Concentration Cells

A concentration cell is an electrochemical cell in which the emf arises from a difference in concentration of a species between two half-cells. Unlike galvanic cells, where electrical energy comes from a chemical reaction, the driving force here is the transfer of matter from a region of higher concentration to lower concentration. The overall process is therefore physical in nature. A classic illustration is a cell made of two chlorine electrodes operating at different chlorine gas pressures.

Concentration cells are classified into two types: electrode concentration cells and electrolyte concentration cells.

In an electrode concentration cell, the concentration difference exists at the electrodes themselves. This may arise from unequal gas pressures in gas electrodes or from different metal concentrations in amalgams. Two identical electrodes at different concentrations are immersed in the same electrolyte.A standard example involves hydrogen electrodes:Pt | H₂(p₁) | H⁺(aq) | H₂(p₂) | Pt.Here, the electrolyte concentration does not affect the overall cell reaction. Applying the Nernst equation shows that the emf is positive when p₂ < p₁, indicating spontaneous flow analogous to gas expansion.

Another example is a lead amalgam cell, Hg–Pb(c₁) | Pb²⁺(aq) | Hg–Pb(c₂). A positive emf corresponds to the transfer of lead from the more concentrated amalgam to the less concentrated one.

In an electrolyte concentration cell, the electrodes are identical, but the electrolyte concentrations differ. A typical example is the zinc concentration cell:Zn | Zn²⁺(a₁) || Zn²⁺(a₂) | Zn.

Electrolyte concentration cells operate in two modes. When solutions are separated by a salt bridge, ion transfer occurs indirectly via salt bridge, producing a concentration cell without transference. When solutions are separated by a porous membrane, they remain in direct contact. Ions migrate across the junction, resulting in a concentration cell with transference.