spheric oxygen in the presence of a catalyst. At the turn of the 20th century, chlorine was produced by the electrolysis of aqueous solutions of the chlorides of alkali metals. About 90–95 percent of the world production of chlorine was obtained by these methods in the 1970’s. Small amounts of chlorine are obtained as a by-product in the production of magnesium, calcium, sodium, and lithium by the electrolysis of molten chlorides. In 1975 the world production of chlorine was about 25 million tons.
The two major methods for the electrolysis of aqueous solutions of NaCl are electrolysis in a diaphragm cell with a solid cathode and electrolysis in a mercury cathode cell. In both methods, chlorine gas is liberated at the graphite anode or titanium oxide-ruthenium oxide anode. In the first method, hydrogen is liberated at the cathode, and a solution of NaOH and NaCl is formed, from which commercial caustic soda is obtained by subsequent treatment. In the second method, sodium amalgam is formed at the cathode. A NaOH solution, hydrogen, and pure mercury are formed upon the decomposition of sodium amalgam by pure water in a separate apparatus. The pure mercury formed is reused in production. Both methods yield 1.125 tons of NaOH per ton of chlorine produced.
Electrolysis in a diaphragm cell is a less expensive process and yields cheaper NaOH. The mercury cathode method permits the production of very pure NaOH, although mercury losses in the course of production pollute the environment. In 1970, 62.2 percent of the world production of chlorine was by the mercury cathode method, while the method using the diaphragm cell accounted for 33.6 percent and other methods for 4.2 percent. Beginning in 1970, electrolysis using a solid cathode and an ion-exchange membrane was used, a method that made possible the production of pure NaOH without mercury.
Uses. Chlorine production is one of the leading branches of the chemical industry. Most of the chlorine produced is converted at the production site into chlorine-containing compounds. Chlorine is stored and transported in liquid form in tanks, cylinders, railroad tank cars, or specially equipped ships. The following consumption of chlorine is characteristic of industrial countries: 60–75 percent is used for the production of chlorine-containing organic compounds, 10–20 percent for the production of chlorine-containing inorganic compounds, 5–15 percent for the bleaching of pulp and fabrics, and 2–6 percent for sanitary purposes and water chlorination.
Chlorine is also used for the chlorination of some ores in order to extract titanium, niobium, and zirconium.
Various chlorine-containing organic and inorganic compounds are discussed in separate articles (see Index).
L. M. IAKIMENKO
Chlorine in organisms. Chlorine is a biogenic element and a component of plant and animal tissues. The content of chlorine in plants ranges from thousandths of 1 percent to several percent (halophytes contain large amounts of chlorine), while the chlorine content in animals ranges from hundredths to tenths of 1 percent. The daily chlorine requirement of an adult human is 2–4 g and is met simply with the ingestion of food. In food, chlorine is usually present in excess in the form of sodium chloride and potassium chloride. Bread, meat, and milk products are especially rich in chlorine.
In animal organisms, chlorine is a major osmotically active substance of blood plasma, lymph, spinal fluid, and some tissues. It is important in water-salt metabolism, facilitating the retention of water by tissues. The regulation of the acid-base equilibrium in tissues is accomplished, in addition to other processes, by a change in the chlorine distribution between blood and other tissues.
In plants, chlorine participates in energy exchange, activating both oxidative phosphorylation and photophosphorylation. It also affects the absorption of oxygen by roots and is necessary for the formation of oxygen in photosynthesis by isolated chloroplasts. Chlorine is not included in the composition of most nutritive media for artificial plant 