Cobalt(redirected from cobalt-57)
Also found in: Dictionary, Thesaurus, Medical.
Cobalt, town, Canada
cobalt, chemical element
a city in Canada, in southeastern Ontario Province on the western shore of Lake Témiscaming. Population, 2,200 (1971). Cobalt has a railroad station and is a center for cobalt and silver mining. The city arose in 1903 and was at its height in 1915, when its population reached 30,000. Mining began to decline in 1929 and ceased entirely from 1935 to 1946, being resumed in 1950 with the increased demand for silver and cobalt.
(Latin, cobaltum), Co, a chemical element of the first triad of Group VIII in the Mendeleev periodic system. Atomic number, 27; atomic weight, 58.9332. A heavy silver metal shot through with pink. There is one naturally occurring stable isotope, 59Co; 60Co is the most important artificially produced radioactive isotope.
Historical information. Cobalt oxide was used in ancient Egypt, Babylonia, and China as a blue coloring for glass and enamel. Zaffer, a gray earthy mass produced by roasting certain ores called cobold ores, began to be used for this purpose in western Europe in the 16th century. These ores released copious toxic fumes when roasted, and it was impossible to smelt out the metal from the roasting products. Medieval miners and metallurgists believed this to be the pranks of mythical beings called cobolds (German Kobold, “sprite” or “gnome”). In 1735 the Swedish chemist G. Brant produced a metal that he named cobold regulus by heating a mixture of zaffer with carbon and flux in a blast furnace. The name was soon changed to “cobolt” and then “cobalt.”
Occurrence in nature. The content of cobalt in the lithosphere is 1.8 × 10-3 percent by weight. In the earth’s crust it migrates in magma, as well as in hot and cold waters. It accumulates primarily in the upper mantle during magmatic differentiation; its average content in hyperbasic rocks is 2 × 10-2 percent. The formation of “liquation” deposits of cobalt ore is related to magmatic processes. Hydrothermal cobalt deposits are formed as a result of ore concentration in hot subterranean waters, where cobalt is bonded to nickel, arsenic, sulfur, and copper. Approximately 30 cobalt minerals are known.
Cobalt is mainly scattered throughout the biosphere; however, cobalt deposits form in regions where plants that are cobalt concentrators grow. Marked differentiation of cobalt is observed in the upper parts of the earth’s crust: the average cobalt content in clay and shale is 2 × 10-3 percent; in sandstones, 3 × 10-5 percent; and in limestones, 1 × 10-5 percent. The sandy soils of wooded areas are highly deficient in cobalt. Very little cobalt is found in surface waters; the world’s oceans contain only 5 × 10 -8 percent cobalt. Since it is a poor water migrant, cobalt is readily converted into sediment, which is absorbed by manganese hydroxides, clay, and other highly disperse minerals.
Physical and chemical properties. At ordinary temperatures (and up to 417°C) the crystal lattice of cobalt is close-packed hexagonal, with lattice constants a = 2.5017 angstroms (Å) and c = 4.614 Å; at higher temperatures the lattice becomes facecentered cubic (a = 3.5370 Å). Atomic radius, 1.25 Å; ion radii, Co2+ 0.78 Å and Co3+ 0.64 Å; density, 8.9 g/cm3 (at 20°C); melting point, 1493°C; boiling point, 3100°C. Specific heat, 0.44 kilojoules per (kg·°K), or 0.1056 calories per (g·°C); thermal conductivity, 69.08 watts per (m·°K), or 165 calories per (cm·sec·°C) for 0°–100°C; specific electrical resistance, 5.68 × 10-8 ohm·m, or 5.68 × 10-6 ohm·cm (at 0°C). Cobalt is ferromagnetic and maintains its ferromagnetism from low temperatures up to the Curie point, Ⓗ= 1121°C.
The mechanical properties of cobalt depend on the methods of mechanical and thermal treatment used. Tensile strength, 500 meganewtons per sq m (MN/m2), or 50 kilograms-force per sq mm (kgf/mm2), for forged and annealed cobalt; 242–260 MN/ m2 for cast cobalt; and 700 MN/m2 for cobalt wire. Brinell hardness, 2.8 GN/m2, or 280 kgf/mm2, for cold-worked metal; 3.0 GN/m2 for metal precipitated by electrolysis; and 1.2–1.3 GN/m2 for annealed cobalt.
