In battery technology, a concentration cell is a limited form of a galvanic cell that has two equivalent half-cells of the same composition differing only in

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...

s. One can calculate the potential developed by such a cell using the Nernst equation.Almost any textbook on

physical chemistry Physical chemistry is the study of macroscopic and microscopic phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistical mech ...

, e.g. by I. N. Levine or P. W. Atkins, and also many general chemistry texts. A concentration cell produces a small

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

as it attempts to reach chemical equilibrium, which occurs when the concentration of reactant in both half-cells are equal. Because an order of magnitude concentration difference produces less than 60 millivolts at room temperature, concentration cells are not typically used for

energy storage Energy storage is the capture of energy produced at one time for use at a later time to reduce imbalances between energy demand and energy production. A device that stores energy is generally called an Accumulator (energy), accumulator or Batte ...

. A concentration cell generates electricity from the reduction in the

thermodynamic free energy In thermodynamics, the thermodynamic free energy is one of the state functions of a thermodynamic system. The change in the free energy is the maximum amount of work that the system can perform in a process at constant temperature, and its ...

of the electrochemical system as the difference in the chemical concentrations in the two half-cells is reduced. The same reaction occurs in the half-cells but in opposite directions, increasing the lower and decreasing the higher concentration. The energy is generated from thermal energy that the cell absorbs as heat, as the electricity flows. This generation of electricity from ambient

thermal energy The term "thermal energy" is often used ambiguously in physics and engineering. It can denote several different physical concepts, including: * Internal energy: The energy contained within a body of matter or radiation, excluding the potential en ...

, without a

temperature gradient A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature spatial gradient is a vector quantity with Dimensional analysis, ...

, is possible because the convergence of the chemical concentrations in the two half-cells increases entropy, and this increase more than compensates for the entropy decrease when heat is converted into electrical energy. Concentration cell methods of

chemical analysis Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separa ...

compare a solution of known concentration with an unknown, determining the concentration of the unknown via the Nernst Equation or comparison tables against a group of standards. The standard reduction and oxidation potentials for a concentration cell is 0V at standard conditions (1M solutions, 298K, 1 atm). This is because a concentration cell is designed to have identical electrodes immersed in solutions with different concentrations of the same ions, and at standard conditions, the electrode potentials of the half-cells are the same. Therefore, there is no net potential difference. Concentration cell

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

occurs when two or more areas of a

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

surface are in contact with different concentrations of the same solution. There are two general types of concentration cells. Concentration cells can be electrode concentration cells or electrolyte concentration cells. Electrolyte Concentration cell - In this particular electrochemical cell, the electrodes within both half-cells consist of identical substances, while the electrolyte comprises a solution of the same substance, albeit with varying concentrations. Electrode Concentration cell - In this particular electrochemical cell, two electrodes composed of the same substance but with differing concentrations are immersed in a common solution.

Metal ion concentration cells

In the presence of

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

, a high concentration of metal ions will exist under faying surfaces and a low concentration of metal ions will exist adjacent to the crevice created by the faying surfaces. An electrical potential will exist between the two points. The area of the metal in contact with the lower concentration of metal ions will be

cathodic A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. Conventional current ...

and will be protected, and the area of metal in contact with the higher metal ion concentration will be anodic and corroded.

Oxygen concentration cells

Water in contact with the metal surface will normally contain dissolved

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse uniformly into the solution, thereby creating a difference in oxygen concentration between two points. Corrosion will occur at the area of low-oxygen concentration, which are anodic.

Active-passive cells

If a metal is protected against corrosion by a tightly adhering passive film (usually an

oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...

) and

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

deposits on the surface in the presence of water, the active metal beneath the film will be exposed to corrosive attack in areas where the passive film is broken. An electrical potential will develop between the large area of the passive film (cathode) and the small area of the exposed active metal (anode). Rapid pitting of the active metal will result.

References

* {{Galvanic cells Battery types

Metal ion concentration cells

Oxygen concentration cells

Active-passive cells

See also

References