Selenium

Selenium: the essentials

 Brief description: selenium can be prepared with either an amorphous or crystalline structure. Crystalline monoclinic selenium is deep red; crystalline hexagonal selenium, the most stable variety, is a metallic grey (see picture above). Elemental selenium is relatively nontoxic and is considered to be an essential trace element. However, hydrogen selenide (H₂Se) and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions. Hydrogen selenide in a concentration of 1.5 ppm is intolerable to man. Selenium occurs in some soils in amounts sufficient to produce serious effects on animals feeding on plants such as locoweed (an American plant) grown in such soils.

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Isolation

Isolation: it is not usually necessary to make selenium in the laboratory as it is commercially available. While there are several selenium ores, most selenium is made as a byproduct of copper refining. It also accumulates in the residues from sulphuric acid manufacture. Extraction is complex since the method emplyed will depend upon what other compounds or elements are present. The first step usually involves an oxidation in the presence of sodium carbonate (soda ash).

Cu₂Se + Na₂CO₃ + 2O₂ → 2CuO + Na₂SeO₃ + CO₂

The selenite Na₂SeO₃ is acidified with sulphuric acid. Any tellurites precipitateout leaving selenous acid, H₂SeO₃, in solution. Selenium is liberated from selenous acid by SO₂

H₂SeO₃ + 2SO₂ + H₂O → Se + 2H₂SO₄

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