Calcium arsenate

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Calcium arsenate
Calcium arsenate
Identifiers
CAS number 7778-44-1 YesY=
PubChem 24501
ChemSpider 22909 N
EC number 233-287-8
KEGG C18647 YesY
RTECS number CG0830000
Jmol-3D images Image 1
Properties
Molecular formula Ca3As2O8
Molar mass 398.072 g/mol
Appearance white powder
Odor odorless
Density 3.62 g/cm3, solid
Melting point 1,455 °C (2,651 °F; 1,728 K) (decomposes)
Solubility in water 0.013 g/100 mL (25 °C)[1]
Solubility in Organic solvents insoluble
Solubility in acids soluble
Hazards
LD50 20 mg/kg (rat, oral)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

Calcium arsenate is the inorganic compound with the formula Ca3(AsO4)2. A colourless solid, it was originally used as a pesticide and as a germicide. It is highly soluble in water, as compared with lead arsenate, which makes it more toxic. The minerals Rauenthalite Ca3(AsO4)2·10H2O and Phaunouxite Ca3(AsO4)2·11H2O are hydrates of calcium arsenate.[2]

Preparation[edit]

Calcium arsenate is commonly prepared from disodium hydrogen arsenate and calcium chloride:

2 Na2H[AsO4] + 3 CaCl2 → 4 NaCl + Ca3[AsO4]2 + 2 HCl

In the 1920s, it was made in large vats by mixing calcium oxide and arsenic oxide.[3] In the United States, 1360 metric tons were produced in 1919, 4540 in 1920, and 7270 in 1922.[1] The composition of commercially available calcium arsenate varies from manufacturer to manufacturer. A typical composition is 80-85% of Ca3(AsO4)2 a basic arsenate probably with a composition of 4CaO.As2O5 together with calcium hydroxide and calcium carbonate.[2]

Use as an herbicide[edit]

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.[4]

Regulation[edit]

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 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.[5]

Toxicity and regulation[edit]

Calcium arsenate is highly toxic, having both carcinogenic and systemic health effects.[6] The Occupational Safety and Health Administration has set a permissible exposure limit at 0.01 mg/m3 over an eight-hour time-weighted average, while the National Institute for Occupational Safety and Health recommends a limit five times less (0.002 mg/m3).[7]

References[edit]

  1. ^ a b Tartar, H.V.; Wood, L; Hiner, E; A Basic Arsenate of Calcium. J. Am. Chem. Soc. 1924, vol. 46, 809-813.
  2. ^ a b Ropp, Richard (2012). Encyclopedia of the Alkaline Earth Compounds. Newnes. p. 76. ISBN 0444595538. 
  3. ^ Smith, C.M.; Murray, C.W.; The Composition of Commercial Calcium Arsenate. Journal of Industrial and Engineering Chemistry; 1931, 23
  4. ^ Robert L. Metcalf "Insect Control" in Ullmann’s Encyclopedia of Industrial Chemistry" Wiley-VCH, Weinheim, 2002. doi:10.1002/14356007.a14_263
  5. ^ Turf insect pest control guide: Urban Phytonarian Series. learningstore.uwex.edu/pdf/A2934.pdf (Accessed 04/16/10)
  6. ^ 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
  7. ^ "Calcium Arsenate". NIOSH Pocket Guide to Chemical Hazards. Centers for Disease Control and Prevention. 4 April 2011. Retrieved 18 November 2013.