Nernst Equation

Vijay Ratna

In electrochemistry the Nernst equation gives the electrode potential (E), relative to the standard electrode potential (E^O) of the electrode couple or equivalently of the half cells of a battery. In physiology the Nernst equation is used for finding the electric potential of a cell membrane w.r.t. one type of ion.

The activities of pure solid or liquid phases are taken as unity. For a solution at RT (25^OC) the following is true :

Where 0.0591 is a constant created from the Gas and Faraday constants, the temperature and a factor to convert from /n to log.

This means that 0.0591 can only be used at 25^OC.

For a cell membrane potential with respect to one cation (for an anion the sign before the logarithm is changed to a minus,

Where

R is the Universal Gas constant – 8.324570 J K^-1 mol^-1

T= Kelvin

a= Chemical activities on the reduced and oxidized side

F= Faraday constant

N= Number of electrons transferred in the half-reaction.

[Red] = concentration of oxidizing agent [the reduced species]

[Ox] = concentration of reducing agent [the oxidized species]

E^O’= formal electrode potential

r= activity coefficient.