Properties & Reactions of Arsenic

Aqueous Inorganic Arsenic
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Though As(III) and As(V) are cations (positively charged ions), they never exist as free cations in solution in any aqueous system. Instead, the cations combine with water to form the oxyanions:
H3AsO3° for As(III)
H3AsO4° for As(V)
The oxidation states of arsenic in these neutral species remain 3+ and 5+, respectively.

Inorganic As(III) species (arsenite)

The inorganic As(III) species (arsenite) of arsenic include:

H3AsO3°
  H2AsO3-  
   HAsO32-   
  AsO33-
These various species are formed by successive hydrolysis reactions:
Reaction # 1
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.

Reaction # 2

H2AsO3- >> H+ + HAsO32-
pKa2 = 12.13

Reaction # 3

HAsO32->> H+ + AsO33- pKa3 = 13.4
The dominant species in the solution is controlled by pH.
The graph below summarizes the impact of pH on prevalence of the various inorganic As(III) species in solution.

Inorganic AS(III) Species

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(III) molecule structure
As(III) molecule structure taking into account actual relative sizes of the atoms

Inorganic 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:

Reaction # 1

H3AsO4° >> H+ + H2AsO4-
pKa1 = 2.2

Reaction # 2

H2AsO4- >> H+ + HAsO32-
pKa2 = 6.97

Reaction #3

HAsO42- >> H+ + AsO33-
pKa3 = 11.53

The graph below summarizes the impact of pH on prevalence of the various inorganic As(V) species in solution.

Inorganic AS(V) Species

As can be seen from the graph, the dominant inorganic As(V) species in solution are H2AsO4- at pH < 7 and HAsO32- at pH > 7.

The graph below indicates the impact of pH on prevalence of the various phosphate species. From a comparison of the graph above and the graph below it is evident that inorganic As(V) and phosphate have very similar hydrolysis behavior.

Inorganic Phosphate(V) Species

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.

Tetrahedral symmetry molecule structure
"Ball & stick" representation of the arsenate anion

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.

redox potential