A cell in which electrical energy is produced by the transference of a substance from a system of high concentration to one at low concentration is known as concentration cells”. Concentration cells are of two types.
(i) Electrode concentration cells:In these cells, the potential difference is developed between two electrodes at different concentrations dipped in the same solution of the electrolyte. For example, two hydrogen electrodes at different gaseous pressures in the same solution of hydrogen ions constitute a cell of this type.
; at If , oxidation occurs at L. H. S. electrode and reduction occurs at R. H. S. electrode.
In the amalgam cells, two amalgams of the same metal at two different concentrations are immersed in the same electrolytic solution.
The emf of the cell is given by the expression, at
(ii) Electrolyte concentration cells: In these cells, electrodes are identical but these are immersed in solutions of the same electrolyte of different concentrations. The source of electrical energy in the cell is the tendency of the electrolyte to diffuse from a solution of higher concentration to that of lower concentration. With the expiry of time, the two concentrations tend to become equal. Thus, at the start the emf of the cell is maximum and it gradually falls to zero. Such a cell is represented in the following manner (is greater then ).
The emf of the cell is given by the following expression,
at 25o C
The concentration cells are used to determine the solubility of sparingly soluble salts, valency of the cation of the electrolyte and transition point of the two allotropic forms of metal used as electrodes, etc.