Cadmium

Category: transition metal Atomic number: 48 Colour: silvery blue Melting point: 321°C (610°F) Boiling point: 767°C (1,413°F) First identified: 1817

Cadmium is a toxic substance, which can cause birth defects and cancer, as well as the disease that the Japanese call itai-itai (meaning ‘ouch-ouch’ because of the joint pain it causes). There was a notorious outbreak of itai-itai in the 1960s in the Jinzu River basin, when rice crops in the area were affected by pollution from a local zinc mine. Our bodies do have some natural defences against cadmium poisoning, but above a certain threshold it can be very dangerous. Cadmium is often produced as a by-product of zinc, to which it has many similarities. It is also similar in some respects to mercury, and all three elements are in a vertical line in the periodic table.

It was identified in 1817 after German apothecaries (or pharmacists) who were making zinc oxide by heating a natural zinc carbonate called cadmia found that the oxide was sometimes discoloured rather than pure white. Friedrich Stromeyer, who was Inspector of Pharmacies at the time, investigated this and separated out a brown oxide, which he then reduced to a new metal by heating it with carbon.

Cadmium has had many uses over the years: for instance, it was used to make the pigment cadmium yellow (one of Monet’s favourite paints) and it could also be produced in brown, red and orange with the addition of other substances such as sulphur or selenium. It was used further (in the form of cadmium sulphide) to give casserole dishes their lovely orangey-red colour.

Old batteries and cathode ray tubes in early colour TV sets contained cadmium. And it is still used today in rechargeable nickel-cadmium batteries. It also absorbs neutrons, so has found uses in nuclear reactors. And many components in heavy machinery, such as oil platforms, contain cadmium. Typically, we are trying to replace most uses of this toxic element, which makes it all the more extraordinary that people still voluntarily inhale it in cigarette smoke, and litter the environment with it when they fail to dispose safely of nickel-cadmium batteries.