Tellurium

Tellurium: the essentials

 Brief description: crystalline tellurium has a silvery-white appearance, and exhibits a metallic lustre when pure (see above). It is brittle and easily pulverised. Tellurium is a p-type semiconductor, and shows varying conductivity with crystal alignment. Its conductivity increases slightly with exposure to light. It can be doped with silver, copper, gold, tin, or other elements.

Humans exposed to as little as 0.01 mg m^-3 in air, or less, develop "tellurium breath", which has a garlic-like odour.

Isolation

Isolation: it is not usually necessary to make tellurium in the laboratory as it is commercially available. While there are some tellurium ores, most tellurium is made as a byproduct of copper refining. 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).

Cu₂Te + Na₂CO₃ + 2O₂ → 2CuO + Na₂TeO₃ + CO₂

The tellurite Na₂TeO₃ is acidified with sulphuric acid and the tellurium precipitates out as the dioxide (leaving and selenous acid, H₂SeO₃, in solution). Tellurium is liberated from the dioxide by dissolving in sodium hydroxide, NaOH, and electroytic reduction.

TeO₂ + 2NaOH → Na₂TeO₃ + H₂O → Te + 2NaOH + O₂

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