The Pourbaix diagram or potential – pH diagrams show the predominance areas of the different species related to one chemical element in aqueous solutions, upon hydrogen potential (abscissa axis, pH unit) and redox-potential (ordinate axis, Volt unit). Areas are defined by lines. Each line corresponds to the thermodynamic equilibrium border between 2 species and is plotted on the basis of the Nernst and the acid-base balance equations. Pourbaix diagrams are widely used to view the element speciation in water chemistry. They bear the name of their inventor, the chemist Marcel Pourbaix (1904–1998). It has to be noted that Pourbaix diagrams are constructed on the basis of thermodynamic calculations and do not take in to account kinetical effects. Hence, the predominance area of one species does not mean the other species are not present in water. Moreover, variations of diagrams may occur upon species concentration, species considered for calculations, water composition and temperature. Other variations are due to database deviations and uncertainty of the software used for the E-pH calculations. For all these reasons, Pourbaix diagrams are always simplified and drawing adapted to the context of the study.

Here below is the Pourbaix diagram of water. The area between lines a and b corresponds to the stability area of water.

Pourbaix diagram of water

Where can I find a Pourbaix diagram or potential – pH diagrams for:

  • Arsenic species in water
  • Chromium in water
  • Lead species in water
  • Mercury species in water
  • Nickel species in water
  • Nitrate (NO₃¯) and Nitrite (NO₂¯) species in water

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