Frost diagram
[1] The free energy ΔG° is related to the standard electrode potential E° shown in the graph by the formula: ΔG° = −nFE° or nE° = −ΔG°/F, where n is the number of transferred electrons, and F is the Faraday constant (F ≈ 96,485 coulomb/(mol e−)).
[1] The Frost diagram shows on its x axis the oxidation state of the species in question, and on its y axis the difference in Gibbs free energy, ΔG°, of the half-reduction reaction of the species multiplied by the sign minus and divided by the Faraday constant, F. The term -ΔG°/F = nE°, i. e., the number, n, of electrons exchanged in the reduction reaction multiplied by the standard potential, E°, expressed in volt.
The standard free-energy scale is measured in electron-volts,[1] and the nE° = 0 value is usually the neutral species of the pure element.
The slope of the line between any two points on a Frost diagram gives the standard reduction potential, E°, for the corresponding half-reaction.
This means that the values for the electrochemical potential rendered in a redox half-reaction, whereby the elements in question change oxidation states are the same whatever the pH conditions under which the procedure is carried out.
The Frost diagram is also a useful tool for comparing the trends of standard potentials (slope) of acidic and basic solutions.
The pure, neutral element transitions to different compounds depending whether the species is in acidic and basic pHs.
He predicts that “the slopes may not be as easily or accurately recognized as they are the direct numerical values of the oxidation potentials [of the Latimer diagram]”.
Frost suggested that the numerical values of standard potentials could be added next to the slopes to provide supplemental information.
Frost diagrams nE° = −ΔG°/F, classically constructed with the standard potential E°, implicitly refers to acid conditions ([ H+] = 1 M, pH = 0).However, in some textbooks the Frost diagram of an element may be confusing for the reader, because the redox potential depends on pH and some notations, or conventions, may differ from the standard conditions and be unclear.
Some textbooks present the reduction potentials calculated under standard conditions, so with [ H+] = 1 M (pH = 0, acid-solution), E° (2 H+ + 2 e− ⇌ H2), while also discussing redox processes occurring in a basic-solution.
Note : as not all chemical species are necessarily indicated on a given Frost diagram, these diagrams can easily exhibit significant differences: e.g. , this diagram does not show hydrazine ( NH 2 -NH 2 ), also a quite unstable compound and the next Frost diagram for nitrogen, here below, does not present the azide species.
