The Electrolytic Process

The electrolytic process requires that an electrolyte, an ionized solution or molten metallic salt, complete an electric circuit between two electrodes. When the electrodes are connected to a source of direct current one, called the cathode, becomes negatively (-) charged while the other, called the anode, becomes positively (+) charged. The positive ions in the electrolyte will move toward the cathode and the negatively charged ions toward the anode. This migration of ions through the electrolyte constitutes the electric current in that part of the circuit. The migration of electrons into the anode, through the wiring and an electric generator, and then back to the cathode constitutes the current in the external circuit.

For example, when electrodes are dipped into a solution of hydrogen chloride (a compound of hydrogen and chlorine) and a current is passed through it, hydrogen gas bubbles off at the cathode and chlorine at the anode. This occurs because hydrogen chloride dissociates (see dissociation) into hydrogen ions (hydrogen atoms that have lost an electron) and chloride ions (chlorine atoms that have gained an electron) when dissolved in water. When the electrodes are connected to a source of direct current, the hydrogen ions are attracted to the cathode, where they each gain an electron, becoming hydrogen atoms again. Hydrogen atoms pair off into hydrogen molecules that bubble off as hydrogen gas. Similarly, chlorine ions are attracted to the anode, where they each give up an electron, become chlorine atoms, join in pairs, and bubble off as chlorine gas.

Commercial Applications of Electrolysis

Various substances are prepared commercially by electrolysis, e.g., chlorine by the electrolysis of a solution of common salt; hydrogen by the electrolysis of water; heavy water (deuterium oxide) for use in nuclear reactors, also by electrolysis of water. A metal such as aluminum is refined by electrolysis. A solution of aluminum oxide in a molten mineral decomposes into pure aluminum at the cathode and into oxygen at the anode. In these examples the electrodes are inert.

In electroplating, the plating metal is generally the anode, and the object to be plated is the cathode. A solution of a salt of the plating metal is the electrolyte. The plating metal is deposited on the cathode, and the anode replenishes the supply of positive ions, thus gradually being dissolved. Electrotype printing plates, silverware, and chrome automobile trim are plated by electrolysis.

The English scientist Michael Faraday discovered that the amount of a material deposited on an electrode is proportional to the amount of electricity used. The ratio of the amount of material deposited in grams to the amount of electricity used is the electrochemical equivalent of the material. Actual electric consumption may be as high as four times the theoretical consumption because of such factors as heat loss and undesirable side reactions.

An electric cell is an electrolytic system in which a chemical reaction causes a current to flow in an external circuit; it essentially reverses electrolysis. A battery is a single electric cell (or two or more such cells linked together for additional power) used as a source of electrical energy. Metal corrosion can take place by electrolysis in an unintentionally created electric cell. The Italian physicist Alessandro Volta discovered the principle of the electric cell (see voltaic cell) in 1800. Within a few weeks William Nicholson and Sir Anthony Carlisle, English scientists, performed the first electrolysis, breaking water down into oxygen and hydrogen.

Process in which electric current passed through a substance causes a chemical change, usually the gaining or losing of electrons (see oxidation-reduction). It is carried out in an electrolytic cell consisting of separated positive and negative electrodes (anode and cathode, respectively) immersed in an electrolyte solution containing ions or in a molten ionic compound. Reduction occurs at the cathode, where electrons are added that combine with positively charged cations in the solution. Oxidation occurs at the anode, where negatively charged anions give up electrons. Both thus become neutral molecules. For historical reasons, electric current is defined to flow in the opposite direction to the flow of electrons. Thus, current is said to flow from the cathode to the anode, even though electrons flow in the opposite direction. Electrolysis is used extensively in metallurgy to extract or purify metals from ores or compounds and to deposit them from solution (electroplating). Electrolysis of molten sodium chloride yields metallic sodium and chlorine gas; that of a strong solution of sodium chloride in water (brine) yields hydrogen gas, chlorine gas, and sodium hydroxide (in solution); and that of water (with a low concentration of dissolved sodium chloride or other electrolyte) yields hydrogen and oxygen.

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