cobalt

iron ← cobalt → nickel - ↑ Co ↓ Rh 27Co Periodic table
Appearance
hard lustrous gray metal
General properties
Name, symbol, number	cobalt, Co, 27
Element category	transition metal
Group, period, block	9, 4, d
Standard atomic weight	58.933195(5) g·mol−1
Electron configuration	[Ar] 4s2 3d7
Electrons per shell	2, 8, 15, 2 (Image)
Physical properties
Color	metallic gray
Density (near r.t.)	8.90 g·cm−3
Liquid density at m.p.	7.75 g·cm−3
Melting point	1768 K, 1495 °C, 2723 °F
Boiling point	3200 K, 2927 °C, 5301 °F
Heat of fusion	16.06 kJ·mol−1
Heat of vaporization	377 kJ·mol−1
Specific heat capacity	(25 °C) 24.81 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k at T/K 1790 1960 2165 2423 2755 3198
Atomic properties
Oxidation states	5, 4 , 3, 2, 1 [1], -1 (amphoteric oxide)
Electronegativity	1.88 (Pauling scale)
Ionization energies (more)	1st: 760.4 kJ·mol−1
	2nd: 1648 kJ·mol−1
	3rd: 3232 kJ·mol−1
Atomic radius	125 pm
Covalent radius	126±3 (low spin), 150±7 (high spin) pm
Miscellanea
Crystal structure	hexagonal
Magnetic ordering	ferromagnetic
Electrical resistivity	(20 °C) 62.4 nΩ·m
Thermal conductivity	(300 K) 100 W·m−1·K−1
Thermal expansion	(25 °C) 13.0 µm·m−1·K−1
Speed of sound (thin rod)	(20 °C) 4720 m/s
Young's modulus	209 GPa
Shear modulus	75 GPa
Bulk modulus	180 GPa
Poisson ratio	0.31
Mohs hardness	5.0
Vickers hardness	1043 MPa
Brinell hardness	700 MPa
CAS registry number	7440-48-4
Most stable isotopes
Main article: Isotopes of cobalt
iso NA half-life DM DE (MeV) DP 56Co syn 77.27 d ε 4.566 56Fe 57Co syn 271.79 d ε 0.836 57Fe 58Co syn 70.86 d ε 2.307 58Fe 59Co 100% 59Co is stable with 32 neutrons 60Co syn 5.2714 years β−,γ,γ 2.824 60Ni
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Cobalt (pronounced /ˈkoʊbɒlt/ KOH-bolt)^[2] is a hard, lustrous, gray metal, a chemical element with symbol Co and atomic number 27. Although cobalt-based colors and pigments have been used since ancient times for making jewelry and paints, and miners have long used the name kobold ore for some minerals, the free metallic cobalt was not prepared and discovered until 1735 by Georg Brandt.

Cobalt occurs in various metallic-lustered ores, for example cobaltite (CoAsS), but is mainly 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 cobalt mined worldwide.

Cobalt is used in the preparation of magnetic, wear-resistant, and high-strength alloys. Smalte (coblat silicate glass) and Cobalt blue (cobalt(II) aluminate, CoAl₂O₄) gives a distinctive deep blue color to glass, ceramics, inks, paints, and varnishes. Cobalt-60 is a commercially important radioisotope, used as a tracer and in the production of gamma rays for industrial use.

Cobalt is an essential trace element for all multicellular organisms as the active center of coenzymes called cobalamins. These include vitamin B-12 which is essential for mammals. Cobalt is also an active nutrient for bacteria, algae, and fungi, and may be a necessary nutrient for all life.

Characteristics

Cobalt is a ferromagnetic metal with a specific gravity of 8.9 (20°C). 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 1115 °C, 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 B₁₂. 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 450 °C, but in practice, the energy difference is so small that random intergrowth of the two is common.^[3]

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(H₂O)₆]²⁺ complex in aqueous solution. Adding excess chloride will change the color from pink to blue, due to the formation of [CoCl₄]²⁻.

Oxygen and chalcogen compounds

Several oxides of cobalt are known. Green cobalt(II) oxide (CoO) has rocksalt crystal structure and is readily oxidized with water and oxygen to brown cobalt(III) hydroxide (Co(OH)₃). 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 as it has not been reported yet.

The chalcogenides of cobalt include two black cobalt(II) sulfides, CoS₂ and Co₂S₃. Orange-colored Cobalt(II) sulfate (CoSO₄) is an example of cobalt compound containing both sulfur and oxygen.

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 the reaction:

Co³⁺ + e⁻ → Co²⁺

is +1.92 V, far beyond the one for chlorine. As a consequence, the only stable cobalt(III) halide is the fluoride.^{[citation needed]}

Coordination compounds

All compounds containing cobalt in the +3 oxidation state are stabilized by complex ion formation, except Cobalt(II,III) oxide (Co₃O₄) and the brown cobalt(III) fluoride (CoF₃), which is instantly hydrolyzed in water.
Examples for the more exotic oxidation states +1, +4 and +5 are the compounds tris(triphenylphosphine)cobalt(I) chloride ((P(C₆H₅)₃)₃CoCl), caesium hexafluorocobaltate (Cs₂CoF₆)) and potassium percobaltate (K₃CoO₄).^[4]

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 complexes showing stereochemistry. The complex can take either right- or left-handed forms of a three-bladed propeller. This complex was first isolated by Werner as yellow-gold needle-like crystals.^[5]

Cobaltocene is a stable cobalt analog to ferrocene.

