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When an element has a range of oxidation states, there exist a number of reduction potentials for conversions between the various oxidation states. So, many redox couples may exist for a given element, and their identities may also depend on the pH of the solution, as different species exist in acidic and basic solutions.
A Frost Diagram is another way of displaying the reduction potentials for the various oxidation states of a given element, X. It shows nE against the oxidation number N: here, E is the reduction potential for the X(N)/X(0) couple, and n is the number of electrons transferred in the conversion of X(N) to X(0).
The use of reduction potentials to predict the course of reactions is limited to thermodynamic considerations only: if the overall reduction potential for the reaction, which is the difference of the reduction potentials of the individual couples, is positive, then there is a negative Gibbs free reaction energy and the reaction is thermodynamically spontaneous.
The use of standard reduction potentials to predict the course of a reaction is somewhat limited, in that they are only valid under standard conditions. However, they can be used to predict the course of redox reactions under nonstandard conditions.
Having defined the half-reaction and a potential associated with the reduction depicted by the half-reaction, we need to determine what the potential is.
When a redox operation proceeds, one species is oxidised and the other is reduced. It is useful to think of these two processes separately.