Cobalt

Cobalt (chemical symbol Co) is an essential trace element. Its most important function in humans is as the active metal atomic center of vitamin B₁₂.

Chemistry

Basic chemistry

Cobalt is a shiny grey-silvery transition metal with an atomic number 27 and an atomic weight of 58.93 Daltons (Da). As with other metals it may be found in elemental, inorganic, and organic forms. Elemental cobalt is a solid, lustrous silver-grey metal with a melting point of 1495°C and a boiling point of 2927°C. Inorganic salts of cobalt occur in two oxidation states; cobaltous (2+) and cobaltic (3+). Organification of cobalt occurs in bacteria yielding cyanocobalamin.

Radiochemistry

A single stable isotope, cobalt-59 accounts for all the naturally-occurring cobalt on earth ⁷.

Dietary

Cobalt is found in fish, nuts, leafy vegetables, and cereal grains.

Toxicity

Cobalt toxicity is protean and variable, ranging from acute (typically enteral) exposures with multisystem organ toxicity to chronic (usually inhalational) toxicity primarily involving the skin and respiratory tract. Sources of exposure to cobalt resulting in toxicity include ⁸:

• tungsten carbide manufacture

  • most common source of chronic, inhalational exposure

  • associated with a pneumoconiosis ("hard metal disease")

• diamond polishing

• wet grinding of Widia tools

• historical erythropoetic therapies

  • cobaltous chloride formerly used therapeutically for anemia

• cement, fly ash

• orthopedic implants ("arthroprosthetic cobaltism")

  • particularly metal-on-metal hip implants

• ceramic tile molds

• chemistry sets

• historical beer foam stabilizers ("beer-drinkers cardiomyopathy")

  • cobaltous sulfate

Clinical manifestations of acute, systemic toxicity include:

• dilated cardiomyopathy

• hypothyroidism, goiter

• polycythemia

• peripheral neuropathy

• retinopathy, hearing loss

• encephalopathy, seizures

Chronic exposure is typically dermal or inhalational in the occupational setting. Manifestations of toxicity include:

• occupational asthma

• hard-metal disease

• dermatitis, acneiform rash

Biological function

• vitamin B₁₂ (cobalamin) is a complex structure and central to it is a cobalt ion within a corrin ring. Vitamin B₁₂ is vital for the normal functioning of every cell in the human body, being key to DNA synthesis. Since vitamin B₁₂ is a hematinic and cobalt is vital for its formation, it follows that cobalt is also one of the hematinics.

Radiological importance

• cobalt-57 is employed as a flood source to perform QA on gamma cameras in nuclear medicine departments

• cobalt-57 spot markers are used as orientation markers during nuclear medicine studies, e.g. thyroid scintigraphy

• cobalt-60 is used in radiotherapy (e.g. Gamma Knife^®)

• cobalt-60 is the sterilization method of choice in the medical device industry

• cobalt-60 was the isotope most commonly employed for the Schilling test which was used to evaluate vitamin B₁₂ deficiency

History and etymology

• Georg Brandt discovered cobalt in 1739 ⁴. It was a difficult metal to extract from its ore cobaltite (an arsenosulfide) and attempts to do so resulted in miners falling ill (likely related to the arsenic found in this ore); these properties were attributed to goblins who had cast evil magic upon the rock. The German word for goblin, kobold, was subsequently adopted for this new element. Interestingly cobalt was the first metal to be discovered that was not known to the ancients.

• Bertrand and Macheboeuf first discovered cobalt in animal cells in 1925, this was later corroborated by more sophisticated spectrographic techniques.