In chemistry, an arsenite is a chemical compound containing an arsenic oxyanion 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]
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]
- As2O4−5 pyro-arsenite, [O2As−O−AsO2]4−
- As3O5−7 a polyarsenite, [O2As−O−As(O)−O−AsO2]5−[3]
- As4O6−9 a polyarsenite, [O2As−O−As(O)−O−As(O)−O−AsO2]6−[3]
- (As6O4−11)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.
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 Na2H2As4O8, NaAsO2·4H2O, Na2HAsO3·5H2O and Na5(HAsO3)(AsO3)·12H2O.[4]
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(AsO3)(As2O5)2;[5] magnussonite, Mn5(OH)(AsO3)3;[2] trippkeite, CuAs2O4;[2] trigonite, Pb3Mn(AsO3)2(HAsO3);[2] tooeleite, Fe6(AsO3)4(SO4)(OH)4·4H2O.[6]
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]
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]
Arsenic in its trioxide, As2O3, (brand name Trisenox, ATO) is used as a chemotherapy drug against acute promyelocytic leukaemia (APL), a type of myeloid leukemia.[10] The detailed mechanism of action is unknown, but it is suspected to speed up apoptosis of cancer cells. Arsenic trioxide triggers morphological changes and DNA fragmentations in NB4 in vitro model for APL. It also degrades retinoic acid receptor alpha (RARA).[11] RARA gene is important regulator of premyelocytic immune cell development, differentiation, and apoptosis.[12]
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. 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. 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. Hawthorne, Frank C. "Schneiderhoehnite, Fe2+
Fe3+
3As3+
5O
13, a densely packed arsenite structure." The Canadian Mineralogist 23.4 (1985): 675–679.
Managing Arsenic in the Environment: From Soil to Human Health, R. Naidu, Csiro Publishing, 2006, ISBN 978-0643068681 EMA (2018-09-17). "Trisenox". European Medicines Agency. Retrieved 2023-08-16.