The galvanic cells in which electrodes are made of same metal but they are immersed in the electrolytes of different concentrations in two half cells are called concentration cells. The difference in the concentrations of electrolytes in the two half cells, creates potential difference across the two electrodes. The electrode which is placed in electrolyte of larger concentration acts as cathode or + terminal. Whereas electrode which is placed in electrolyte of lower concentration acts as anode or -ve terminal.
The cell can be represented as Cu/CuSO4 (0.01 M) ? CuSO4 (1 M)/Cu or simply as Cu/Cu2+ (0.01 M) ? Cu2+ (1 M)/Cu.
Similarly, hydrogen ion concentration cell can be constituted by connecting hydrogen electrodes placed in hydrochloric acid solutions of different concentration. It can be represented as
Pt, H2 (1 bar)/H+ (0.01 M) ? H+ (1 M)/H2 (1 bar), Pt.
Ecell of concentration cell
Consider the consideration cell of the type M/Mn+ (C1) ? Mn+(C2)/M. since standard electrode potentials of the two electrodes cancel each other, the emf of the cell can be written on the basis of Nerst equation as
Ecell = (0.059/n) log C2/C1
Evidently, for Ecell to be positive C2 > C1.