What Is a Concentration Cell?

A concentration cell is a unique type of electrochemical cell where two half-cells contain the same components, but at different concentrations. This difference creates a voltage, as nature's drive to reach equilibrium propels electrons from the higher to the lower concentration side. Intrigued? Discover how this principle is harnessed in applications ranging from batteries to environmental sensors. What could this mean for future technology?
Andrew Kirmayer
Andrew Kirmayer

A concentration cell is split into two different sections, which are linked by a component to allow electron particles to pass, such as a salt bridge. These two half cells contain electrodes and the solutions in each are different in their concentrations. When the concentration cell is activated, the concentrated solution typically becomes more diluted while the dispersed one adds molecules, until both are equal. Usually as equilibrium is neared, a voltage is generated. The strength of this reaction can be measured by calculating the cell’s potential, using a mathematical formula called the Nernst Equation.

Calculating the potential of a concentration cell usually requires a voltmeter. The value provided by the instrument, which can be positive or negative, can then be put into the equation, and used with the concentration level to determine the potential. Electrons flow from one substance to another in a concentration cell. The section that loses particles is called the oxidation side, while the reduction side gains them.

Scientist with beakers
Scientist with beakers

Metal parts are often used in these types of cells, and different areas of them can be exposed to varying concentrations of material. Some sections of the metal can have more electric potential than others, which can cause corrosion. While a concentration cell can be cleaned to prevent this from happening, this is often difficult when the system is placed in soil, for example.

Corrosion can occur because of metal ions. If the cell contains or is exposed to water, is not sealed, or there is no protective coating, then the areas near a high level of metal ions may corrode. Uneven oxygen concentrations, which can occur on metal surfaces under water, can lead to corrosion where the gas levels are low. Points in between adjoining parts, and wood, rubber, or plastic near a metal surface, are typically where degradation happens. An active-passive concentration cell develops when there is oxygen and a passive film is punctured below a layer of salt, causing pitting in the metal.

Concentration cells are often used as meters for measuring acidity and basicity of soil. These usually have one end exposed to the soil and another end contained in a solution of potassium chloride. Comparison of the two measurements can then be made. Generally the higher the voltage produced, the more acidic the sample is. A concentration cell can also be used to analyze chemicals as well as test different materials for susceptibility to corrosion.

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      Scientist with beakers