Selenium is a group 16 element with the atomic number 34. It is a metalloid, and shares several of its properties with oxygen and sulfur, which are the most well-known members of this group.
Jons Jakob Berzelius discovered the new element selenium when he was asked to find the reason for a spate of illnesses in workers manning a sulfuric acid factory in Gripsholm. His colleague, Martin Klaproth, found that a substance had contaminated the sulfuric acid solution, making it red.
Since the plant had recently switched from the use of imported pyrites to locally available ores, Berzelius investigated the imported ores for the presence of arsenic or tellurium impurities.
Instead, he found, after a lengthy and careful investigation, that the acid production process yielded a sludge with a high concentration of a new element called selenium. Its properties were similar to tellurium. Its name derives from the Greek word for moon, which is “selene”.
However, it is possible to trace references to a substance which might have been selenium, such as the mention of “red sulfur” in the writings of Arnold of Villanova, in the 13th century.
It was Thressa Stadtman who identified the biological importance of selenium in the synthesis of the redox enzyme glycine reductase. Further study showed her that selenium in protein occurred in the form of an amino acid called selenocysteine. This was confirmed by the detection of selenocysteine in the important redox enzyme glutathione peroxidase. These findings fueled more intensive investigation into the role of selenium in various body processes.
Selenium has several allotropes, including crystalline hexagonal, trigonal and non-crystalline forms. The latter is red, and is the form in which most selenium is obtained. It is usually obtained as a by-product from the electrolysis of copper, as part of the anode mud. In nature, it is found in pyrites and other sulfide ores, where it partly replaces sulfur.
Selenium is part of the unique group of compounds which are called semiconductors. It conducts some electricity, though not as freely as a conductive metal. At the same time it is able to convert the light energy incident on it into electrical energy, which is called photoconductivity. An increase in the intensity of light falling on it causes the electrical conductivity of the element to increase as well.
The photoconductive nature of selenium has brought it into use in photocells, exposure meters widely employed by photographers, solar cells and various kinds of optical printers and photocopiers. It is also used in rectifiers, because it can convert alternating current into direct current.
Another interesting use is in glassmaking, where the presence of selenium causes other impurities in glass to lose their color. It, however, can turn glass a beautiful shining red. It is used in the manufacture of alloys, including stainless steel.
Selenium is also an important trace element in food, and is a vital part of many enzymes that take part in redox reactions. Despite its early discovery, it was largely ignored until it was found to form toxic compounds that caused serious disease in livestock on free range pastures. This was in the 1930’s.
It was in 1957 that the German scientist Schwarz found that a mysterious substance present in bakers’ yeast, but not in brewers’ yeast, was absolutely essential as a protein component in the diet of laboratory animals, and its absence caused liver necrosis. This was identified as selenium.
This exciting research resulted in further identification of its beneficial, and in fact essential, role in animal health. Selenium deficiency was found to result in white muscle disease in young animals, pancreatic necrosis in chickens, ulcers of the esophagus and stomach in wine, stillbirth and resorption of embryos in sheep, and abnormal sperm motility in sheep. These were all found to be treatable by selenium supplementation, which was especially rewarding since these diseases had, up to that time, had no cure.