the aggregate of electrochemical oxidation-reduction processes occurring at electrodes immersed in an electrolyte upon the passage of an electric current through the electrolyte. Electrolysis forms the basis of the electrochemical method of the laboratory and industrial production of various substances, both simple substances (electrolysis in the narrow sense of the word) and complex ones (see).
The first studies and applications of electrolysis date to the end of the 18th and beginning of the 19th centuries, when electrochemistry was becoming established as a science (see). Of great importance in the development of the theoretical basis of electrolysis was M. Faraday’s discovery (1833–34) of the exact relationship between the amount of electric current consumed during electrolysis and the amount of substance produced at each electrode (seeFARADAYS LAWS.) The industrial use of electrolysis became possible after the development of powerful direct-current generators in the 1870’s.
A unique feature of electrolysis is the spatial separation of the oxidation and reduction processes. The electrochemical oxidation occurs at the anode, and the reduction occurs at the cathode. Electrolysis is performed in special apparatus called electrolyzers. Electrolysis is induced by the energy of direct current supply and the energy released during chemical transformations at the electrodes. The power consumed in electrolysis is expended on raising the Gibbs free energy of the system in the course of the formation of the desired products and is partially dissipated in the form of heat in the course of overcoming the resistance in the electrolyzer and in other parts of the electric circuit (seeGIBBS FREE ENERGY).
As a result of electrolysis, the reduction of the ions or molecules of the electrolyte with the formation of new products occurs at the cathode. The cations accept electrons and are transformed into ions of lower oxidation state or into atoms, for example, in the reduction of iron ions (Fe3+ + e– → Fe2+) and the electrodeposition of copper (Cu2+ + 2e– → Cu). Neutral molecules may participate in transformations at the cathode directly, or they may react with the intermediate products of the cathodic process. The oxidation of the ions or molecules found in the electrolyte or within the anode material (the anode dissolves or is oxidized) occurs at the anode, for example, the liberation of oxygen (4OH– → 4e– + 2H2O + O2), the liberation of chlorine (2Cl– → 2e– + Cl2), the formation of chromate (Cr3+ + 3OH– + H2O→CrO42– + 5H+ + 3e–), the dissolution of copper (Cu → Cu2+ + 2e–), and the oxidation of aluminum (2A1 + 3H2O → Al2O3 + 6H+ + 6e–). The electrochemical production of a substance (in the atomic, molecular, or ionic state) involves the transfer of one or several charges from the electrode to the electrolyte (or vice versa) according to the chemical reaction equation. In the latter case, this is achieved, as a rule, through a sequence of one-electron steps, with the formation at the electrode of intermediate ions or radical species, which frequently remain at the electrode in the adsorbed state.
The rates of the electrode reactions depend on the composition and concentration of the electrolyte, on the electrode material, on the electrode potential, and on temperature, among other factors. The rate of each reaction is determined by the rate of electric charge transfer per unit of electrode surface per unit time. Thus, the current density serves as a measure of the rate.
The amount of product formed in electrolysis is determined by Faraday’s laws. If a number of products are formed concurrently at each electrode as a result of several electrochemical reactions, the amount of current (given in percent) consumed in the formation of the product of each of these reactions is called the current efficiency of the given product.
The advantages of electrolysis over chemical methods for the production of various products lie in the relative ease of control of the rate and selectivity of reaction by regulating the current. Electrolysis conditions are easily controlled. As a consequence, processes may be performed in either the “mildest” or “most vigorous” oxidation or reduction conditions and produce the strongest oxidizing and reducing agents used in science and industry. Electrolysis is the principal method for the industrial production of aluminum, chlorine, and sodium hydroxide and an important method for the production of fluorine, alkali metals, and alkaline-earth metals. It is an efficient method of refining metals. Hydrogen and oxygen are produced by the electrolysis of water. The electrochemical method is used for the synthesis of various classes of organic compounds and many oxidizing agents (persulfates, permanganates, perchlorates, and perfluoro-organic compounds). The use of electrolysis for the treatment of surfaces includes cathodic electroplating processes in machine building, instrument-making, and the aviation, electrical engineering, and electronics industries, as well as in anodic polishing and etching processes, anodic mechanical sizing, and the oxidation (anodizing) of metal articles. Metal installations and structures are protected from corrosion by electrolysis under controlled conditions (anodic and cathodic protection).
E. V. KASATKIN