This is a method of conveying large amounts of reduction potential information about a series of reductions for a given element.
Simple redox reactions:
VO2+(aq) + 2 H+(aq) + e– → VO2+(aq) + H2O(l) Eored = +1.000 V
VO2+(aq) + 2 H+(aq) + e– → V3+(aq) + H2O(l) Eored = +0.337 V
V3+(aq) + e– → V2+(aq) Eored = –0.255 V
V2+(aq) + 2 e– → V(s) Eored = –1.13 V
More complex redox reactions (one of many possible examples)
V3+(aq) + 3 e– → V(s)
Disproportionation (one of many possible examples)
2 V3+(aq) + H2O(l) → V2+(aq) + VO2+(aq) + 2 H+(aq)
Eo = –0.255 + –0.337 = –0.592 V
(disfavored, rather, the comproportionation reaction will occur)
These are plots of NEored vs N, where N is the oxidation number of the redox active atom and Eored is defined for the reaction:
X(N) + N e– → X(0)
The vanadium example:
The minimum on the Frost diagram is the most stable species. The slope between points is the reduction potential for the couple.
These are phase diagrams for the stability of various species as a function of both pH and reduction potential. Like any phase diagram, the Pourbaix diagram gives the predominant species at the given equilibrium conditions, in this case the pH and Eo.
pH < 3, only +7, +4, +2, and 0 oxidation states available
pH between 4 and 13, +7, +4, +3, +2, and 0 oxidation states
only above pH = 13 can +6 oxidation state be found