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Cobalt
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Cobalt is a hard, lustrous, grey metal, a chemical element with symbol Co and atomic number 27. Although cobalt-based colors and pigments have been used since ancient times, and miners have long used the name kobold ore for some minerals, cobalt was only discovered in 1735 by Georg Brandt. It is found in various metallic-lustred ores for example cobaltite (CoAsS), but it is produced as a by-product of copper and nickel mining.
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Encyclopedia
Cobalt is a hard, lustrous, grey metal, a chemical element with symbol Co and atomic number 27. Although cobalt-based colors and pigments have been used since ancient times, and miners have long used the name kobold ore for some minerals, cobalt was only discovered in 1735 by Georg Brandt. It is found in various metallic-lustred ores for example cobaltite (CoAsS), but it is produced as a by-product of copper and nickel mining. The copper belt in the Democratic Republic of the Congo and Zambia yields most of the worldwide mined cobalt. Cobalt is used in the preparation of magnetic, wear-resistant, and high-strength alloys. Cobalt blue (cobalt(II) aluminate, CoAl2O4) gives a distinctive deep blue color to glass, ceramics, inks, paints, and varnishes.
Characteristics
Cobalt is a ferromagnetic metal. Pure cobalt is not found in nature, but compounds of cobalt are common. Small amounts of it are found in most rocks, soil, plants, and animals. It is the element of atomic number 27. The Curie temperature is 1388 K, and the magnetic moment is 1.6–1.7 Bohr magnetons per atom. In nature, it is frequently associated with nickel, and both are characteristic minor components of meteoric iron. Mammals require small amounts of cobalt which is the basis of vitamin B. Cobalt-60, an artificially produced radioactive isotope of cobalt, is an important radioactive tracer and cancer-treatment agent. Cobalt has a relative permeability two thirds that of iron. Metallic cobalt occurs as two crystallographic structures hcp and fcc. The ideal transition temperature between hcp and fcc structures is 722 K, but in practice, the energy difference is so small that random intergrowth of the two is common. Cobalt has a hardness of 5.5 on the Mohs scale of mineral hardness.
Isotopes
59Cobalt is the only stable cobalt isotope. 22 radioisotopes have been characterized with the most stable being 60Co with a half-life of 5.2714 years, 57Co with a half-life of 271.79 days, 56Co with a half-life of 77.27 days, and 58Co with a half-life of 70.86 days. All of the remaining radioactive isotopes have half-lives that are less than 18 hours and the majority of these have half-lives that are less than 1 second. This element also has 4 meta states, all of which have half-lives less than 15 minutes.
The isotopes of cobalt range in atomic weight from 50 u (50Co) to 73 u (73Co). The primary decay mode for isotopes with atomic mass unit values less than that of the most abundant stable isotope, 59Co, is electron capture and the primary mode of decay for those of greater than 59 atomic mass units is beta decay. The primary decay products before 59Co are element 26 (iron) isotopes and the primary products after are element 28 (nickel) isotopes.
Cobalt radioisotopes in medicine
Cobalt-60 (Co-60 or 60Co) is a radioactive metal that is used in radiotherapy. It produces two gamma rays with energies of 1.17 MeV and 1.33 MeV. The 60Co source is about 2 cm in diameter and as a result produces a geometric penumbra, making the edge of the radiation field fuzzy. The metal has the unfortunate habit of producing a fine dust, causing problems with radiation protection. Cobalt-60 has a radioactive half-life of 5.27 years. This decrease in activity requires periodic replacement of the sources used in radiotherapy. This is one more reason why cobalt machines have been largely replaced by linear accelerators (linacs) in modern radiation therapy.
Cobalt-57 (Co-57 or 57Co) is a radioactive metal that is used in medical tests; it is used as a radiolabel for vitamin B uptake. It is useful for the Schilling test.
Industrial uses for radioactive isotopes
Cobalt-60 (Co-60 or 60Co) is useful as a gamma ray source because it can be produced—in predictable quantity, and high activity—by simply exposing natural cobalt to neutrons in a reactor for a given time. It is used for
Cobalt-57 is used as a source in Mössbauer spectroscopy.
Cobalt-57 is used as one of several possible sources in XRF devices (Lead Paint Spectrum Analyzers).
History
Cobalt compounds have been used for centuries to impart a rich blue color to glass, glazes, and ceramics. Cobalt has been detected in Egyptian sculpture and Persian jewelry from the third millennium BC, in the ruins of Pompeii (destroyed AD 79), and in China dating from the Tang dynasty (AD 618–907) and the Ming dynasty (AD 1368–1644). Cobalt glass ingots have been recovered from the Uluburun shipwreck, dating to the late 14th century BC.
