Pourbaix Diagram - Diagram

Diagram

Pourbaix diagrams are also known as Eh-pH diagrams due to the labeling of the two axes. The vertical axis is labeled Eh for the voltage potential with respect to the standard hydrogen electrode (SHE) as calculated by the Nernst equation. The "h" stands for Hydrogen, although other standards may be used, and they are for room temperature only.

The horizontal axis is labeled pH for the -log function of the H+ ion activity.

The lines in the Pourbaix diagram show the equilibrium conditions, that is, where the activities are equal, for the species on each side of that line. On either side of the line, one form of the species will instead be said to be predominant.

In order to draw the position of the lines with the Nernst equation, the activity of the chemical species at equilibrium must be be defined. Usually, the activity of a species is approximated as equal to the concentration (for soluble species) or partial pressure (for gases). The same values should be used for all species present in the system.

For soluble species, the lines are often drawn for concentrations of 1 M or 10−6 M. Sometimes additional lines are drawn for other concentrations.

If the diagram involves the equilibrium between a dissolved species and a gas, the pressure is usually set to P0 = 1 atm = 101 325 Pa, the minimum pressure required for gas evolution from an aqueous solution at standard conditions.

While such diagrams can be drawn for any chemical system, it is important to note that the addition of a metal binding agent (ligand) will often modify the diagram. For instance, carbonate has a great effect upon the diagram for uranium. (See diagrams at right.) The presence of trace amounts of certain species such as chloride ions can also greatly affect the stability of certain species by destroying passivating layers.

In addition, changes in temperature and concentration of solvated ions in solution will shift the equilibrium lines in accordance with the Nernst equation.

The diagrams also do not take kinetic effects into account, meaning that species shown as instable might not react to any significant degree in practice.

A simplified Pourbaix diagram indicates regions of "Immunity", "Corrosion" and "Passivity", instead of the stable species. They thus give a guide to the stability of a particular metal in a specific environment. Immunity means that the metal is not attacked, while corrosion shows that general attack will occur. Passivation occurs when the metal forms a stable coating of an oxide or other salt on its surface, the best example being the relative stability of aluminium because of the alumina layer formed on its surface when exposed to air.