Latimer diagram
Representation of the standard electrode potential data of an element
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Through Standard electrode potential, Redox and Electron
Construction
In a Latimer diagram, because by convention redox reactions are shown in the direction of reduction (gain of electrons), the most highly oxidized form of the element is on the left side, with successively lower oxidation states to the right side. The species are connected by arrows, and the numerical value of the standard potential (in volts) for the reduction is written at each arrow. For example, for oxygen, the species would be in the order O2 (0), H2O2 (–1), H2O (-2):
The arrow between O2 and H2O2 has a value +0.68 V over it, it indicates that the standard electrode potential for the reaction:
- O2(g) + 2 H+ + 2 e− ⇌ H2O2(aq)
is 0.68 volts.
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Application
Latimer diagrams can be used in the construction of Frost diagrams, as a concise summary of the standard electrode potentials relative to the element. Since ΔrGo = -nFEo, the electrode potential is a representation of the Gibbs energy change for the given reduction. The sum of the Gibbs energy changes for subsequent reductions (e.g. from O2 to H2O2, then from H2O2 to H2O) is the same as the Gibbs energy change for the overall reduction (i.e. from O2 to H2O), in accordance with Hess's law. This can be used to find the electrode potential for non-adjacent species, which gives all the information necessary for the Frost diagram.
It must be stressed that standard reduction potentials are not additive values. They cannot be directly summed up, or subtracted, from the values (in volts) indicated in a Latimer diagram. If needed, their calculation must be performed via the difference in Gibbs free energies. The easiest way to proceed is simply to use energies (nE) directly expressed in electron-volts (eV), because the Faraday constant F and the sign minus simplifies on both side of the equation. So, the values of Eo in volts must be simply multiplied by the number (n) of electrons transferred in the half-reaction. Since the Faraday constant can disappear from the equation, no need to calculate ΔrGo expressed in joules.
A Latimer diagram can also indicate if a species will disproportionate in solution under the conditions for which the electrode potentials are given: if the same number of electrons are exchanged in each reaction, and if the potential to the right of the species is higher than the potential on the left, it will disproportionate. Therefore, hydrogen peroxide H2O2 is unstable and will disproportionate into O2 and H2O.