Frost diagram

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A Frost diagram is an Oxidation State Free Energy Diagram (O.S.F.E), also known as an Eh-pH diagram, due to its axial labellings, and is used in electrochemistry to illustrate the relative stability of a number of different oxidation states of a particular substance. Frost diagrams will be different at different pHs so the pH must be specified.

The pH dependence is given by the factor -0.059m/n per pH unit, where m relates to the number of protons in the equation, and n the number of electrons exchanged. Electrons are always exchanged in electrochemistry, but not necessarily protons. If there is no proton exchange in the reaction equilibrium, the reaction is said to be pH independent. This means that the values for the electrochemical potential rendered in a redox half-reaction whereby the element/s in question change/s oxidation state are the same whatever the pH conditions under which the procedure is carried out.

On a Frost diagram the element is generally set as the reference point at 0 Volts.

The lower a point is on the Frost Diagram, the more stable it is. This indicates the most stable oxidation state for that element. Points found high in the upper left indicate an element that will act as a strong oxidizing agent at that oxidation state [1].

The gradient of the line between any two points on a Frost Diagram give the potential for the reaction. A species which lies in a peak, above the gradient of the two points on either side, denotes a species unstable with respect to disproportionation, and a point which falls below the gradient of the line joining its two adjacent points lies in a thermodynamic sink, and is intrinsically stable.

The axes of the Frost diagram show (horizontally) the oxidation state of the species in question and (vertically) the electron exchange number multiplied by the voltage (nE) OR the Gibbs free energy per unit of the Faraday constant, ΔG/F.

An example of a Frost diagram: [2]