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Electrolysis

Electroplating

Another important use of electrolytic cells is in the electroplating of silver, gold, chromium and nickel. Electroplating produces a very thin coating of these expensive metals on the surfaces of cheaper metals, to give them the appearance and the chemical resistance of the expensive ones.

In silver plating, the object to be plated (e.g., a spoon) is made from the cathode of an electrolytic cell. The anode is a bar of silver metal, and the electrolyte (the liquid in between the electrodes) is a solution of silver cyanide, AgCN, in water. When a direct current is passed through the cell, positive silver ions (Ag+) from the silver cyanide migrate to the negative anode (the spoon), where they are neutralized by electrons and stick to the spoon as silver metal:

Meanwhile, the silver anode bar gives up electrons to become silver ions:

Thus, the anode bar gradually dissolves to replenish the silver ions in the solution. The net result is that silver metal has been transferred from the anode to the cathode, in this case the spoon. This process continues until the desired coating thickness is built up on the spoon-usually only a few thousandths of an inch-or until the silver bar has completely dissolved.

In electroplating with silver, silver cyanide is used in the electrolyte rather than other compounds of silver such as silver nitrate, AgNO3, because the cyanide ion, CN-, reacts with silver ion, Ag+, to form the complex ion Ag(CN) -. This limits the supply of free Ag+ ions in the solution, so they can deposit themselves only very gradually onto the cathode. This produces a shinier and more adherent silver plating. Gold plating is done in much the same way, using a gold anode and an electrolyte containing gold cyanide, AuCN.


Resources

Books

Chang, Raymond. Chemistry. New York: McGraw-Hill, 1991.

Sherwood, Martin, and Christine Sutton, eds. The Physical World. New York: Oxford, 1991.


Robert L. Wolke

KEY TERMS

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Complex ion

—A big ion that is made up of smaller ions, combined with each other or with other atoms or molecules

Faraday

—A unit of electrical charge equal to the amount of charge carried by one mole of electrons. One faraday is equivalent to 96,485 coulombs.

Oxidation

—The process in which an atom's oxidation state is increased, by its losing one or more electrons.

Reduction

—The process by which an atom's oxidation state is decreased, by its gaining one or more electrons.

Additional topics

Science EncyclopediaScience & Philosophy: Dysprosium to Electrophoresis - Electrophoretic TheoryElectrolysis - Electrolysis Of Water, Production Of Sodium And Chlorine, Production Of Magnesium, Production Of Sodium Hydroxide, Chlorine And Hydrogen