Calcium arsenate is the
inorganic compound
An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bondsthat is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''.
Inorgan ...
with the formula Ca
3(AsO
4)
2. A colourless salt, it was originally used as a
pesticide
Pesticides are substances that are used to control pests. They include herbicides, insecticides, nematicides, fungicides, and many others (see table). The most common of these are herbicides, which account for approximately 50% of all p ...
and as a
germicide. It is highly soluble in water, in contrast to
lead arsenate, which makes it more toxic. Two minerals are hydrates of calcium arsenate:
rauenthalite Ca
3(AsO
4)
2·10H
2O and
phaunouxite Ca
3(AsO
4)
2·11H
2O. A related mineral is ferrarisite (.
Preparation
Calcium arsenate is commonly prepared from
disodium hydrogen arsenate and
calcium chloride
Calcium chloride is an inorganic compound, a Salt (chemistry), salt with the chemical formula . It is a white crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with cal ...
:
:2 Na
2H
sO4 + 3 CaCl
2 → 4 NaCl + Ca
3 sO4sub>2 + 2 HCl
In the 1920s, it was made in large vats by mixing calcium oxide and arsenic oxide. In the United States, 1360 metric tons were produced in 1919, 4540 in 1920, and 7270 in 1922.
The composition of commercially available calcium arsenate varies from manufacturer to manufacturer. A typical composition is 80-85% of Ca
3(AsO
4)
2 a basic arsenate probably with a composition of 4CaO.As
2O
5 together with calcium hydroxide and calcium carbonate.
Use as a herbicide
It was once a common herbicide and insecticide. 38,000,000 kilograms were reported to be produced in 1942 alone, mainly for protection of cotton crops. Its high toxicity led the development of
DDT.
[Robert L. Metcalf "Insect Control" in ''Ullmann’s Encyclopedia of Industrial Chemistry'' Wiley-VCH, Weinheim, 2002. ]
Regulation
Calcium arsenate use is now banned in the UK, and its use is strictly regulated in the United States. It is currently the active ingredient in TURF-Cal manufactured by Mallinckrodt, it is one of the few herbicides – used mainly for the control of ''
Poa annua
''Poa annua'', or annual meadow grass (known in America more commonly as annual bluegrass or simply poa), is a widespread low-growing turfgrass in temperate climates. Notwithstanding the reference to annual plant in its name, perennial bio-types ...
'' and
crabgrass- that hinders earthworm activity. Its label states that it will "reduce and inhibit earthworm activity and survival" and is only recommended against serious earthworm infestations in places such as golf course greens.
Toxicity and regulation
Calcium arsenate is highly toxic, having both
carcinogen
A carcinogen () is any agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and biologic agents such as viruse ...
ic and systemic health effects.
[Tchounwou, P.B.; Patlolla, A.K.; Centeno, J.A.; Carcinogenic and Systematic Health Effects Associated with Arsenic – A Critical Review. ''Toxicologic Pathology''; 2003, ''31'', 575-588] The
Occupational Safety and Health Administration
The Occupational Safety and Health Administration (OSHA; ) is a regulatory agency of the United States Department of Labor that originally had federal visitorial powers to inspect and examine workplaces. The United States Congress established ...
has set a
permissible exposure limit
The permissible exposure limit (PEL or OSHA PEL) is a legal limit in the United States for exposure of an employee to a chemical substance or physical agents such as high level noise. Permissible exposure limits were established by the Occupational ...
at 0.01 mg/m
3 over an eight-hour time-weighted average, while the
National Institute for Occupational Safety and Health
The National Institute for Occupational Safety and Health (NIOSH, ) is the List of United States federal agencies, United States federal agency responsible for conducting research and making recommendations for the prevention of work-related occ ...
recommends a limit five times less (0.002 mg/m
3).
It is classified as an
extremely hazardous substance in the United States as defined in Section 302 of the U.S.
Emergency Planning and Community Right-to-Know Act
The Emergency Planning and Community Right-to-Know Act of 1986 is a United States federal law passed by the 99th United States Congress located at Title 42, Chapter 116 of the U.S. Code, concerned with emergency response preparedness.
On Octobe ...
(42 U.S.C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.
Other natural occurrences
Weilite is the monohydrogenated counterpart, Ca(HAsO
4), while švenekite – the dihydrogenated one, Ca(H
2AsO
4)
2. Hydrated analogues of weilite are haidingerite (monohydrate) and pharmacolite (dihydrate), with the latter name reflecting arsenic-related toxicity. Examples of more complex, hydrated Ca arsenates with some anions hydrogenated, are ferrarisite, guérinite, sainfeldite, vladimirite, and jeankempite.
References
{{DEFAULTSORT:Calcium Arsenate
Arsenates
Calcium compounds
Insecticides
Mutagens
Carcinogens