The configuration of the outer electron shells of a cobalt atom is 3d74s2. In compounds cobalt exhibits alternating valence. In simple compounds, Co (II) has the highest degree of stability, and Co (III) is most stable in complex compounds. Only a few complex compounds of Co (I) and Co (IV) have been produced. Cobalt is resistant to water and air at room temperature. Finely crushed cobalt, which is produced by reduction of cobalt oxide with hydroxide at 250°C (pyrophoric cobalt), undergoes spontaneous combustion upon exposure to air and is converted into CoO. Compacted cobalt begins to oxidize in air at temperatures above 300°C, and it dissociates water vapor at red heat: Co + H2O = CoO + H 2. Cobalt combines readily with halogens upon heating to form halides, CoX2. When heated, cobalt reacts with sulfur, selenium, phosphorus, arsenic, antimony, carbon, silicon, and boron; however, the composition of the compounds produced sometimes does not satisfy the valence states mentioned above (for example, Co2P, Co2AS, CoSb2, Co3C, and CoSi3). Cobalt is dissolved slowly in dilute hydrochloric and sulfuric acids, with the liberation of hydrogen and the formation of cobalt chloride, CoCl2, and cobalt sulfate, CoSO4, respectively. Cobalt dissolves in dilute nitric acid, releasing nitric oxide and forming a nitrate, Co(NO3)2. Concentrated HNO3 passivates cobalt. The salts of Co (II) mentioned above are readily soluble in water [at 25°C, 100 g H2O dissolves 52.4 g CoCl2, 39.3 g CoSO4, and 136.4 g Co(NO3)2]. Caustic alkalies precipitate a blue hydroxide, Co(OH)2, from Co2+ salt solutions; the hydroxide gradually turns brown as it undergoes oxidation by atmospheric oxygen into Co(OH)3. Upon heating to 400°–500°C in the presence of oxygen, CoO is transformed into a black oxide, Co3O4, or CoO·Co2O3 (a spinel compound). A compound of this type, such as CoAl2O4or blue CoO·Al2O3 (cobalt blue, or Thénard’s blue, discovered by L. J. Thénard in 1804), is produced upon calcination of a mixture of CoO and Al2O3 at a temperature of about 1000°C.
Only a few simple Co (III) compounds are known to exist. The brown fluoride CoF3 is formed by treating Co or CoCl2 powder with fluorine at 300°–400°C. Complex compounds of Co (III) are highly stable and can be produced easily. For example, KNO 2 precipitates the yellow, poorly soluble product potassium hexanitrocobaltate (III), K3 [Co(NO2)6], from solutions of Co (II) salts containing CH3 COOH. Cobaltammines (formerly known as cobaltiacs), which are complex Co (III) compounds containing ammonia or certain organic amines, are extremely abundant.
Preparation and use. Cobalt minerals are rare and do not form any significant accumulations of ore. Nickel ores, which contain cobalt as an impurity, are the main source for the industrial extraction of cobalt. The processing of these ores is extremely complex, and the method used depends on the composition of the ore. A solution of cobalt chlorides and nickel chlorides, containing the impurities Cu2+, Pb2+, Bi3+, is obtained as a final result. Copper, lead, and bismuth sulfides are precipitated by H2S, after which Fe (II) is converted into Fe (III) by the passage of chlorine; Fe(OH)3 and CaHAsO4 are subsequently precipitated by the addition of CaCO3. Cobalt is separated from nickel by the reaction 2CoCl2 + NaCIO + 4NaOH + H2O = 2Co(OH)3↓ + 5NaCl. Almost all the nickel remains in the solution. The black residue Co(OH)3 is calcined to remove water, and the resultant oxide Co3O4 is reduced with hydrogen or carbon. Metallic cobalt that contains up to 2–3 percent impurities (nickel, iron, copper, and so on) can be purified by electrolysis.
