Cobalt ( ) 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, CoAl
2O
4) 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 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 450 °C, but in practice, the energy difference is so small that random intergrowth of the two is common.
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(H
2O)
6]
2+ complex in aqueous solution. Adding excess chloride will change the color from pink to blue, due to the formation of .
Chalcogen compounds
Several oxides of cobalt are known. Green
cobalt(II) oxide (CoO) has NaCl 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 (Co
3O
4), which has
spinel structure. The brown
cobalt(III) oxide (Co
2O
3) is the least stable of the oxides. Cobalt oxides are
antiferromagnetic at low
temperature: CoO (
Neel temperature 291 K) and Co
3O
4 (Neel temperature: 40 K), which is analogous to
magnetite (Fe
3O
4), with a mixture of +2 and +3 oxidation states. The oxide Co
2O
3 is probably unstable; it has never been synthesized.
The sulfur compounds are the two black
cobalt(II) sulfide (CoS
2) and
cobalt(III) sulfide (Co
2S
3).
Halogen compounds
The halogen compounds of cobalt are
cobalt(II) fluoride (CoF
2),
cobalt(II) chloride (CoCl
2),
cobalt(II) bromide (CoBr
2),
cobalt(II) iodide (CoI
2), and
cobalt(III) fluoride (CoF
3). 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:
+ →
is +1.92 V, far beyond the one for
chlorine. As a consequence, the only stable cobalt(III) halide is the fluoride.
Coordination compounds
Other than Co
3O
4 and the brown fluoride CoF
3 (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 ((P(C
6H
5)
3)
3CoCl), caesium hexafluorocobaltate (Cs
2CoF
6)) and potassium percobaltate (K
3CoO
4).
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 CoCl
3(NH
3)
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 propeller. This complex was first isolated by Werner as yellow-gold needle-like crystals.
Cobaltocene is a fairly 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.
Cobalt-57 (Co-57 or
57Co) is a cobalt radioisotope most often used in medical tests, as a radiolabel for
vitamin B uptake, and 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 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.
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 As
4O
6, 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 (CoAs
2) and
skutterudite (CoAs
3) 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': Co
3(AsO
4)
2·8H
2O) and
sphaerocobaltite (CoCO
3).
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.
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.
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.
All the above-mentioned processes yield copper compounds which are transformed into the cobalt oxide Co
3O
4. The reduction to the metal is done either by the
aluminothermic reaction or reduction with carbon in a
blast furnace.
In 2008, The
London Metal Exchange announced that Cobalt would be traded as a commodity on the London Metal Exchange.
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.
Special cobalt-
chromium-
molybdenum alloys are used for prosthetic parts such as hip and knee replacements. Cobalt alloys are also used for dental prosthetics, where they are useful to avoid allergies to nickel. 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.
Batteries
Lithium cobalt oxide (LiCoO
2) is widely used in
Lithium ion battery electrodes.
(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.
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.
The same carboxylates are used to improve the adhesion of the steel to rubber in steel-belted radial tires.
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. Smalt was widely used for the coloration of glass and as pigment for paintings. In 1780
Sven Rinman discovered
cobalt green and in 1802
Louis Jacques Thénard discovered
cobalt blue. 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.
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. 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.
Other uses
Biological role
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 B.
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 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.
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). 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.
Powdered cobalt in metal form is a
fire hazard. After nickel and
chromium, cobalt is a major cause of
contact dermatitis.