Swedish chemist Georg Brandt (1694–1768) is credited with isolating cobalt circa 1735. He was able to show that cobalt was the source of the blue color in glass, which previously had been attributed to the bismuth found with cobalt. The word cobalt is derived from the German , from kobold meaning "goblin", a term used for the ore of cobalt by miners. The first attempts at smelting the cobalt ores to produce cobalt metal failed, yielding cobalt(II) oxide instead. Also,because the primary ores of cobalt always contain arsenic, smelting the ore oxidized into the highly toxic and volatile oxide As4O6, which was inhaled by workers.
During the 19th century, cobalt blue was produced at the Norwegian Blaafarveværket (70–80% of world production), led by the Prussian industrialist Benjamin Wegner.
In 1938, John Livingood and Glenn Seaborg discovered cobalt-60. This isotope was famously used at Columbia University in the 1950s to establish parity violation in beta decay.
Applications
Alloys
The cobalt based superalloys consume most of the produced cobalt. The temperature stability of these alloys make them suitable for turbine blades within gas turbines and jet aircraft engines. The nickel-based single crystal alloys surpass the cobalt based in temperature stability, but the cobalt based are still in use. These alloys are also corrosion and wear-resistant. Special cobalt chromium molybdenum alloys are used for prosthetic parts such as hip and knee replacements. Some high speed steels also use cobalt to increase heat and wear-resistance. The special alloys of aluminium, nickel and cobalt, known as Alnico, and of samarium and cobalt (samarium-cobalt magnet) are used in permanent magnets,
Batteries
Lithium cobalt oxide (LiCoO2) is widely used in Lithium ion battery electrodes. Nickel-cadmium (NiCd) and nickel metal hydride (NiMH) batteries also contain significant amounts of cobalt.
Catalyst
Several cobalt compounds are used in several chemical reactions as catalyst. Cobalt acetate is used for the production of terephthalic acid as well as dimethyl terephthalic acid, which are the key compound for the production of PET. The steam reforming and hydrodesulfuration for the production of petroleum, which uses mixed cobalt molybdenum aluminium oxides as catalyst is another important application.
Cobalt and its compounds; especially cobalt caroxylates, known as cobalt soaps; are good oxidation catalysts. They are used in paints, varnishes, and ink as drying agents in which the oxidation of certain compounds leads to the drying. The same carboxylates are used to improve the adhesion of the steel to rubber in Steel-belted radial tires.
Pigments and coloring
The use of cobalt for blue pigments was the predominant use before the 19th century. The coloring of glass and the pigments cobalt blue and cobalt green are some examples for these uses.
Cobalt has been used to color glass since the bronze age. The excavation of the Uluburun shipwreck yielded an ingot of blue glass which was cast in the 14th century BC. Blue glass items from Egypt are colored with copper, iron, or cobalt. The oldest cobalt colored glass was from the time of the Eighteenth dynasty (1550–1292 BC). The location where the cobalt compounds used for the coloring were found is still unknown.
Other use
Occurrence
Cobalt occurs in copper and nickel minerals and in combination with sulfur and arsenic in the sulfidic cobaltite (CoAsS), safflorite (CoAs2) and skutterudite (CoAs3) minerals. The mineral cattierite is similar to pyrite and occurs together vaesite in the copper deposits in the Katanga Province. If the sulfides come in contact with the atmosphere weathering starts transforming the minerals by oxidation. The products of the oxidation are for example pink erythrite ('cobalt glance': Co3(AsO4)2·8H2O) and sphaerocobaltite (CoCO3).
Production
Cobalt is not found as a native metal but generally found in the form of ores. Cobalt is usually not mined alone, and tends to be produced as a by-product of nickel and copper mining activities. The main ores of cobalt are cobaltite, erythrite, glaucodot, and skutterudite.
In 2005, the copper deposits in the Katanga Province (former Shaba province) of the Democratic Republic of the Congo was the top producer of cobalt with almost 40% world share, reports the British Geological Survey. The problematic political situation in the Congo influences the price of cobalt significantly, best example was the Shaba crisis in 1978.
For the separation of the cobalt from copper and nickel exist several methods depending on the concentration of cobalt and the exact composition of the used ore. The first possible separation step is the froth flotation of the ore, in which special surfactats yield in an enrichment of cobalt. The following roasting of the ores can be conducted in a way that the cobalt sulfide is oxidized to the cobalt sulfate, while the copper and the iron is oxidized to the oxide. The leaching with water extracts the sulphate together with the arsenates. The residues are further leached with sulfuric acid yielding a solution of copper sulfate. They also present iron nickel and cobalt salts can be precipitated by chlorine or hypochloride. If the copper is not produced by leaching and electrowinning but by the pyrometallurgic process, the cobalt can be leached from the slag of the copper smelter.
