Pourbaix diagram

In electrochemistry, and more generally in solution chemistry, a Pourbaix diagram, also known as a potential/pH diagram, EH–pH diagram or a pE/pH diagram, is a plot of possible thermodynamically stable phases (i.e., at chemical equilibrium) of an aqueous electrochemical system. Boundaries (50 %/50 %) between the predominant chemical species (aqueous ions in solution, or solid phases) 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.

Naming The diagrams are named after Marcel Pourbaix (1904–1998), the Belgian engineer who invented them. Pourbaix diagrams are also known as potential-pH diagrams or EH-pH diagrams due to the labeling of the two axes.

Diagram 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. For a reversible redox reaction described by the following chemical equilibrium:

a A + b B ⇌ c C + d D With the corresponding equilibrium constant K:

K = [ C ] c [ D ] d [ A ] a [ B ] b , {\displaystyle K={\frac {[C]^{c}[D]^{d}}{[A]^{a}[B]^{b}}},}

The Nernst equation is:

E H = E 0 − R T z F ln ⁡ K , {\displaystyle E_{\text{H}}=E^{0}-{\frac {RT}{zF}}\ln {K},}

E H = E 0 − R T z F ln ⁡ [ C ] c [ D ] d [ A ] a [ B ] b , {\displaystyle E_{\text{H}}=E^{0}-{\frac {RT}{zF}}\ln {\frac {[C]^{c}[D]^{d}}{[A]^{a}[B]^{b}}},}

sometimes formulated as:

E H = E 0 − V T λ z log ⁡ [ C ] c [ D ] d [ A ] a [ B ] b , {\displaystyle E_{\text{H}}=E^{0}-{\frac {V_{T}\lambda }{z}}\log {\frac {[C]^{c}[D]^{d}}{[A]^{a}[B]^{b}}},}

or, more simply directly expressed numerically as:

E H = E 0 − 0.05916 z log ⁡ [ C ] c [ D ] d [ A ] a [ B ] b , {\displaystyle E_{\text{H}}=E^{0}-{\frac {0.05916}{z}}\log {\frac {[C]^{c}[D]^{d}}{[A]^{a}[B]^{b}}},}

where:

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