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Cobalt                                                                                     

Standard Sate: solid at 298 K

Color: lustrous, metallic, greyish tinge     

Cobalt is a brittle, hard, transition metal with magnetic properties similar to those of iron. Cobalt is present in meteorites.

(Co), chemical element, ferromagnetic metal of Group VIII of the
periodic table, used especially for heat-resistant and magnetic alloys.

The metal was isolated (c. 1735) by a Swedish chemist, Georg
Brandt, though cobalt compounds had been used for centuries to
impart a blue colour to glazes and ceramics. Cobalt has been
detected in Egyptian statuettes and Persian necklace beads of the 3rd
millennium BC, in glass found in the Pompeii ruins, in China as early
as the T'ang dynasty (ad 618-907) and later in the blue porcelain of
the Ming dynasty. The name kobold was first applied (16th century)
to ores thought to contain copper but eventually found to be
poisonous, arsenic-bearing cobalt ores. Brandt finally determined
(1742) that the blue colour of these ores was due to the presence of
cobalt.

Occurrence, properties, and uses.


Cobalt, though widely dispersed, makes up only 0.001 percent of the
Earth's crust. It is found in small quantities in terrestrial and meteoritic
native nickel-iron, in the Sun and stellar atmospheres, and combined
with other elements in natural waters, in nodules beneath the oceans,
in soils, in plants and animals, and in such minerals as cobaltite,
linnaeite, skutterudite, smaltite, heterogenite, and erythrite. Traces of
cobalt are present in many ores of iron, nickel, copper, silver,
manganese, zinc, and arsenic, from which it is often recovered as a
by-product.

For information on the mining, refining, and recovery of cobalt, see
Industries, Extraction and Processing: Cobalt.

Cobalt is a trace element essential in the nutrition of ruminants
(cattle, sheep) and in the maturation of human red blood cells in the
form of vitamin B12, the only vitamin known to contain such a heavy
element.

Polished cobalt is silver-white with a faint bluish tinge. Two
allotropes are known: the close-packed-hexagonal structure stable
below 417 C (783 F) and the face-centred-cubic, stable at high
temperatures. It is ferromagnetic up to 1,121 C (2,050 F, the highest
known Curie point of any metal or alloy) and may find application
where magnetic properties are needed at elevated temperatures.
Natural cobalt is all stable isotope cobalt-59, from which the longest
lived artificial radioactive isotope cobalt-60 (5.3-year half-life) is
produced by neutron irradiation in a nuclear reactor. Gamma radiation
from cobalt-60 has been used in place of X rays or alpha rays from
radium in the inspection of industrial materials to reveal internal
structure, flaws, or foreign objects; in cancer therapy; in sterilization
studies; and in biology and industry as a radioactive tracer. It is in turn
being replaced in both industrial and medical radiology by cesium-137
because of the long (30-year) half-life of the latter.

Most of the cobalt produced is used for special alloys. A relatively
large percentage of the world's production goes into magnetic alloys
such as the Alnicos for permanent magnets. Sizable quantities are
utilized for alloys that retain their properties at high temperatures and
superalloys that are used near their melting points (where steels
would become too soft). Cobalt is also employed for hard-facing
alloys, tool steels, low-expansion alloys (for glass-to-metal seals), and
constant-modulus (elastic) alloys (for precision hairsprings). Cobalt is
the most satisfactory matrix for cemented carbides.

Finely divided cobalt ignites spontaneously. Larger pieces are
relatively inert in air, but above 300 C (570 F) extensive oxidation
occurs.


Compounds.


In its compounds cobalt nearly always exhibits a +2 or +3 oxidation
state, although states of +4, +1, 0, and -1 are known. The compounds
in which cobalt is divalent (Co2+, the ion being stable in water) are
called cobaltous, while those in which cobalt is trivalent (Co3+) are
termed cobaltic. (See chemical compound.)

Both the divalent and trivalent states form numerous coordination
compounds, or complexes. Cobalt(III) forms more known complex
ions than any other metal except platinum. The coordination number
of the complexes is generally six.

Cobalt forms two well-defined binary compounds with oxygen:
cobaltous, or cobalt(II) oxide, CoO, and tricobalt textroxide, or
cobalto-cobaltic oxide, Co3O4. The latter contains cobalt in both
valences and comprises up to 40 percent of the commercial cobalt
oxide used in the manufacture of ceramics, glass, and enamel and in
the preparation of catalysts and cobalt metal powder.

One of the more important salts of cobalt is the sulfate CoSO4,
which is employed in electroplating, in preparing drying agents, and
for pasture top-dressing in agriculture. Other cobaltous salts have
significant applications in the production of catalysts, driers, cobalt
metal powders, and other salts. Cobaltous chloride (CoCl26H2O in
commercial form), a pink liquid that changes to blue as it dehydrates,
is utilized in catalyst preparation and as an indicator of humidity.
Cobaltous phosphate, Co3(PO4)28H2O, is used in painting porcelain
and colouring glass. atomic number 27 atomic weight 58.9332 melting
point 1,495 C (2,723 F) boiling point 2,870 C (5,198 F) density 8.9
(20 C) valence 2, 3 electronic config. 2-8-15-2 or (Ar)3d74s2

"cobalt" Encyclop�dia Britannica Online.

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