Cobalt is used primarily in the form of alloys based on it (cobalt alloys), as well as alloys based on other metals in which cobalt is the alloying element. Cobalt alloys are used as heat-resistant and scaling-resistant materials and in the manufacture of permanent magnets and cutting tools. Powdered cobalt and Co3O4 are used as catalysts. The fluoride CoF3 is used as a strong fluorinating agent; Thénard’s blue and particularly the silicate of cobalt and potassium are used as paints in the ceramics and glass-making industries. Cobalt salts are used as trace-element fertilizers in agriculture, as well as in fodder for fattening animals.
S. A. POGODIN
The most important artificially radioactive cobalt isotope is 60Co, with half-life T½ = 5.27 years. It is widely used as a gamma emitter in gamma-ray flaw detection. In medicine, 60Co is used primarily in radiation therapy on tumors, as well as for sterilizing medicines. It also serves to destroy insects in grain and fruits and is used in preserving food products. Other radioactive isotopes—for example, 56Co (T½ = 77 days), 57Co (270 days), and 58Co (72 days), which are less harmful because of their short half-life—are used as isotope tracers in the study of metabolism, particularly in studying the distribution of cobalt in the bodies of animals (for example, the permeability of the placenta was studied with the aid of radioactive cobalt).
Cobalt in the organism. Cobalt, which is always present in animal and plant tissues, participates in metabolic processes. The cobalt content in the bodies of animals is related to the cobalt content in food plants and soils. The average cobalt concentration in plants growing in pastures and meadows is 2.2 × 10-5 to 4.5 × 10-5 percent dry weight. Cobalt accumulation in leguminous plants exceeds that in herbaceous and oleraceous plants. Because of the great ability of seaweed to accumulate cobalt its cobalt content is virtually the same as that in land plants, although there is considerably less cobalt in seawater than in the soil. The daily cobalt requirement for humans is approximately 7–15 micrograms and is satisfied by its intake with food. The cobalt requirement for animals depends on their species, age, and productivity. Ruminants require the most cobalt, since it is essential for the development of the symbiotic microflora in the stomach (primarily in the rumen). The daily cobalt requirement for milch cows is 7–20 mg; for sheep, about 1 mg. In cases of cobalt-deficient diets, the productivity of animals decreases, metabolism and hematopoiesis are disrupted, and ruminants suffer from an endemic disease, cobalt deficiency. The biological activity of cobalt is determined by its participation in the structuring of the vitamin B12 molecule and its coenzymatic forms and of the enzyme transcarboxylase. Cobalt is necessary for the activity of several enzymes. It affects protein metabolism, the synthesis of nucleic acids, carbohydrate and fat metabolism, and oxidation-reduction reactions in animals. Cobalt is a powerful activator of hematopoiesis and the synthesis of erythropoietins. It takes part in the enzyme systems of symbiotic bacteria that fix atmospheric nitrogen. Cobalt also stimulates growth, development, and productivity of leguminous and other plants.
IU. I. RAETSKAIA
REFERENCESPerel’man, F. M. “Kobal’t.” In Kratkaia khimicheskaia entsiklopediia, vol. 2. Moscow, 1963.
Nekrasov, B. V. Osnovy obshchei khimii, vol. 3. Moscow, 1970.
Gudima, N. V. “Metallurgiia kobal’ta.” In Spravochnik metallurga po tsvetnym metallam, vol. 2. Moscow, 1947.
Iastrebov, A. P. “Deistvie kobal’ta na obrazovanie eritrotsitov.” In Patofiziologiia eritropoeza (Tr. 7 konferentsii Ural’skogo Mezhoblastnogo obshchestva patofiziologov). Sverdlovsk, 1965.
Iagodin, B. A. Kobal’t v zhizni rastenii. Moscow, 1970.
Trace Element Metabolism in Animals: Proceedings of International Symposium, Aberdeen, Scotland, July 1969. Edinburgh-London, 1970.
Northrop, D. B. “Transcarboxylase.” Journal of Biological Chemistry, 1969, vol. 244, no. 21, pp. 5808–27.