion An oxyanion, or oxoanion, is an ion with the generic formula (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determine ...
where arsenic has oxidation state +3. Note that in fields that commonly deal with groundwater chemistry, arsenite is used generically to identify soluble As
III anions. IUPAC have recommended that arsenite compounds are to be named as arsenate(III), for example ortho-arsenite is called trioxidoarsenate(III).
Ortho-arsenite contrasts to the corresponding anions of the lighter members of group 15,
phosphite which has the structure and
nitrite
The nitrite ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name nitrite als ...
, which is bent.
A number of different arsenite anions are known:
* ortho-arsenite, an ion of
arsenous acid, with a pyramidal shape
* meta-arsenite, a polymeric chain anion.
* pyro-arsenite,
* a polyarsenite,
* a polyarsenite,
*, a polymeric anion
In all of these the geometry around the As
III centers are approximately trigonal, the lone pair on the arsenic atom is stereochemically active.
Well known examples of arsenites include
sodium meta-arsenite which contains a polymeric linear anion, , and silver ortho-arsenite, , which contains the trigonal anion.
Preparation of arsenites
Some arsenite salts can be prepared from an aqueous solution of
. Examples of these are the meta-arsenite salts and at low temperature, hydrogen arsenite salts can be prepared, such as , , and .
Arsenite minerals
A number of minerals contain arsenite anions:
reinerite, ;
, ;
, ;
, (contains a complex polymeric anion);
, FeFe;
magnussonite, ;
, ;
, ;
, .
Arsenites in the environment
Arsenic can enter
groundwater
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolid ...
due to naturally occurring arsenic at deeper levels or from mine workings. Arsenic(III) can be removed from water by a number of methods, oxidation of As
III to As
V for example with chlorine followed by coagulation with for example iron(III) sulfate. Other methods include ion-exchange and filtration. Filtration is only effective if arsenic is present as particulates, if the arsenite is in solution it passes through the filtration membrane.
Uses
Sodium arsenite is used in the
water gas shift reaction
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
to remove carbon dioxide.
Fowler's solution first introduced in the 18th century was made up from as a solution of potassium meta-arsenite, .
Bacteria using and generating arsenite
Some species of
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
obtain their energy by
oxidizing
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
various fuels while
reducing 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 re ...
s to form arsenites. The
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...
s involved are known as
arsenate reductases.
In 2008, bacteria were discovered that employ a version of
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
with arsenites as
electron donor
In chemistry, an electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process.
Typical reducing agents undergo permanent che ...
s, producing arsenates (just like ordinary photosynthesis uses water as electron donor, producing molecular oxygen). The researchers conjectured that historically these photosynthesizing organisms produced the arsenates that allowed the arsenate-reducing bacteria to thrive.
"Arsenic-loving bacteria rewrite photosynthesis rules"
''Chemistry World'', 15 August 2008
In humans, arsenite inhibits pyruvate dehydrogenase
Pyruvate dehydrogenase is an enzyme that catalyzes the reaction of pyruvate and a lipoamide to give the acetylated dihydrolipoamide and carbon dioxide. The conversion requires the coenzyme thiamine pyrophosphate.
Pyruvate dehydrogenase i ...
(PDH complex) in the pyruvate
Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell.
Pyruvic ac ...
-acetyl CoA
Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized f ...
reaction, by binding to the –SH group of lipoamide
Lipoamide is a trivial name for 6,8-dithiooctanoic amide. It is the functional form of lipoic acid, i.e the carboxyl group is attached to protein via an amine with an amide linkage. Illustrative of the biochemical role of lipoamide is in the conv ...
, a participant coenzyme. It also inhibits the oxoglutarate dehydrogenase complex by the same mechanism. The inhibition of these enzymes disrupts energy production.
References
{{Reflist
External links
Case Studies in Environmental Medicine - Arsenic Toxicity
Oxyanions
*
Arsenic(III) compounds