A concentration cell is an electrochemical cell where the two electrodes are the same material, the electrolytes on the two half-cells involve the same ions, but the electrolyte concentration differs between the two half-cells.
An example is an electrochemical cell, where two copper electrodes are submerged in two copper(II) sulfate solutions, whose concentrations are 0.05 M and 2.0 M, connected through a salt bridge. This type of cell will generate a potential that can be predicted by the Nernst equation. Both can undergo the same chemistry (although the reaction proceeds in reverse at the anode)
:Cu + 2 e → Cu
Le Chateliers principle indicates that the reaction is more favorable to reduction as the concentration of Cu ions increases. Reduction will take place in the cells compartment where the concentration is higher and oxidation will occur on the more dilute side.
The following cell diagram describes the concentration cell mentioned above:
:Cu | Cu (0.05 M) || Cu (2.0 M) | Cu
where the half cell reactions for oxidation and reduction are:
:Oxidation: Cu → Cu (0.05 M) + 2 e
:Reduction: Cu (2.0 M) + 2 e → Cu
:Overall reaction: Cu (2.0 M) → Cu (0.05 M)
The cell's emf is calculated through the Nernst equation as follows:
The value of E° in this kind of cell is zero, as electrodes and ions are the same in both half-cells.
After replacing values from the case mentioned, it is possible to calculate cell's potential:
or by:
However, this value is only approximate, as reaction quotient is defined in terms of ion activities which can be approximated with the concentrations as calculated here.
The Nernst equation plays an important role in understanding electrical effects in cells and organelles. Such effects include nerve synapses and cardiac beat as well as the resting potential of a somatic cell.