Chemistry Reference
In-Depth Information
CHAPTER
25
Cobalt
DOMINIQUE LISON
ABSTRACT
1 PHYSICAL AND CHEMICAL PROPERTIES
Cobalt is a relatively rare element of the earth's
crust, which is essential to mammals in the form of
cobalamin (vitamin B
12
). The adult human body con-
tains approximately 1 mg of cobalt, 85% of which is
in the form of vitamin B
12
. Human dietary intake of
cobalt varies between 5 and 50
Cobalt (Co): atomic weight, 58.9; atomic number,
27; density, 8.9; melting point, 1495°C; boiling point,
2870°C; crystalline form, silver-grey metal, cubic. The
main radioactive isotopes
57
Co,
58
Co, and
60
Co decay
with respective half-lives of 272 days, 71 days, and
5.271 years, and emit
g/day, and most of
the cobalt ingested by humans is inorganic, vitamin
B
12
representing only a small fraction. The oral bio-
availability of inorganic cobalt varies with the solubil-
ity of the cobalt compound (5-45%). In occupational
settings, workers are exposed to cobalt compounds by
inhalation of dusts. The main industrial use of cobalt
is for the manufacture of alloys and hard metals. The
absorption rate of inhaled cobalt also varies with
the species considered. Cobalt does not accumulate
in the organism and is rapidly excreted in urine.
The concentration of cobalt in urine or in blood is
proposed as a biomarker of recent exposure to solu-
ble cobalt species. The respiratory system is the main
target organ (asthma, fi brosing alveolitis, lung cancer)
on inhalation exposure to cobalt, with a higher risk
of fi brosing alveolitis (hard metal disease) and lung
cancer in the hard metal industry, where workers are
exposed to cobalt metal mixed with tungsten carbide
particles. A physicochemical interaction leading to
the formation of reactive oxygen species may account
for the increased toxicity of this mixture of particles.
Other target organs include the hematopoietic system,
the myocardium, the thyroid gland, and possibly the
reproductive system.
µ
-rays. This document only deals
with the stable isotope. Pure cobalt metal is steel-grey,
shiny, hard, ductile, brittle, and has magnetic proper-
ties. Cobalt compounds have oxidation states (II) or
(III), the former being the most stable. The main com-
pounds of toxicological interest are the metallic form
(cobalt metal and its alloys and composite materials),
the oxides (cobalt oxide and tetroxide), and the salts
(cobalt(II) chloride, sulfi de and sulfate). The most
important Co(II) salts of carboxylic acids include for-
mate, acetate, citrate, naphtenate, linoleate, oleate,
oxalate, resinate, stearate, succinate, sulfamate, 2-
ethylhexanoate, and cobalt carbonyl. A biologically
important cobalt compound is vitamin B
12
, or cyano-
cobalamin, in which cobalt is complexed with four
pyrrole nuclei joined in a ring called corrin, similar to
porphyrins.
The main characteristics of the most common cobalt
compounds are summarized in Table 1.
Physicochemical studies have shown that cobalt metal,
and not its Co(II) ionic species, is thermodynamically
able to reduce ambient oxygen in reactive oxygen spe-
cies (ROS); the kinetics of this process is, however, slow
as a result of the poor oxygen-binding capacity at the sur-
face of cobalt metal particles. In the presence of tungsten
γ
