Pourbaix diagram

Pourbaix diagram of iron.^[1]

In chemistry, a Pourbaix diagram, also known as a potential/pH diagram, maps out possible stable (equilibrium) phases of an aqueous electrochemical system. Predominant ion boundaries are represented by lines. As such a Pourbaix diagram can be read much like a standard phase diagram with a different set of axes.

The diagrams are named after Marcel Pourbaix (1904–1998), the Russian-born chemist who invented them.

Diagram

The Pourbaix diagram for uranium in a non-complexing aqueous medium (eg perchloric acid / sodium hydroxide).^[2]

The Pourbaix diagram for uranium in carbonate solution.^[2]

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.

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

pH = − log[H ⁺ ]

The lines are drawn for ions at unit activity (about 1 M) and represent equilibrium for that concentration. Additional lines may be drawn for other concentrations, e.g., 10^-3 M or 10^-6 M.

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.)

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

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.

See also

• Frost diagram

Footnotes

References

• Denny A. Jones, Principles and Prevention of Corrosion, 2nd edition, 1996, Prentice Hall, Upper Saddle River, NJ. ISBN 0-13-359993-0 Page 50-52
• Pourbaix, M., Atlas of electrochemical equilibria in aqueous solutions. 2d English ed. 1974, Houston, Tex.: National Association of Corrosion Engineers.

External links

Wikimedia Commons has media related to: Pourbaix diagrams

• Marcel Pourbaix — Corrosion Doctors

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