- Frost diagram
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 differentoxidation state s of a particular substance. Frost diagrams will be different at differentpH s 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 ofprotons in the equation, and n the number ofelectrons exchanged.Electrons are always exchanged inelectrochemistry , but not necessarilyprotons . If there is noproton exchange in the reaction equilibrium, the reaction is said to be "pH independent". This means that the values for the electrochemical potential rendered in aredox half-reaction whereby theelement /s in question change/soxidation 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 [http://www.wou.edu/las/physci/ch462/redox.htm] .
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 athermodynamic 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 theFaraday constant , ΔG/F.An example of a Frost diagram: [http://facstaff.unca.edu/gheard/chem328/notes/V_frost.gif]
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