Concentration cell
A galvanic cell generating voltage from concentration differences alone.
Last updated
A concentration cell is a specific type of galvanic cell where both half-cells are chemically identical, differing only in the concentration of their reactants. The cell generates a small voltage as it drives toward chemical equilibrium, which is reached when the reactant concentrations in the two half-cells are equal. Since a tenfold concentration difference produces less than 60 millivolts at room temperature, these cells are rarely used for practical energy storage.
Electricity is produced as the system reduces its thermodynamic free energy by lowering the concentration difference between the two half-cells. The same chemical reaction occurs in both half-cells but in opposite directions—one side increases its concentration while the other decreases it. The energy for this process comes from thermal energy absorbed as heat from the surroundings, and the flow of electricity is possible because the entropy increase from mixing the concentrations outweighs the entropy decrease from converting heat into electrical energy.
In chemical analysis, concentration cells compare a solution of known concentration with an unknown one. The unknown concentration is determined using the Nernst equation or by referencing comparison tables against a set of standards.
At standard conditions (1 M solutions, 298 K, 1 atm), the standard reduction and oxidation potentials for a concentration cell are both 0 V. This is because the identical electrodes are immersed in solutions of the same ions at different concentrations, and under standard conditions, the half-cell potentials are equal, resulting in no net potential difference.
Oxygen concentration cells
Concentration cell corrosion occurs when different areas of a metal surface contact varying concentrations of the same solution. Two main types exist: electrolyte concentration cells and electrode concentration cells.
In an electrolyte concentration cell, the electrodes in both half-cells are made of the same material, and the electrolyte is a solution of the same substance but at different concentrations.
In an electrode concentration cell, two electrodes of the same material but with different concentrations are placed in a common solution.
Metal ion concentration cells
Metal ion concentration cells arise in the presence of water. A high concentration of metal ions develops under faying surfaces, while a low concentration exists adjacent to the crevice they create.
Quick Facts
- Voltage per order-of-magnitude differenc
- less than 60 mV at room temperature
- Types
- Electrolyte concentration cell, Electrode concentration cell
- Corrosion types
- Metal ion concentration cells
- Oxygen concentration cells
- Active-passive cells
Facts from the source article.
Lore & Background
A concentration cell generates electricity from the reduction in thermodynamic free energy as the difference in chemical concentrations in the two half-cells is reduced. The same reaction occurs in both half-cells but in opposite directions, increasing the lower concentration and decreasing the higher one. The energy is generated from thermal energy that the cell absorbs as heat while electricity flows; this conversion from ambient thermal energy without a temperature gradient is possible because the convergence of concentrations increases entropy, which more than compensates for the entropy decrease when heat is converted into electrical energy.
Concentration cells are also used in chemical analysis, comparing a solution of known concentration with an unknown via the Nernst equation or comparison tables against standards. In corrosion, concentration cell corrosion occurs when two or more areas of a metal surface contact different concentrations of the same solution. Three types exist: metal ion concentration cells, oxygen concentration cells, and active-passive cells.
In metal ion cells, the area with higher metal ion concentration becomes anodic and corrodes, while the lower-concentration area is cathodic and protected. In oxygen cells, corrosion occurs at the area of low oxygen concentration (anodic). Active-passive cells develop when a passive film is broken, creating a potential between the large passive area (cathode) and the small exposed active metal (anode), leading to rapid pitting.
Reader's Guide
The concentration cell is notable primarily as a fundamental electrochemical concept rather than a practical energy storage device. Its small voltage output—less than 60 millivolts per order-of-magnitude concentration difference at room temperature—makes it unsuitable for typical battery applications. However, its principle underlies important phenomena in corrosion science, where concentration differences on metal surfaces drive localized corrosion. The three recognized types—metal ion, oxygen, and active-passive cells—describe common corrosion mechanisms in aqueous environments.
Additionally, concentration cells serve as analytical tools, enabling the determination of unknown ion concentrations through the Nernst equation. The ability to generate electricity from ambient thermal energy without a temperature gradient, by exploiting entropy increase from concentration equalization, illustrates a unique thermodynamic pathway. Related concepts include the liquid junction potential, membrane potential, and thermogalvanic cell, and practical applications include the zirconia oxygen sensor. Despite its limited role in energy storage, the concentration cell remains a key teaching example in electrochemistry and a basis for understanding corrosion and analytical methods.
More in Battery Types
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Concentration cell (CC BY-SA 4.0).
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced