The Challenges of Arsenic in Agriculture and the Environment

Properties & Reactions of Arsenic

Aqueous Inorganic Arsenic
Previous	Page 3 of 10	Next

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^-

HAsO₃^2-

AsO₃^3-

These various species are formed by successive hydrolysis reactions:

Reaction # 1

H₃AsO₃° >> H⁺ + H₂AsO₃^-

pK_a1 = 9.22

The pK_a₁ 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⁺ + HAsO₃^2-

pK_a2 = 12.13

Reaction # 3

HAsO₃^2->> H⁺ + AsO₃^3- pK_a3 = 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 taking into account actual relative sizes of the atoms

Inorganic As(V) species (arsenate)

Inorganic As(V) species (arsenate) of arsenic include:

H₃AsO₄°

H₂AsO₄-

HAsO₄^2-

AsO₄^3-

The various species are formed by successive hydrolysis reactions:

Reaction # 1

H₃AsO₄° >> H⁺ + H₂AsO₄^-

pK_a1 = 2.2

Reaction # 2

H₂AsO₄^- >> H⁺ + HAsO₃^2-

pK_a2 = 6.97

Reaction #3

HAsO₄^2- >> H⁺ + AsO₃^3-

pK_a3 = 11.53

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

As can be seen from the graph, the dominant inorganic As(V) species in solution are H₂AsO₄^- at pH < 7 and HAsO₃^2- 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.

"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.