All the above mentioned processes yield copper compounds which are transformed into the cobalt oxide Co3O4. The reduction to the metal is done either by the aluminothermic reaction or reduction with carbon in a blast furnace.
Compounds
Common oxidation states of cobalt include +2 and +3, although compounds with oxidation state +1 are also known. The most stable oxidation state for simple compounds is +2. Cobalt(II) salts form the red-pink [Co(OH2)6]2+ complex in aqueous solution. Adding excess chloride will change the color from pink to blue, due to the formation of [CoCl4]2-.
Chalcogen compounds
The several oxides of cobalt are known. The green cobalt(II) oxide (CoO) has NaCl structure is readily oxidized with water and oxygen to the brown cobalt(III) hydroxide (Co(OH)). At temperatures of 400–500 °C the CoO is oxidized to the blue cobalt(II,III) oxide (Co3O4), which has spinel structure. The brown cobalt(III) oxide (Co2O3) is the least stable of the oxides. Cobalt oxides are antiferromagnetic at low temperature: CoO (Neel temperature 291 K) and Co3O4 (Neel temperature: 40 K), which is analogous to magnetite (Fe3O4), with a mixture of +2 and +3 oxidation states. The oxide Co2O3 is probably unstable; it has never been synthesized.
The sulfur compounds are the two black cobalt(II) sulfide (CoS2) and cobalt(III) sulfide (Co2S3).
Halogen compounds
The halogen compounds of cobalt are cobalt(II) fluoride (CoF2), cobalt(II) chloride (CoCl2), cobalt(II) bromide (CoBr2), cobalt(II) iodide (CoI2), and cobalt(III) fluoride (CoF3). Cobalt(II) chloride is commonly found as an indicator of dryness in silica gel beads used as a desiccant. Anhydrous cobalt(II) chloride is blue, while the hexahydrate is red. The reduction potential for Co3+ + e- ? Co2+ (+1.92 V) is far beyond the one for chlorine. As a concequence, only the fluoride is able to exist in the cobalti-status.
Coordination compounds
Other than Co3O4 and the brown fluoride CoF3 (which is instantly hydrolyzed in water), all compounds containing cobalt in the +3 oxidation state are stabilized by complex ion formation. Examples for the more exotic oxidation states +1, +4 and +5 are the compounds tris(triphenylphosphine)cobalt(I) chloride ((C6H6P)3CoCl), caesium hexafluorocobaltate (Cs2CoF6)) and potassium percobaltate (K3CoO4).
The class of vitamin B12 compounds are coordination complexes of elaborated corrin rings with a central cobalt atom.
Alfred Werner, a pioneer in coordination chemistry, worked with compounds of empirical formula CoCl3(NH3)6; one of the isomers determined was cobalt(III) hexammine chloride. This coordination complex, a "typical" Werner-type complex, consists of a central cobalt atom coordinated by six ammine ligands orthogonal to each other, and three chloride counteranions.
Using chelating ethylenediamine ligands in place of ammonia gives tris(ethylenediamine)cobalt(III) chloride ([Co(en)3]Cl), which was one of the first coordination complex showing stereochemistry. The complex can take either right- or left-handed forms of a three-bladed propellor. This complex was first isolated by Werner as yellow-gold needle-like crystals.
Cobaltocene is a fairly stable cobalt analog to ferrocene.
Biological role
Cobalt in small amounts is essential to many living organisms, including humans. Having 0.13 to 0.30 mg/kg of cobalt in soils markedly improves the health of grazing animals. Cobalt is a central component of the vitamin cobalamin, or vitamin B.
Although cobalt proteins are less common than proteins containing metals like manganese, iron, or zinc, several proteins are known. Most of the cobalt proteins use a cofactor based on the corrin cobalt, derived from vitamin B, but there are also a few proteins known in which cobalt is directly coordinated by the protein structure; Methionine aminopeptidase 2 and Nitrile hydratase are two examples of these proteins.
Precautions
Although cobalt is an essential element for life in minute amounts, at higher levels of exposure it shows mutagenic and carcinogenic effects similar to nickel (see Cobalt Poisoning). The addition of cobalt compounds to stabilize the beer foam in Canada in 1966 led to Cardiomyopathy, which came to be known as beer drinker's cardiomyopathy. Powdered cobalt in metal form is a fire hazard.
After nickel and chromium, cobalt is a major cause of contact dermatitis.
Cobalt-60
60Co is a high-energy gamma ray emitter. Acute high-dose exposures to the gamma emissions can cause severe burns and death. Extended exposures increase the risk of morbidity or mortality from cancer. Nuclear weapon designs could intentionally incorporate 59Co, some of which would be activated in a nuclear explosion to produce 60Co. The 60Co, dispersed as nuclear fallout, creates what is sometimes called a dirty bomb or cobalt bomb.
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