Concentration Cells

In a concentration cell, the emf arises from the difference in concentrations between two half-cells, which creates unequal electrode potentials that drive a spontaneous flow of electrons and ions.

Concentration cells are divided into electrode concentration cells and electrolyte concentration cells.

In an electrode concentration cell, the electrodes are identical, but the concentration of the electroactive species differs at each electrode.

A standard example is the hydrogen electrode, where identical platinum electrodes are exposed to hydrogen gas at different pressures in the same hydrogen ion solution. According to the Nernst equation, the cell operates spontaneously and produces a positive emf when p₂ is less than p₁.

On the other hand, electrolyte concentration cells consist of identical electrodes immersed in the same electrolyte at different concentrations. These cells operate without transference or with transference.

For example, a zinc concentration cell uses zinc electrodes in solutions with different Zn²⁺ ion concentrations. According to the Nernst equation, the emf is positive, and the process is spontaneous when a₂ exceeds a₁.