page_title: Pourbaix diagram
section_title: The stability region of water
hierarchical_section_title: Pourbaix diagram / The stability region of water
caption_attribution_description: English: Pourbaix diagram for water, including equilibrium regions for water, oxygen and hydrogen.
context_page_description: In electrochemistry, a Pourbaix diagram, also known as a potential/pH diagram, EH-pH diagram or a pE/pH diagram, maps out possible stable 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. Similarly to phase diagrams, they do not allow for reaction rate or kinetic effects. Beside potential and pH, the equilibrium concentrations are also dependent upon, e.g., temperature, pressure, and concentration. Pourbaix diagrams are commonly given at room temperature, atmospheric pressure, and molar concentrations of 10⁻⁶ and changing any of these parameters will yield a different diagram.
  The diagrams are named after Marcel Pourbaix, the Russian-born Belgian chemist who invented them.
context_section_description: In many cases, the possible conditions in a system are limited by the stability region of water. In the Pourbaix diagram for uranium, the limits of stability of water are marked by the two dashed green lines, and the stability region for water falls between these lines.
  Under highly reducing conditions (low EH/pE) water will be reduced to hydrogen according to
  or
  Using the Nernst equation, setting E⁰ = 0 V and the hydrogen gas fugacity (corresponding to activity) at 1, the equation for the lower stability line of water in the Pourbaix diagram will be:
  at standard temperature and pressure. Below this line, water will be reduced to hydrogen, and it will usually not be possible to pass beyond this line as long as there is still water present to be reduced.
  Correspondingly, under highly oxidizing conditions (high EH/pE) water will be oxidized to oxygen gas according to
  Using the Nernst equation as above, but with an E⁰ =-ΔG⁰H2O/2F = 1.229 V, gives an upper stability limit of water at
  at standard temperature and pressure. Above this line, water will be oxidized to form oxygen gas, and it will usually not be possible to pass beyond this line as long as there is still water present to be oxidized.
caption: Pourbaix diagram for water, including equilibrium regions for water, oxygen and hydrogen at STP. The vertical scale is the electrode potential of a hydrogen or non-interacting electrode relative to an SHE electrode, the horizontal scale is the pH of the electrolyte (otherwise non-interacting). Assuming no overpotential, above the top line the equilibrium condition is oxygen gas, and oxygen will bubble off of the electrode until equilibrium is reached. Likewise, below the bottom line, the equilibrium condition is hydrogen gas, and hydrogen will bubble off of the electrode until equilibrium is reached.
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