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 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 and is one reason why cobalt machines have been largely replaced by linear accelerators in modern radiation therapy. Cobalt from radiotherapy machines has been a serious hazard when not disposed of properly, and one of the worst radiation contamination accidents in North America occurred in 1984, after a discarded cobalt-60 containing radiotherapy unit was mistakenly disassembled in a junkyard in Juarez, Mexico.^[6][7]

Cobalt-57 (Co-57 or ⁵⁷Co) is a cobalt radioisotope most often used in medical tests, as a radiolabel for vitamin B₁₂ uptake, and for the Schilling test.^[8]

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 period. Its uses include sterilization of medical supplies and medical waste, radiation treatment of foods for sterilization (cold pasteurization), industrial radiography (e.g., weld integrity radiographs), density measurements (e.g., concrete density measurements), and tank fill height switches. Cobalt-57 is used as a source in Mössbauer spectroscopy and is one of several possible sources in XRF devices (Lead Paint Spectrum Analyzers).

Cobalt-60 as weapon

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.^[9]

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)^[10]. Cobalt glass ingots have been recovered from the Uluburun shipwreck, dating to the late 14th century BC.^[11]

Swedish chemist Georg Brandt (1694–1768) is credited with isolating cobalt circa 1735.^[12] 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 kobalt, 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.

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.^[4] The mineral cattierite is similar to pyrite and occurs together vaesite in the copper deposits in the Katanga Province.^[13] 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 ore

Cobalt output in 2005

World production trend

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.^[14][15]

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.^[16] The problematic political situation in the Congo influences the price of cobalt significantly, best example was the Shaba crisis in 1978.^[17]

There are several methods which can be used to separate cobalt from copper and nickel. They depend 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 surfactants 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 are oxidized to the oxide. The leaching with water extracts the sulfate 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.^[18]

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.^[4]

In 2008, The London Metal Exchange announced that Cobalt would be traded as a commodity on the London Metal Exchange.^[19]

Applications

Alloys

Cobalt-based superalloys consume most of the produced cobalt. The temperature stability of these alloys makes them suitable for use in turbine blades for gas turbines and jet aircraft engines, though nickel-based single crystal alloys surpass them in this regard. Cobalt-based alloys are also corrosion and wear-resistant.^[20] Special cobalt-chromium-molybdenum alloys are used for prosthetic parts such as hip and knee replacements.^[21] Cobalt alloys are also used for dental prosthetics, where they are useful to avoid allergies to nickel.^[22] Some high speed steels also use cobalt to increase heat and wear-resistance. The special alloys of aluminium, nickel, cobalt and iron, known as Alnico, and of samarium and cobalt (samarium-cobalt magnet) are used in permanent magnets.^[23]

Batteries

Lithium cobalt oxide (LiCoO₂) is widely used in Lithium ion battery electrodes.^[24] Nickel-cadmium (NiCd) and nickel metal hydride (NiMH) batteries also contain significant amounts of cobalt.

Catalyst

Several cobalt compounds are used in chemical reactions as catalysts. Cobalt acetate is used for the production of terephthalic acid as well as dimethyl terephthalic acid, which are key compounds in the production of Polyethylene terephthalate. The steam reforming and hydrodesulfuration for the production of petroleum, which uses mixed cobalt molybdenum aluminium oxides as a catalyst, is another important application.^[24] Cobalt and its compounds, especially cobalt carboxylates (known as cobalt soaps), are good oxidation catalysts. They are used in paints, varnishes, and inks as drying agents through the oxidation of certain compounds.^[24] The same carboxylates are used to improve the adhesion of the steel to rubber in steel-belted radial tires.^[24]

Pigments and coloring

Cobalt blue glass

Before the 19th century, the predominant use of cobalt was as pigment. Since the midage the production of smalt a blue colored glas was known. Smalt is produced by melting a mixture of the roasted mineral smaltite, quartz and potassium carbonate, yielding a dark blue silicate glass which is grinded after the production.^[25] Smalt was widely used for the coloration of glass and as pigment for paintings.^[26] In 1780 Sven Rinman discovered cobalt green and in 1802 Louis Jacques Thénard discovered cobalt blue.^[27] The two colors cobalt blue, a cobalt aluminate, and cobalt green, a mixture of cobalt(II) oxide and zinc oxide, were used as pigments for paintings due to their superior stability.^[28][29]

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 during the 14th century BC.^[30] 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 were obtained is unknown.^[31][32]

Other uses

• Electroplating due to its appearance, hardness, and resistance to oxidation
• Ground coats for porcelain enamels
• Purification of histidine-tagged fusion proteins in biotechnology applications

Biological role

Please help improve this article by expanding it. Further information might be found on the talk page. (September 2008)

Cobalamin

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 B₁₂.

Although cobalt proteins are less common than proteins containing metals like manganese, iron, or zinc, several are known. Most cobalt proteins use a cofactor based on the corrin cobalt, derived from vitamin B₁₂, 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.^[33]

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).^[34] In 1966, the addition of cobalt compounds to stabilize beer foam in Canada led to cardiomyopathy, which came to be known as beer drinker's cardiomyopathy.^[35] Powdered cobalt in metal form is a fire hazard. After nickel and chromium, cobalt is a major cause of contact dermatitis.^[36]

References

External links

Wikimedia Commons has media related to: Cobalt

Look up cobalt in Wiktionary, the free dictionary.

• National Pollutant Inventory - Cobalt fact sheet
• WebElements.com – Cobalt
• London celebrates 50 years of Cobalt-60 Radiotherapy