Properties & Reactions of Arsenic
Aqueous Inorganic Arsenic
H3AsO3° for As(III) H3AsO4° for As(V)
Inorganic As(III) species (arsenite)
The inorganic As(III) species (arsenite) of arsenic include:
H3AsO3° H2AsO3- HAsO32- AsO33-
H3AsO3° >> H+ + H2AsO3-
pKa1 = 9.22
The pKa1 of 9.22 for this reaction indicates that at pH 9.22 there would be approximately equal amounts of H3AsO3° and H2AsO3- in solution. Therefore, at pH < 9.22, H3AsO3° is the dominant species; at pH > 9.22, H2AsO3- is the dominant species in solution.
At pH < 8, neutral H3AsO3° is
the dominant As(III) species in solution. The charged H2AsO3- species
plays a significant role only at pH > 8.
As(III) species have pyramidal symmetry, in which the central cat ion, As3+, resides at the apex of the pyramid and is bound to three oxygen's at the corners of the base of the pyramid as shown below at left. The oxygen atoms are much larger than the arsenic atoms. The figure below (right) shows the arsenite anion, taking into account the actual relative sizes of the arsenic and oxygen atoms.
As(V) species (arsenate)
Inorganic As(V) species (arsenate) of arsenic include:
H3AsO4° H2AsO4- HAsO42- AsO43-
The various species are formed by successive hydrolysis reactions:
Both As(V) and phosphate have tetrahedral symmetry, in which the central cat ion, As5+ or P5+, is at the center of the tetrahedron and is surrounded by the four oxygen's at the corners of the tetrahedron, as shown below at left. The figure below (right) is a “ball and stick” representation of the arsenate anion. The balls represent atoms and the sticks represent bonds between atoms.
Impact of Redox Potential
The graph below indicates the regions of prevalence of the various As(III) and As(V) species. Low pe values indicate more highly reduced conditions, as might occur under flooded conditions as in a rice paddy. High pe values indicate oxidized conditions, as might occur when the soil is relatively dry.