Arsenate by itself is the trioxide oxyanion of arsenic with an oxidation state of five, denoted As(V); arsenates refer to the group of compounds containing this anion. Common arsenates are the fully protonated form – arsenic acid (H3AsO4), sodium arsenate (Na3AsO4), lead arsenate (PbHAsO4), and a host of naturally occurring minerals containing arsenate. Arsenate is the predominant arsenic species in oxidizing environments – such as bodies of water – though many forms of arsenic exist in waters depending on the conditions.
Arsenate forms a tetrahedral geometry with the four oxygens. The arsenic at the center of the arsenate anion is in the pentavalent state. Though there is one double bond and three single bonds depicted between the oxygens and the arsenic, it is understood that each bond from arsenic to an oxygen is identical because each of the lone pairs on the oxygens and the pi bond are delocalized, thus, there is a partial negative charge on each oxygen and each oxygen-arsenic bond has a slight double-bond character. The bond lengths reflect this. The structure of arsenate is similar to the structure of the biochemically abundant phosphate. This is simply due to the similarities between elemental arsenic and phosphorous, which arise from their relatedness on the periodic table.
Arsenic acid (H3AsO4) is the protonated form of the arsenate ion. Arsenic acid is a triprotic acid. With the following mass action equlibria and acid dissociation constants:
Similarities between phosphoric acid and arsenic acid are apparent; this stems from the similarities between phosphate and arsenate and thus from elemental properties of phosphorous and arsenic. Their three acid dissociation constants are of similar orders of magnitude. Although As has a greater van der Waals radius, the hydrated structures of both phosphorous and arsenic systems demonstrate close similarity (within 0.25 Å in the interaction distance between P/As and O of water and within a few kJ/mol in binding energy) due to the increased induction energies by the more polar arsenic acid or anions and slightly higher dispersion energy because of its greater size. When arsenic acid or one of its associated anions complexes with metal cations such as Na+, K+, Mg2+, and Ca2+, it has a larger binding distance (by 0.07-1.0 Å) and weaker binding energy than its analogous phosphorus system because arsenic (V) ion's radius is slightly larger than the phosphorous (V) ion's, and the electrostatic interaction is the prevalent characteristic in these systems.1)
One of the first uses of arsenate as an insecticide is the compound Paris green, which has the chemical formula Cu(C2H3O2)2·3Cu(AsO2)2 . It was used in 1867 on the Colorado potato beetle, then later by fruit growers against the codling moth to protect their apples, and even internationally against mosquitoes where it was directly mixed with bodies of water or wet sand. 2) Lead arsenate was first used in 1892 in Massachusetts to get rid of the gypsum moth but soon spread across the world as a popular insecticide. Lead arsenate was not as phytotoxic as Paris green and its desired effects were longer lasting. Lead arsenate became popular in two forms: basic lead arsenate (Pb5OH(AsO4)3) in California and acid lead arsenate (PbHAsO4) everywhere else. During the First World War calcium arsenate replaced lead arsenate due to the increasing price of lead. It wasn’t until 1947 that the toxic arsenate insecticides were replaced by a synthetic organic insecticide, though some use of lead arsenate persisted through the mid 20th century.3)
Since arsenic and many of its derivatives are naturally occurring and many of the man-made derivatives are also spread throughout the environment, it is of importance to observe how this affects the environment knowing that arsenic is poisonous to humans. Arsenate (AsO4-3) and arsenite (AsO3-3) are the most abundant forms of arsenic in soil.4) Plants can tolerate any where from 1 mg arsenic per kg soil to 50 mg arsenic per kg soil depending on the species. Arsenite is more toxic than arsenate, and both of them are more toxic than organic arsenic-containing compounds.5) 6) While arsenic is toxic to plants and animals some extremophiles thrive in arsenic rich environments and the debate is still out whether arsenic-based life is feasible.
There are many forms of arsenic in the environment of which some natural and some man-made are dangerous to life. Arsenate – as arsenic and its inorganic forms – is considered a carcinogen and highly poisonous to humans.7) Since arsenate is chemically similar to phosphate it takes the place of phosphate in ATP synthesis in phosphorus biochemistry, thus uncoupling oxidative phosphorylation.8)
by Jess Coulter