Galvanic cell

The Galvanic cell, named after Luigi Galvani, consists of two metals connected by an electrolyte which forms a salt bridge between the metals. It is also known as a voltaic cell and an electrochemical cell. In 1780, Luigi Galvani discovered that when two different metals (copper and zinc for example) were connected together and then both touched to different parts of a nerve of a frog leg at the same time, they made the leg contract. He called this "animal electricity". The Voltaic pile, invented by Alessandro Volta in the 1800's, is a similar concept. These discoveries paved the way for all electrical batteries.



Missing image
Scheme of a galvanic cell

The Galvanic cell's metals dissolve in the electrolyte at a different rate, leaving some electrons in the rest of the metal, which charges it. The different dissolving rate thus causes an unequal number of electrons in the two metals. This results in an electric potential between the two metals. If an electrical connection, such as a wire or direct contact, is formed between the two, an electric current flows. At the same time, ions of the more active metal, which forms the anode, are transferred through the electrolyte to the less active metal, the cathode, and deposited there as a plating. In this way the anode is consumed or corroded. A similar process is used in electroplating.

There is a flow of electrons from the oxidised ion at the anode to the reduced atom (formerly an ion) at the cathode. It is this flow, due to this redox reaction which constitutes the current.

Electric Potential of a Galvanic Cell

The electric potential of a cell can be easily determined by use of a standard reduction potential table. An oxidation potential table could also be used, but the reduction table is more common. The first step is to identify the two metals reacting in the cell. Then one looks up the Eo (standard electrode potential, in volts) for each of the two half reactions. The electric potential for the cell is equal to the more positive Eo value plus the opposite of the more negative Eo value. The reason you add the opposite of the more negative Eo value is that that reaction is not going to be a reduction reaction, but instead an oxidation reaction.

For example, in the picture above the solutions are CuSO4 and ZnSO4. Each solution has a corresponding metal strip in it, and a salt bridge connecting the two solutions and allowing SO42- ions to flow freely between the copper and zinc solutions. In order to calculate the electric potential one looks up copper and zinc's half recations and finds that:

Cu2+ + 2e- → Cu E = .34 V

Zn2+ + 2e- → Zn E = -.76 V

Thus the reaction that is going on is really

Cu2+ + Zn + 2e- → Cu + Zn2+ + 2e-

The electric potential is then .34 V + -( -.76 V ) = 1.10 V

A easy way to determine the voltage a cell is producing is to use a voltmeter. A more in-depth explanation of the whys and wherefores can be found under Electrode potential.

Galvanic corrosion

Unwanted galvanic cells are formed whenever two metals are in contact in the presence of an electrolyte, such as salt water, resulting in the galvanic corrosion of the more active metal. There are two common ways of preventing this. One is to insulate the two metals from each other, for example plastic or fibre washers are used to separate steel water pipes from copper-based fittings. The other is the use of sacrificial anodes.

See also

External links

de:Galvanische_Zelle nl:Galvanische cel pl:Ogniwo galwaniczne


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