Arsenite

In chemistry, an arsenite is a chemical compound containing an arsenic oxoanion where arsenic has oxidation state +3. Note that in fields that commonly deal with groundwater chemistry, arsenite is used generically to identify soluble AsIII anions. IUPAC have recommended that arsenite compounds are to be named as arsenate(III), for example ortho-arsenite is called trioxidoarsenate(III). Ortho-arsenite contrasts to the corresponding anions of the lighter members of group 15, phosphite which has the structure HPO2−
3 and nitrite, NO−
2 which is bent.[1]

For the supporters of Arsenios Autoreianos in the Byzantine Empire, see Arsenite Schism.

A number of different arsenite anions are known:

  • AsO3−
    3     ortho-arsenite, an ion of arsenous acid, with a pyramidal shape[1]
  • [AsO−
    2    ]
    n     meta-arsenite, a polymeric chain anion.[2]
  • As
    2    O4−
    5     pyro-arsenite, O2As–O–AsO2
  • As
    3    O5−
    7     a polyarsenite, [O2As–O–As(O)–O–AsO2][3]
  • As
    4    O6−
    9     a polyarsenite, [O2As–O–As(O)–O–As(O)–O–AsO2][3]
  • [As
    6    O4−
    11    ]
    n    , a polymeric anion

In all of these the geometry around the AsIII centers are approximately trigonal, the lone pair on the arsenic atom is stereochemically active.[1] Well known examples of arsenites include sodium meta-arsenite which contains a polymeric linear anion, [AsO−
2]
n, and silver ortho-arsenite, Ag3AsO3, which contains the trigonal AsO3−
3 anion.

Preparation of arsenites

Some arsenite salts can be prepared from an aqueous solution of As2O3. Examples of these are the meta-arsenite salts and at low temperature, hydrogen arsenite salts can be prepared, such as Na2(H2As4O8), NaAsO2·4H2O, Na2(HAsO3)·5H2O and Na5(HAsO3)(AsO3)·12H2O [4]

Arsenite minerals

A number of minerals contain arsenite anions: reinerite, Zn3(AsO3)2;[2] finnemanite, Pb5Cl(AsO3)3;[2] paulmooreite, Pb2As2O5;[2] stenhuggarite, CaFeSbAs2O7 (contains a complex polymeric anion);[2] schneiderhöhnite, FeII
FeIII
3(As2O5)2AsO3;[5] magnussonite, Mn5(OH)(AsO3)3;[2] trippkeite, CuAs2O4;[2] trigonite, Pb3Mn(AsO3)2(HAsO3);[2] tooeleite, Fe6(AsO3)4SO4(OH)4·4H2O.[6]

Arsenites in the environment

Arsenic can enter groundwater due to naturally occurring arsenic at deeper levels or from mine workings. Arsenic(III) can be removed from water by a number of methods, oxidation of AsIII to AsV for example with chlorine followed by coagulation with for example iron(III) sulfate. Other methods include ion-exchange and filtration. Filtration is only effective if arsenic is present as particulates, if the arsenite is in solution it passes through the filtration membrane.[7]

Uses

Sodium arsenite is used in the water gas shift reaction to remove carbon dioxide. Fowler's solution first introduced in the 18th century was made up from As2O3 [8] as a solution of potassium meta-arsenite, KAsO2.[9]

Bacteria using and generating arsenite

Some species of bacteria obtain their energy by oxidizing various fuels while reducing arsenates to form arsenites. The enzymes involved are known as arsenate reductases.

In 2008, bacteria were discovered that employ a version of photosynthesis with arsenites as electron donors, producing arsenates (just like ordinary photosynthesis uses water as electron donor, producing molecular oxygen). The researchers conjectured that historically these photosynthesizing organisms produced the arsenates that allowed the arsenate-reducing bacteria to thrive.[10]

In humans, arsenite inhibits pyruvate dehydrogenase (PDH complex) in the pyruvate–acetyl CoA reaction, by binding to the –SH group of lipoamide, a participant coenzyme. It also inhibits the oxoglutarate dehydrogenase complex by the same mechanism. The inhibition of these enzymes disrupts energy production.

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References
  1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  2. Carmalt, C.J & Norman, N.C. (1998). "Chapter 1: Arsenic, antimony and bismuth". In Norman, N.C. (ed.). Chemistry of Arsenic, Antimony and Bismuth. Blackie Academic and Professional. pp. 118–121. ISBN 07514-0389-X.
  3. Hamida, M. Ben; Wickleder, M. S. (2006). "Die neuen Catena-Polyarsenite [As3O7]5− und [As4O9]6−". Zeitschrift für anorganische und allgemeine Chemie. 632 (12–13): 2109. doi:10.1002/zaac.200670065. ISSN 0044-2313.
  4. Sheldrick, W. S.; Häusler, H.-J. (1987). "Zur Kenntnis von Natriumarseniten im Dreistoffsystem Na2O–As2O3–H2O bei 6 °C". Zeitschrift für anorganische und allgemeine Chemie. 549 (6): 177–186. doi:10.1002/zaac.19875490618. ISSN 0044-2313.
  5. Hawthorne, Frank C. "Schneiderhoehnite, Fe2+
    Fe3+
    3    As3+
    5    O
    13    , a densely packed arsenite structure." The Canadian Mineralogist 23.4 (1985): 675–679.
  6. Morin, G.; Rousse, G.; Elkaim, E. (2007). "Crystal structure of tooeleite, Fe6(AsO3)4SO4(OH)4•4H2O, a new iron arsenite oxyhydroxy-sulfate mineral relevant to acid mine drainage". American Mineralogist. 92 (1): 193–197. Bibcode:2007AmMin..92..193M. doi:10.2138/am.2007.2361. ISSN 0003-004X.
  7. EPA, United states Environmental Protection Agency, Report 815R00012, "Technologies and Costs for the Removal of Arsenic From Drinking Water", December 2000 http://water.epa.gov/drink/info/arsenic/upload/2005_11_10_arsenic_treatments_and_costs.pdf
  8. Managing Arsenic in the Environment: From Soil to Human Health, R. Naidu, Csiro Publishing, 2006, ISBN 978-0643068681
  9. Jolliffe, D. M. (1993). "A history of the use of arsenicals in man". Journal of the Royal Society of Medicine. 86 (5): 287–289. PMC 1294007. PMID 8505753.
  10. "Arsenic-loving bacteria rewrite photosynthesis rules", Chemistry World, 15 August 2008
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