Cobalt: Encyclopedia - Cobalt

Cobalt

Cobalt is a chemical element in the periodic table that has the symbol Co and atomic number 27.

Cobalt - Notable characteristics

Cobalt is a hard ferromagnetic silver-white element. The Curie temperature is of 1388 K with 1.6~1.7 Bohr magnetons per atom. It is frequently associated with nickel, and both are characteristic ingredients of meteoric iron. Mammals require small amounts of cobalt salts. 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 commonly presents a mixture of two crystallographic structures hcp and fcc with a transition temperature hcp→fcc of 722 K.

Common oxidation states of cobalt include +2, and +3, though +1 is also seen.

Cobalt - Applications

• Alloys, such as:
  • Superalloys, for parts in gas turbine aircraft engines.
  • Corrosion- and wear-resistant alloys.
  • High-speed steels.
  • Cemented carbides (also called hard metals) and diamond tools.
• Magnets and magnetic recording media.
  • Alnico magnets.
• Catalysts for the petroleum and chemical industries.
• electroplating because of its appearance, hardness, and resistance to oxidation.
• Drying agents for paints, varnishes, and inks.
• Ground coats for porcelain enamels.
• Pigments (cobalt blue and cobalt green).
• Battery electrodes.
• Steel-belted radial tires.
• Cobalt-60 has multiple uses as a gamma ray source:
  • It is used in radiotherapy.
  • It is used in radiation treatment of foods for sterilization (cold pasteurization).
  • It is used in industrial radiography to detect structural flaws in metal parts.

Co-60 is useful as a gamma ray source partially because it can be produced - in known quantity, and very large amounts - by simply exposing natural cobalt to neutrons in a reactor for a given time.

Cobalt - Use 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. The ⁶⁰Co source is useful for about 5 years but even after this point is still very radioactive, and so cobalt machines have fallen from favor in the Western world where linacs are common. The first ⁶⁰Co therapy machine (the "cobalt bomb") was built and first used in Canada. In fact the first machine is on display in the Saskatoon Cancer Centre – look up when entering the lobby. The second machine is out beside the walkway into the Centre.

Cobalt - History

Cobalt was known in ancient times through its compounds, which would color glass a rich blue.

George Brandt (1694-1768) is credited with the discovery of cobalt. The date of discovery varies depending on the source, but is between 1730 and 1737. He was able to show that cobalt was the source of the blue color in glasses, which previously had been attributed to the bismuth found with cobalt.

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.

The word cobalt comes from the German kobalt or kobold, meaning evil spirit, the metal being so called by miners, because it was poisonous and troublesome (it polluted and degraded the other mined elements, like nickel). Other sources cite the origin as stemming from silver miners' belief that cobalt had been placed by kobolds who had stolen the silver. Some also think the name may derive from Greek kobalos, which means 'mine', and which may have common roots with kobold, goblin, and cobalt.

Cobalt - 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-12.

Cobalt - Occurrence

Cobalt is not found as a free metal and is 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. The world's major producers of cobalt are the Democratic Republic of the Congo, mainland China, Zambia, Russia and Australia. It is also found in Finland, Azerbaijan, and Kazakhstan. It is also produced in the town of Cobalt, Ontario as a byproduct of the silver mining.

Cobalt - Compounds

Due to the various oxidation states, there is an abundant number of compounds. Oxides are antiferromagnetic at low temperature CoO (Neel temperature: 291 K) and Co₃O₄ (Neel temperature: 40 K).

Cobalt - Isotopes

Naturally occurring cobalt is composed of 1 stable isotope, 59-Co (⁵⁹Co). 22 radioisotopes have been characterized with the most stable being ⁶⁰Co with a half-life of 5.2714 years, 57-Co (⁵⁷Co) with a half-life of 271.79 days, and 56-Co (⁵⁶Co) with a half-life of 77.27 days, and 58-Co (⁵⁸Co) with a half life of 70.86 days. All of the remaining radioactive isotopes have half-lifes 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 amu (⁵⁰Co) to 73 amu (⁷³Co). The primary decay mode before the most abundant stable isotope, ⁵⁹Co, is electron capture and the primary mode after 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 - Precautions

Powdered cobalt in metal form is a fire hazard. Cobalt compounds should be handled with care due to cobalt's slight toxicity.

Cobalt-60 is a powerful gamma ray emitter and exposure to ⁶⁰Co is therefore a cancer risk. Ingestion of ⁶⁰Co will lead to incorporation of some cobalt into tissues, which is released very slowly. Cobalt-60 is a risk factor in a nuclear confrontation because neutron emissions will convert iron into this isotope. Some nuclear weapon designs could intentionally increase the amount of Cobalt-60 dispersed as nuclear fallout – this is sometimes called a dirty bomb or cobalt bomb, once predicted by a leading scientist as being capable of wiping out all life on earth. The risk in the absence of a nuclear war comes from improper handling (or theft) of medical radiotherapeutic units. Nevertheless, the gamma radiation emitted from cobalt-60 is used to kill bacteria on fruit and vegetables thus increasing their shelf life. This process is referred to as irradiation.

See also

• Benjamin Wegner

Categories: Chemical elements | Transition metals

Adapted from the Wikipedia article "Cobalt", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki