cadmium

(redirected from cadmiums)
Also found in: Dictionary, Thesaurus, Medical.

cadmium

(kăd`mēəm) [from cadmia, Lat. for calamine, with which cadmium is found associated], metallic chemical element; symbol Cd; at. no. 48; at. wt. 112.411; m.p. 321°C;; b.p. 765°C;; sp. gr. 8.65 at 20°C;; valence +2. Cadmium is a lustrous, silver-white, ductile, very malleable metal. It belongs to Group 12 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
..... Click the link for more information.
, and resembles zinczinc,
metallic chemical element; symbol Zn; at. no. 30; at. wt. 65.38; m.p. 419.58°C;; b.p. 907°C;; sp. gr. 7.133 at 25°C;; valence +2. Zinc is a lustrous bluish-white metal. It is found in Group 12 of the periodic table.
..... Click the link for more information.
 in its chemical properties. Like zinc, it tarnishes in moist air. Cadmium oxide, a brown powder formed by burning the metal in air, is used in electroplating; it is also made by heating cadmium hydroxide. Cadmium forms a carbonate, a chloride, and several complex ions. Cadmium yellow (the sulfide) is a very durable yellow pigment used in paints. The major use of cadmium is as a coating that is electroplated on iron and steel to prevent corrosion; it is preferable to zinc for protection from alkalies. Cadmium is also used in so-called fusible metals, which are low-melting alloys such as Wood's metal, used in automatic fire sprinklers and alarm systems. Cadmium is used in alkaline nickel-cadmium electric storage cells, which have a greater storage capacity than an equal weight of lead-acid storage cells. It has also found some use in the control of nuclear reactions, since it absorbs neutrons. Cadmium does not occur uncombined in nature; greenockite, a cadmium sulfide mineral first found in Scotland, is the only commercial ore. Cadmium is obtained principally as a byproduct of the smelting and refining of ores of zinc, especially zinc sulfides, and of lead and copper. The element was discovered in 1817 by Friedrich Stromeyer.

Cadmium

 

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.

REFERENCES

Pogodin, 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

cadmium

[′kad·mē·əm]
(chemistry)
A chemical element, symbol Cd, atomic number 48, atomic weight 112.40.
(metallurgy)
A tin-white, malleable, ductile metal capable of high polish; principal use is in the plating of iron and steel, and to a much less extent of copper, brass, and other alloys, to protect them from corrosion and improve solderability and surface conductivity, and as a control absorber and shield in nuclear reactors.

cadmium

a malleable ductile toxic bluish-white metallic element that occurs in association with zinc ores. It is used in electroplating, alloys, and as a neutron absorber in the control of nuclear fission. Symbol: Cd; atomic no.: 48; atomic wt.: 112.411; valency: 2; relative density: 8.65; melting pt.: 321.1°C; boiling pt.: 767°C
References in periodicals archive ?
Still, suppliers and compounders agree that if cost is not a concern, there is no application for which a cadmium alternative cannot be used.
Most suppliers now offer full lines of "drop-in" liquid and powder non-cadmium substitutes with the same compatibility, odor, clarity and aging resistance characteristics as cadmium versions.
Cadmium stabilizer substitutes tend to be very formula-specific and less suitable for use in a wide variety of applications unless other components are added to the formula.
We think we can pretty much duplicate the performance of cadmium in just about any application," says Edward Freshman, director of sales and marketing for the Bedford Chemical Div.
Suppliers suggest the end to cadmium use is more likely to be market driven.
The resulting compounds, Flammer says, have the same performance and processing properties as PVC containing cadmium stabilizers and are reportedly not sold at premium prices because of a manufacturing process that enables Vista to produce the new material for just about the same cost as cadmium-stabilized compounds.
Banning cadmium would devastate the art industry," French contends, noting that these pigments provide the brightest tones of colors ranging from yellow to deep maroon.
Though painters seem to agree that a sweeping ban on cadmium would deprive them of pigments that offer a unique combination of vividness, opacity and long-term stability, some admit they could get by without them.
Some artists, particularly jewelers and enamelers, face health threats from some of the same cadmium compounds they seek to keep on the shelves of art supplies stores.
advise artists against using cadmium pigments in airbrushes, which turn the paints into easily inhaled aerosols.
For painters who use hand brushes and buy prepared pigments rather than mix their own, McCann says the risk of kidney-damaging cadmium exposure would be low.
Jewelers and enamelers brave far greater cadmium risks than painters, he adds: "The major concern is soldering with low-melting silver solders that contain about 20 to 30 percent cadmium.