An equation that relates the electrode potential E of an electrode that is in contact with an ionic solution to the ionic concentration c. The equation is:

where E^⦵ is the standard electrode potential, R is the gas constant, T is the absolute temperature, z is the valence of the ion, and F is the Faraday constant. The equation was derived by Walther Nernst in 1889. It is fundamental to the thermodynamics of electrochemical cells.