Electrochemistry, that part of the science of chemistry that deals with the interrelationship of electrical currents, or voltages, and chemical reactions, and with the mutual conversion of chemical and electrical energy. In the broadest sense, electrochemistry is the study of chemical reactions that produce electrical effects and of the chemical phenomena that are caused by the action of currents or voltages.

Electric Current and Ion Movement

Most inorganic and some organic chemical compounds, when in a molten state or when dissolved in water or other liquids, become ionized; that is, their molecules become dissociated into positively and negatively charged components, which have the property of conducting an electric current. If a pair of electrodes is placed in a solution of an electrolyte, or an ionizable compound, and a source of direct current is connected between them, the positive ions in the solution move toward the negative electrode and the negative ions toward the positive. On reaching the electrodes, the ions may gain or lose electrons and be transformed into neutral atoms or molecules, the nature of the electrode reactions depending on the potential difference, or voltage, applied.

The action of a current on an electrolyte can be understood from a simple example. If the salt copper sulfate is dissolved in water, it dissociates into positive copper ions and negative sulfate ions. When a potential difference is applied to the electrodes, the copper ions move to the negative electrode, are discharged, and are deposited on the electrode as metallic copper. The sulfate ions, when discharged at the positive electrode, are unstable and combine with the water of the solution to form sulfuric acid and oxygen. Such decomposition caused by an electric current is called electrolysis.

In all cases, the quantity of material evolved at each electrode when current is passed through an electrolyte follows a law discovered by the British chemist and physicist Michael Faraday. This law states that the quantity of material transformed at each electrode is proportional to the quantity of electricity passed through the electrolyte; and that the weight of the elements transformed is proportional to the equivalent weights of the elements, that is, to the atomic weights of the elements divided by their valences.

All chemical changes involve a regrouping or readjustment of the electrons in the reacting substances; hence all such changes may be said to be electrical in character. To produce an electrical current from a chemical reaction, it is necessary to have a reducible substance, that is, a substance that can gain electrons easily; and an oxidizable substance, one that can give up electrons easily. A reaction of this kind can be understood from the operation of a simple type of electrochemical cell, or battery. If a zinc rod is placed in a dilute solution of sulfuric acid, the zinc, which oxidizes readily, will lose electrons, and positive zinc ions will be liberated into the solution. The free electrons stay in the zinc rod. If the rod is connected through a conductor to an inert-metal electrode placed in the sulfuric acid solution, the electrons will flow around this circuit into the solution, where they will be taken up by the positive hydrogen ions of the dilute acid. The combination of the electrons and the ions produces hydrogen gas, which appears as bubbles on the surface of the electrode. The reaction of the zinc rod and sulfuric acid thus produces a current in the external circuit. An electrochemical cell of this kind is known as a primary cell, or voltaic cell.

In the storage battery or accumulator, commonly known as a secondary cell, electrical energy is fed to the cell from an outside source and stored within in the form of chemical energy. The chemical reaction of a secondary cell is reversible, proceeding in one direction when the cell is being charged, and in the opposite direction when it is discharging. Because the reaction is of this type, a secondary cell can be discharged again and again.

Industrial Applications

Electrolytic decomposition is the basis for a number of important extractive and manufacturing processes in modern industry. Caustic soda, an important chemical in the manufacture of paper, rayon, and photographic film, is produced by the electrolysis of a solution of common salt in water (see Alkalies). The reaction produces chlorine and sodium. The sodium in turn reacts with the water in the cell to yield caustic soda. The chlorine evolved is used in pulp and paper manufacture.

An important industrial use of electrolysis is in the electrolytic furnace, which is employed in the manufacture of aluminum, magnesium, and sodium. In this furnace the resistance of a charge of metallic salts is used to heat the charge until it becomes molten and ionizes. The metal is then deposited electrolytically.

Electrolytic methods are also employed in the refining of lead, tin, copper, gold, and silver. The advantage of extracting or refining metals by electrolytic processes is that the deposited metal is of great purity. Electroplating, another industrial application of electrolytic deposition, is used to deposit films of precious metals on base metals and to deposit metals and alloys, as strengthening or wear-resistant coating, on metal parts. Recent advances in electrochemistry include the development of new techniques for placing layers of material on electrodes to increase their efficiency and endurance. Electrodes made out of polymers are now also possible, through the discovery of polymers that can conduct electricity.