Ladder Diagrams: Redox Equilibria

Ladder diagrams representing redox equilibria use electrochemical potential, E, as their scale.

Consider the half-cell reaction between Fe³⁺ and an electron to yield Fe²⁺. At equilibrium, E equals its standard state potential,  E^o of +0.771V.

The concentration of Fe³⁺ is higher for more positive potentials than E^o, while Fe²⁺ dominates at potentials more negative than E^o.

Now, consider the half-cell reaction between Sn⁴⁺ and an electron to yield Sn²⁺ with E^o of +0.154V. Above this point, Sn⁴⁺ dominates, whereas Sn²⁺ is the dominant species below this point. 

Adding excess Sn²⁺ to the system reduces Fe³⁺, changing the solution potential to +0.154V.

Generally, the electrochemical potential varies with the pH of the solution.

For instance, the ladder diagram of the half-cell reaction between UO₂²⁺ and two electrons to give U⁴⁺ shows that at pH zero, E equals +0.327V.

If the pH of the solution changes, the E value changes, affecting the concentration of the predominant species.