Cadmium  

The essentials

Description   

Here is a brief description of cadmium.

• Standard state: solid at 298 K
• Colour: silvery grey metallic
• Classification: Metallic
• Availability: cadmium is available in many forms including foil, granules, pellets, sheet, powder, rod, shot, sticks, wire and "mossy cadmium". Small and large samples of cadmium foil, sheet, and wire can be purchased from via their web catalogue.

Cadmium is a soft, bluish-white metal and is easily cut with a knife. It is similar in many respects to zinc. Interestingly, aa characteristic cadmium "scream" is heard on bending a cadmium bar (such as that illustrated above). Cadmium and its compounds are highly toxic. Silver solder, which contains cadmium, should be handled with care.

cartoon included by kind permission of Nick Kim.

Isolation

It is rare that preparation of cadmium in the laboratory should be required bacause of environmental concerns about cadmium. The isolation of cadmium is associated with zinc recovery as cadmium is an impurity in zinc ores. Most zinc production is based upon sulphide ores. These are roasted in industrial plants to form zinc oxide, ZnO. This may be reduced with carbon to form zinc metal, but in practice ingenious technology is required to ensure that the resulting zinc does not contain oxide impurities.

ZnO + C Zn + CO

ZnO + CO Zn + CO₂

CO₂ + C 2CO

After this process, zinc may be refined by distillation under vacuum and this process also allows the separation of any cadmium present in the crude zinc.

The other type of extraction of zinc is electrolytic. Dissolution of crude zinc oxide, ZnO, in sulphuric acid gives zinc sulphate, ZnSO₄ in solution. Before electrolysis to produce zinc, the cadmium impurity and is removed as a precipitate by the addition of zinc dust as cadmium sulphate.