User:Mac239/Arsenate

Article Draft arsenate

Lead

The arsenate ion has the formula ${\ce {AsO^{3}4-}}$. Bonding in arsenate consists of a central arsenic atom, with oxidation state +5, double bonded to one oxygen atom and single bonded to a further three oxygen atoms. The four oxygen atoms orient around the arsenic atom in a tetrahedral geometry. The ion's 3^- charge is dispersed across all four oxygen atoms through resonance.

Arsenate readily reacts with metals to form arsenate metal compounds. Arsenate is a moderate oxidizer and an electron acceptor, with an electrode potential of +0.56 V for its reduction to arsenite. Due to arsenic having the same valency and similar atomic radius to phosphorus, arsenate shares similar geometry and reactivity with phosphate. Arsenate can replace phosphate in biochemical reactions and is toxic to most organisms.

Speciation

Depending on the pH, arsenate can be found as arsenic acid (${\textstyle {\ce {H3AsO4}}}$) dihydrogen arsenate (${\ce {H2AsO4-}}$), hydrogen arsenate (${\ce {HAsO^{2}4-}}$), or arsenate (${\ce {AsO^{3}4-}}$). Trihydrogen arsenate is also known as arsenic acid. At a given pH, the distribution of these arsenate species can be determined from their respective acid dissociation constants.

Pourbaix diagram showing the distribution of arsenate and arsenite species in water. Oxygenated waters have a high pe value and arsenate species dominate while in deoxygenated water, with low pe, arsenite species dominate.

H₃AsO₄ + H₂O ⇌ H
₂AsO⁻
₄ + H₃O⁺ (log K₁ = −2.19)

H
₂AsO⁻
₄ + H₂O ⇌ HAsO²⁻
₄ + H₃O⁺ (log K₂ = −6.94)

HAsO²⁻
₄ + H₂O ⇌ AsO³⁻
₄ + H₃O⁺H (log K₃ = −11.5)

These values are similar to those of phosphoric acid. Hydrogen arsenate and dihydrogen arsenate predominate in aqueous solution near neutral pH.

Uses

Arsenate based pesticides such as lead hydrogen arsenate were commonly used until their replacement by newer pesticides such as DDT and subsequent ban by multiple regulatory bodies due to health concerns.

Cobalt violet pigment

Egyptian blue pigment

Chromated copper arsenate wood

Contamination

Water; rivers

Food; honey

Occurrence[edit]

Adamite, one of the many arsenate minerals that occur naturally.

Arsenates occur naturally, in hydrated and anhydrous form, in a variety of minerals. Examples of arsenate-containing minerals include adamite, alarsite, annabergite, erythrite and legrandite. Where two arsenate ions are required to balance the charge in a formula, it is called diarsenate for example zinc diarsenate, Zn₃(AsO₄)₂.

Arsenate poisoning[edit]

Arsenate is harmful to humans and animals as it interferes with the normal functioning of glycolysis and the Krebs cycle. Arsenate replaces inorganic phosphate in the step of glycolysis that produces 1,3-bisphosphoglycerate from glyceraldehyde 3-phosphate. This yields 1-arseno-3-phosphoglycerate instead, which is unstable and quickly hydrolyzes, forming the next intermediate in the pathway, 3-phosphoglycerate. 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, arsenate binds to lipoic acid, inhibiting the conversion of pyruvate into acetyl-CoA, blocking the Krebs cycle and therefore resulting in further loss of ATP.