The arsenate

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

is . An arsenate (compound) is any

compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with defensive struc ...

that contains this ion. Arsenates are

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

s or

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...

s of

arsenic acid Arsenic acid or trihydrogen arsenate is the chemical compound with the formula . More descriptively written as , this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as ...

. The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As(V). Arsenate resembles

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 ...

in many respects, since

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, ...

and

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

occur in the same group (column) of the periodic table. Arsenates are moderate oxidizers, with an

electrode potential In electrochemistry, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrode ( ...

of +0.56 V for reduction to

arsenite In chemistry, an arsenite is a chemical compound containing an arsenic oxyanion where arsenic has oxidation state +3. Note that in fields that commonly deal with groundwater chemistry, arsenite is used generically to identify soluble AsIII anions. ...

Occurrence

Arsenates occur naturally in a variety of

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

s. Those minerals may contain

hydrated Drinking is the act of ingesting water or other liquids into the body through the mouth, proboscis, or elsewhere. Humans drink by swallowing, completed by peristalsis in the esophagus. The physiological processes of drinking vary widely among o ...

or anhydrous arsenates. Unlike phosphates, arsenates are not lost from a mineral during

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...

. Examples of arsenate-containing minerals include adamite,

alarsite Alarsite (AlAsO4) is an aluminium arsenate mineral with its name derived from its composition: aluminium and arsenate. It occurs as brittle subhedral grains which exhibit trigonal symmetry. It has a Mohs hardness of 5-5.5 and a specific gravity o ...

annabergite Annabergite is an arsenate mineral consisting of a hydrous nickel arsenate, Ni3(AsO4)2·8H2O, crystallizing in the monoclinic system and isomorphous with vivianite and erythrite. Crystals are minute and capillary and rarely met with, the mineral ...

erythrite Erythrite or red cobalt is a secondary hydrated cobalt arsenate mineral with the formula . Erythrite and annabergite, chemical formula , or nickel arsenate form a complete series with the general formula . Erythrite crystallizes in the monocli ...

and legrandite. Where two arsenate ions are required to balance the charge in a formula, it is called diarsenate for example trizinc diarsenate, Zn₃(AsO₄)₂.

Ions

The word arsenate is derived from

, H₃AsO₄. This moderately strong acid converts to dihydrogen arsenate (), hydrogen arsenate (), and arsenate (), depending on pH. The quantitative relationship between these species is defined by the acid dissociation constants: :H₃AsO₄ + H₂O + H₃O⁺(log ''K''₁ = −2.19) : + H₂O + H₃O⁺(log ''K''₂ = −6.94) : + H₂O + H₃O⁺(log ''K''₃ = −11.5) These values are similar to those of the hydrogen phosphates. Hydrogen arsenate and dihydrogen arsenate predominate in aqueous solution near neutral pH.

Arsenate poisoning

Arsenate can replace inorganic

in the step of glycolysis that produces 1,3-bisphosphoglycerate from

glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is a metabolite that occurs as an intermediate in several central pathways of all organisms.Nelson, D ...

. This yields 1-arseno-3-phosphoglycerate instead, which is unstable and quickly hydrolyzes, forming the next intermediate in the pathway,

3-phosphoglycerate 3-Phosphoglyceric acid (3PG, 3-PGA, or PGA) is the conjugate acid of 3-phosphoglycerate or glycerate 3-phosphate (GP or G3P). This glycerate is a biochemically significant metabolic intermediate in both glycolysis and the Calvin-Benson cycle. The ...

. Therefore, glycolysis proceeds, but the ATP molecule that would be generated from 1,3-bisphosphoglycerate is lost – arsenate is an uncoupler of glycolysis, explaining its toxicity. As with other arsenic compounds, arsenite binds to lipoic acid, inhibiting the conversion of pyruvate into acetyl-CoA, blocking the

Krebs cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and protein ...

and therefore resulting in further loss of ATP.

References

{{Reflist Oxyanions * Arsenic(V) compounds

Occurrence

Ions

Arsenate poisoning

See also

References