**4. Arsenite and arsenate as acids**

Given the arsenite's high pKa1 of 9.2, the dominant arsenite species will be H3AsO3 [14]. H2AsO3 −1 is a weak acid with a pKa2 = 12.7, thus HAsO3 −2 has a small activity within the normal alkaline pH range of most soil environments. Conversely, arsenate (H3AsO4) readily deprotonates to form H2AsO4 −1 (pKa1 = 2.3). Additionally, H2AsO4 −1 will deprotonate to form HAsO4 −2 (pKa2 = 6.8), thus H2AsO4 −1 and HAsO4 −2 are the dominant arsenate species in most soils. HAsO4 −2 will deprotonate to form AsO4 −3 (pKa3 = 11.6); however, this species will only exist in the most extreme alkaline soil environments. Monomethylarsonic acid (MMA or CH3AsO(OH)2 with pK1 = 3.6 and pK2 = 8.2) and dimethylarsenic acid (DMA or (CH3)2AsO(OH) with pK1 = 6.2) may also readily exist in soil environments [17].

*An Emerging Global Understanding of Arsenic in Rice (*Oryza sativa*) and Agronomic Practices… DOI: http://dx.doi.org/10.5772/intechopen.105500*
