The pH condition of the environment is not sufficient for predicting the form in which an element will exist in natural waters. Consider whether the aqueous environment is well aerated (oxidizing) or polluted with organic wastes (reducing). In order to add this variable, we must expand the predominance diagram to include the reduction potential of the environment as well as the pH. This type of predominance diagram is known as a Pourbaix diagram.Eo-pH diagram, or pE-pH diagram.. Low E or pE values represent a reducing environment. High E values represent an oxidizing environment. The pE scale is intended to represent the concentration of the standard reducing agent (the e-) analogously to the pH scale representing the concentration of standard acid (H⁺). PE values are obtained from reduction potentials by dividing E^oby 0.059.

Key to features on the diagram

line a (see following diagrams) separate species related by acid-base equilibria

• line a shows the pH at which half of the 1 M iron is Fe3+ and half is precipitated as Fe(OH)2
• Pourbaix diagrams incorporate Z1/r calculations and acid-base equilibria
• the position of an acid-base equilibrium is dependent on the total concentration of iron
  
  • reducing the total concentration of Fe3+ will reduce the driving force of the precipitation
  • reducing the total iron concentration from 1 M to 10-6 M (more realistic concentrations for geochemists and corrosion engineers) shifts the boundary from pH 1.7 to pH 4.2
  • In general, in more dilute solutions, the soluble species have larger predominance areas.

Double lines, lines c & d separate species related by redox equilibria

• redox equilibria of species not involving hydrogen or hydroxide ions appear as horizontal boundaries (line b)
• redox species of species involving hydrogen or hydroxide appear as diagonal boundaries becuase they are in part acid-base equilibria (line c)
  
  • diagonal boundaries slope from upper left to lower right because basic solutions tend to favor the more oxidized species

Longer lines d & f enclose the theoretical region of stability of the water to oxidation or reduction while shorter lines e & g enclose the practical region of stability of the water

• Line d represents the potential of water saturated with dissolved O₂at 1 atm (very well aerated water).
• above this potential water is oxidized to oxygen: 2 H₂O + 4 H⁺ (aq) O₂ + 4 e^- E^o = +1.229 V
  
  • theoretically water should be oxidized by any dissolved oxidizing agent E^o> 1.229
  • in practice, about 0.5 V of additional potential is required to overcome the overvoltage of oxygen formation (line e)

Line f represents the potential of water saturated with dissolved H₂ at 1 atm pressure (high level or reducing agents in solution).

Below this potential water is reduced to hydrogen: 2 H⁺ + 2 e^- E^o = +1.229 V

• in practice, an overvoltage effect prevents significant release of hydrogen until the lower dashed line g is reached

Uses of Pourbaix Diagrams

Any point on the diagram will give the termodynamically most stable (theoretically the most abundant) form of the element for that E and pH.

• o E=+0.8 V and pH = 14
• predominant form is FeO₄2.

The diagram gives a visual representation of the oxidizing and reducing abilities of the major stable compounds of an element

• o Strong oxidizing agents and oxidizing conditions are found only at the top of the diagram.
• The lower boundaries of strong oxidizing agents are high on the diagram.
• o Reducing agents and reducing conditions are found at the bottom of a diagram and nowhere else.
• Strong reducing agents have boundaries that are low on the diagram.
• o A species that prevails from top to bottom at the pH in question has no oxidizing or reducing properties at all within that range.

EXAMPLE
On the Pourbaix diagram for iron find:

• the chemical form of iron that is the strongest oxidizing agent.
• the form of iron that is the strongest reducing agent
• the form of iron that would predominate in a neutral solution at a potential of 0.00V
• the standard reduction potential for the reduction of Fe2+ to Fe metal