Chemistry of Arsenic
Arsenic is located in group 5A and has 33 atomic number. It occurs in a variety of forms and oxidation states (-3,0,+3,+5). Both inorganic and organic species are consisted in drinking water. The two common oxidation states in drinking water are trivalent (As(III), arsenite) and pentavalent (As(V), arsenate). Pentavalent arsenic is thermodynamically stable form of inorganic arsenic in the oxic water and generally predominates in surface water. Both As(III) and As(V) are observed in several protonate forms; at neutral pH, the predominant species are H3AsO3 for As(III) and H2AsO4- and HAsO4- for As(V). It means that As(V) shows an anion character at typical pH in natural water (pH 5 – 8) while As(III) remains as a neutral molecule at the same pH range. As(V) has a negative charge over pH 9.2 .[3]. Figure 1 shows structural differences between arsenite and arsenate.


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