Latimer diagram

A Latimer diagram of a chemical element is a summary of the standard electrode potential data of that element. This type of diagram is named after Wendell Mitchell Latimer (1893–1955), an American chemist.

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.

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 Δ_rG^o = -nFE^o, 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 volt 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-volt (eV), because the Faraday constant F and the sign minus simplifies on both side of the equation. So, the values of E in volt must be simply multiplied by the number (n) of electron transferred in the considered half-reaction. Since the Faraday constant can disappear from the equation, no need to calculate Δ_rG^o expressed in joule.

A simple examination of 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 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 in O₂ and H₂O.

• Frost diagram
• Pourbaix diagram
• Ellingham diagram