cadmium(redirected from Cadmium compounds)
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Cd, a chemical element of Group II of the periodic system of Mendeleev. Atomic number, 48; atomic weight, 112.40. Cadmium is a white, shiny, heavy, soft, ductile metal. The element consists of a mixture of eight stable isotopes with the mass numbers 106 (1.215 percent), 108 (0.875 percent), 110 (12.39 percent), 111 (12.75 percent), 112 (24.07 percent), 113 (12.26 percent), 114 (28.86 percent), and 116 (7.58 percent).
Historical information. During inspection of a drugstore in 1817 the german chemist F. Strohmeyer found that the zinc carbonate there contained an unknown metal as an impurity, which was precipitated in the form of a yellow sulfide on treatment with hydrogen sulfide in an acid solution. Strohmeyer called the discovered metal cadmium (from the Greek kadmeia, impure zinc oxide, also zinc ore). Independently from him, the German scientists K. Hermann, K. Karsten, and W. Meissner discovered cadmium in Silesian zinc ores in 1818.
Distribution in nature. Cadmium is a rare, widely scattered element with a clarke value of 1.3 × 10-5 percent by weight in the lithosphere. Migration in hot underground waters with zinc and other chalcophilic elements is characteristic of cadmium as is its concentration in hydrothermal deposits. The mineral sphalerite, ZnS, contains up to 0.5-1 percent cadmium in some locations; the maximum content is 5 percent. Greenockite, CdS, is found less frequently. Cadmium is concentrated in marine sedimentary rocks—shales (Mansfeld, GDR)—as well as in sandstones, in which it is also combined with zinc and other chalcophilic elements. Three very rare well-identified cadmium minerals are known in the biosphere: the carbonate CdCC>3 (ota-vite), the oxide CdO, and the selenide CdSe.
Physical and chemical properties. The cadmium crystal lattice is hexagonal, a = 2.97311 Å, c = 5.60694 Å (at 25°C). The atomic radius measures 1.56 Å, and the ionic radius of Cd2+measures 1.03 Å. The density is 8.65 g/cm3 (at 20°C); the melting point, 320.9°C; the boiling point, 767°C; the coefficient of thermal expansion, 29.8 × 10-6 (at 25°C); thermal conductivity (at 0°C), 97.55 W/(m.°K), or 0.233 cal/(cm.sec• °C); specific heat (at 25°C), 225.02 joules per kilogram • degree Kelvin, or 0.055 cal/(g.°C); specific electrical resistivity (at 20°C), 7.4 × 10-8 ohm«m (7.4 × 10-6 ohm.cm); temperature coefficient of electrical resistivity, 4.3 × 10-3 (0°-100°C); tensile strength, 64 meganewtons per square meter (MN/m2), or 6.4 kilograms-force per square millimeter (kgf/mm2); relative elongation, 20 percent; and Brinell hardness, 160 MN/m2, or 16 kgf/mm2.
In accordance with the external electron configuration of the atom 4dl05s2, the valence of cadmium in compounds is equal to 2 (there are, however, indications of the formation of Cd22+ ions during the solution of cadmium in molten CdCl2). Cadmium loses its luster on exposure to air with formation of a thin coating of the oxide CdO, which protects the metal from further oxidation. Intense heating in air leads to the combustion of cadmium to give the oxide CdO, a crystalline powder of light brown to dark brown color and density of 8.15 g/cm3; CdO sublimes without melting at 700°C. Cadmium combines directly with the halogens, and the resulting compounds are colorless. CdCl2, CdBr2, and Cdl2 are readily soluble in water (about one part anhydrous salt in one part water at 20°C), whereas CdF2 is less soluble (one part in 25 parts water). Cadmium reacts with sulfur to yield the sulfide CdS, which is lemon-yellow to orange-red in color and insoluble in water and dilute acids. Cadmium readily dissolves in nitric acid with the evolution of nitrogen oxides and the formation of a nitrate, which gives the hydrate Cd(N0 3)2. 4H2). Cadmium slowly liberates hydrogen from hydrochloric and dilute sulfuric acids; upon evaporation of the solutions hydrates of chloride, 2CdCl2. 5H2O, and sulfate, 3CdSO4. 8H2, crystallize out. Solutions of cadmium salts have acidic reactions owing to hydrolysis. Alkali hydroxides precipitate from these solutions the white hydroxide Cd(OH)2, which is insoluble in an excess of reagent. However, the action of concentrated caustic solutions on Cd(OH)2 yields hydroxocadmiates, for example, Na2[Cd(OH)2]. The Cd2+ cation readily forms complex ions with ammonia—[Cd(NH3)4]2+—and with cyanide—[Cd(CN)4]2+ and [Cd(CN)6]4-. Numerous basic, double, and complex cadmium salts are known. Cadmium compounds are toxic; inhalation of its oxide vapors is particularly dangerous.
Preparation and uses. Cadmium is prepared from the byproducts of the processing of zinc, lead-zinc, and copper-zinc ores. These products, containing 0.2-7 percent cadmium, are treated with dilute sulfuric acid, which dissolves the oxides of cadmium and zinc. Cadmium is precipitated from the solution by the addition of zinc dust. The spongy residue (mixture of cadmium and zinc) is dissolved in dilute sulfuric acid, and cadmium is separated by electrolysis of this solution. Electrolytic cadmium is remelted under a layer of sodium hydroxide and cast into sticks. The purity of the metal is not less than 99.98 percent.
Metallic cadmium is used in nuclear reactors, for anticorrosion and decorative coatings, and in storage batteries. Cadmium is the basis for some bearing alloys, and it is a component of low-melting alloys (for example, Wood’s alloy). Low-melting alloys are used for joining glass to metal, in automatic fire extinguishing systems, for fine and complex castings in gypsum molds, and for other purposes. Cadmium sulfide (cadmium yellow) is a pigment used in art. Cadmium sulfate and amalgam are used in the Weston standard cell.
Cadmium in the organism. The cadmium content of plants is 10-4 percent (on a dry basis), whereas the cadmium content of some animals (sponges, coelenterates, worms, echinoderms, and tunicates) is 4 × 10-5-3 × 10-3 percent (on a dry basis). Cadmium has been detected in all vertebrates. The liver is particularly rich in cadmium. The physiological importance of cadmium has not been adequately investigated, but it is known that cadmium affects carbohydrate metabolism, the synthesis of hip-puric acid in the liver, and the activity of some enzymes.
REFERENCESPogodin, S. A. “Cadmium.” In Kratkaia khimicheskaia entsiklopediia, vol. 2. Moscow, 1963.
Remy, H. Kurs neorganicheskoi khimii, vol. 2. Moscow, 1966. Pages 476-86. (Translated from German.)
S. A. POGODIN