Arsenate is a negatively charged ion with the formula AsO₄^3–. It has one arsenic atom in the center and four oxygen atoms around it. Arsenate forms from arsenic acid (H₃AsO₄) when hydrogen ions are removed in water. Arsenate easily combines with metal ions to make salts, like sodium arsenate (Na₃AsO₄) and calcium arsenate (Ca₃(AsO₄)₂). [1-2]

Structure and Bonding

The arsenate ion (AsO₄^3–) has a tetrahedral geometry in which the four oxygen atoms are located at the corners of a tetrahedron, resulting in bond angles close to 109.5°. Its Lewis structure is shown below [3].

The negative charge is delocalized over all four oxygen atoms due to resonance. This means:

• All As–O bonds are nearly the same length.
• The ion is more stable than if the charge were localized.
• It can form stable salts with metal ions.
• It binds to mineral surfaces in soils and interacts with biological molecules.

Because arsenic and phosphorus have the same valency and comparable atomic sizes, arsenate closely resembles phosphate in both shape and chemical behavior. As a result, arsenate can substitute for phosphate in many biological reactions. However, it is toxic to most living organisms.

Occurrence in Nature

Arsenate in the environment is mainly produced from natural geological processes. When arsenic-bearing minerals undergo weathering, arsenic is released and is rapidly oxidized to arsenate in the presence of oxygen. [5]

A common source mineral is arsenopyrite (FeAsS). Other arsenate-containing minerals include:

• scorodite (FeAsO₄·2H₂O)
• adamite (Zn₂AsO₄OH)
• erythrite (Co₃(AsO₄)2·8H₂O)
• annabergite (Ni₃(AsO₄)₂·8H₂O)
• legrandite (Zn₂AsO₄OH·H₂O)

Volcanoes and hot springs also release arsenic into the air and water. In oxygen-rich environments, arsenic is oxidized to arsenate, which can migrate through soil, rocks, and groundwater.

Arsenate can bind to iron minerals in soil, which may slow its movement. However, under alkaline conditions it becomes more soluble and can spread into drinking-water sources. Because of its toxicity, arsenic contamination is a serious environmental problem in parts of South Asia, South America, and China.

Why is Arsenate Toxic

Arsenate is a poisonous substance. It interferes with the way cells produce energy. It can replace phosphate in glycolysis, causing a step that normally makes ATP to fail. Glycolysis still continues, but less ATP is produced. Arsenate also blocks the Krebs cycle by preventing pyruvate from being converted into acetyl-CoA. As a result, cells lose a lot of their energy supply, leading to toxic effects. [4]

Common Arsenate Compounds [6]

Several arsenates listed were historically used as pesticides but are now restricted or discontinued in many countries due to toxicity.

Acid–Base Behavior

Arsenic can participate in acid-base reactions in water. Arsenic acid (H₃AsO₄) is a triprotic acid that loses hydrogen ions step by step: [7]

H₃AsO₄ ⇌ H₂AsO₄^– + H⁺

H₂AsO₄^– ⇌ HAsO₄^2– + H⁺

HAsO₄^2– ⇌ AsO₄^3– + H⁺

Different forms exist at different pH values:

• Acidic solutions: H₃AsO₄
• pH 2–7: H₂AsO₄^–
• pH 7–11: HAsO₄^2–
• Strongly alkaline: AsO₄^3–

In most natural waters, H₂AsO₄^– and HAsO₄^2– are the main forms present.

This pH-dependent change is called speciation. Speciation controls the solubility of arsenate in water, its binding affinity to soil particles, and its ability to enter living cells. Understanding its behavior helps scientists design better arsenic-removal methods, predict how arsenic moves through soils and aquifers, and reduce risks to human health.