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Jai Ganesh

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Cadmium

Cadmium

Gist

Cadmium (Cd) is a chemical element, a soft, bluish-white metal. It's a heavy metal, known for its toxicity and its presence in the Earth's crust. Cadmium is used in various industrial applications like batteries, pigments, and as a coating to prevent corrosion, but its use is often limited due to its toxicity.

Cadmium is primarily used in rechargeable batteries, pigments, and electroplating for corrosion protection. It's also used in alloys, nuclear reactors, and some specialized applications like solar cells.

Summary

Cadmium is a chemical element; it has symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of its compounds, and like mercury, it has a lower melting point than the transition metals in groups 3 through 11. Cadmium and its congeners in group 12 are often not considered transition metals, in that they do not have partly filled d or f electron shells in the elemental or common oxidation states. The average concentration of cadmium in Earth's crust is between 0.1 and 0.5 parts per million (ppm). It was discovered in 1817 simultaneously by Stromeyer and Hermann, both in Germany, as an impurity in zinc carbonate.

Cadmium occurs as a minor component in most zinc ores and is a byproduct of zinc production. It was used for a long time in the 1900s as a corrosion-resistant plating on steel, and cadmium compounds are used as red, orange, and yellow pigments, to color glass, and to stabilize plastic. Cadmium's use is generally decreasing because it is toxic and nickel–cadmium batteries have been replaced with nickel–metal hydride and lithium-ion batteries. Because it is a neutron poison, cadmium is also used as a component of control rods in nuclear fission reactors. One of its few new uses is in cadmium telluride solar panels.

Although cadmium has no known biological function in higher organisms, a cadmium-dependent carbonic anhydrase has been found in marine diatoms.

Details

Cadmium (Cd), chemical element, a metal of Group 12 (IIb, or zinc group) of the periodic table.

Element Properties

atomic number  :  48
atomic weight  :  112.414
melting point  :  321 °C (610 °F)
boiling point  :  765 °C (1,409 °F)
specific gravity  :  8.65 at 20 °C (68 °F)
oxidation state  :  +2

Properties, occurrence, and uses

Silver-white and capable of taking a high polish, cadmium is nearly as soft as tin and, like tin, emits a crackling sound when bent; it can be rolled out into sheets. Cadmium melts and boils at relatively low temperatures; its vapor is deep yellow and monatomic. The metal is permanent in dry air, becomes coated with the oxide in moist air, burns on heating to redness, and is readily soluble in mineral acids. Poisoning results from the inhalation of vapour or dust of cadmium. Friedrich Stromeyer, a German chemist, discovered the element (1817) in a sample of zinc carbonate, and, in the same year, K.S.L. Hermann and J.C.H. Roloff found cadmium in a specimen of zinc oxide. Both zinc compounds were being examined because their purity as pharmaceuticals was suspect.

A rare element (about 0.2 gram per ton in Earth’s crust), cadmium occurs in a few minerals and in small quantities in other ores, especially zinc ores, from which it is produced as a by-product. The chief zinc ore, zinc blende, or sphalerite, consists mainly of zinc sulfide, containing from 0.1 to 0.3 percent cadmium. All methods of zinc production begin with the conversion of the sulfide into zinc oxide by roasting: the cadmium becomes concentrated in the fumes, which are treated in various steps until a product is obtained containing over 99.9 percent cadmium. Some lead ores also contain small quantities of cadmium, and, if it is present in sufficient quantity, it is recovered by a cycle of operations similar to that used by zinc smelters. Zinc producers who use the electrolytic process recover cadmium in a somewhat different way, but again the principle is the same, beginning with the roasting of zinc sulfide, followed by the treatment of the flue dusts. Most cadmium is recovered in one of these three processes. China, South Korea, Canada, Japan, and Kazakhastan led the world in cadmium refinement in the early 21st century.

Most cadmium produced is electroplated onto steel, iron, copper, brass, and other alloys to protect them from corrosion. Cadmium plating is especially resistant to attack by alkali. Cadmium is physically similar to zinc but is denser and softer. The plated cadmium has a smaller grain size than electro-zinc coatings, and deposits tend to be more uniform and smooth. Consequently, good protection is afforded by thin coatings of cadmium, and thus, in spite of its high price, it is frequently used for the protection of precision parts. Its resistance to marine atmospheres is also superior to that of zinc.

An important application of cadmium is its use as the anode with either nickel or silver oxide as the cathode and a caustic potash electrolyte in rechargeable electrical storage batteries for uses in which lower weight, longer life, and stability upon storage in discharged condition are desirable as in aircraft.

Cadmium combines with many heavy metals to yield alloys; the most important are bearing alloys and low-melting alloys used for brazing. The small quantities of cadmium added to the heavy metals strengthen them. One percent added to copper increases its strength and hardness with only a small reduction in electrical conductivity. Alloyed with zinc, cadmium forms solders with good shear strength. Because it efficiently absorbs thermal neutrons, it is used in control rods for some nuclear reactors.

Natural cadmium is a mixture of eight isotopes: 106Cd (1.2 percent), 108Cd (0.9 percent), 110Cd (12.4 percent), 111Cd (12.8 percent), 112Cd (24.0 percent), 113Cd (12.3 percent), 114Cd (28.8 percent), and 116Cd (7.6 percent).

Additional Information:

Appearance

Cadmium is a silvery metal with a bluish tinge to its surface.

Uses

Cadmium is a poison and is known to cause birth defects and cancer. As a result, there are moves to limit its use.

80% of cadmium currently produced is used in rechargeable nickel-cadmium batteries. However, they are gradually being phased out and replaced with nickel metal hydride batteries.

Cadmium was often used to electroplate steel and protect it from corrosion. It is still used today to protect critical components of aeroplanes and oil platforms.

Other past uses of cadmium included phosphors in cathode ray tube colour TV sets, and yellow, orange and red pigments.

Cadmium absorbs neutrons and so is used in rods in nuclear reactors to control atomic fission.

Biological role

Cadmium is toxic, carcinogenic and teratogenic (disturbs the development of an embryo or foetus). On average we take in as little as 0.05 milligrams per day. But it accumulates in the body, and so on average we store about 50 milligrams.

Before the dangers of cadmium were fully understood, welders and other metal workers were at risk of becoming ill. In 1966 some welders working on the Severn Road Bridge became ill from breathing in cadmium fumes.

Natural abundance

The only mineral containing significant quantities of cadmium is greenockite (cadmium sulfide). It is also present in small amounts in sphalerite. Almost all commercially produced cadmium is obtained as a by-product of zinc refining.

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