Cadmium

Cadmium is a chemical elementin the periodic tablethat has the symbol Cd and atomic number48. A relatively rare, soft, bluish-white, toxic transition metal, cadmium occurs with zincores and is used largely in batteries.

Production

Cadmium is a common impurity in zinc, and it is most often isolated during the production of zinc. Zinc sulfide ores are roasted in the presense of oxygen converting the zinc sulfide to the oxide. Zinc metal is produced either by smelting the oxide with carbon or by electrolysis in sulfuric acid. Cadmium is isolated from the zinc metal by vacuum distillation if the zinc is smelted, or cadmium sulfate is precipitated out of the electrolysis solution. (ref 1)

Notable characteristics

Cadmium is a soft, malleable, ductile, bluish-white bivalent metalwhich can be easily cut with a knife. It is similar in many respects to zincbut lends itself to more complex compounds.

The most common oxidation stateof cadmium is +2, though rare examples of +1 can be found.

Applications

About three-fourths of cadmium is used in batteries(especially Ni-Cdbatteries) and most of the remaining one-fourth is used mainly for pigments, coatings and plating, and as stabilizers for plastics. Other uses;

• Used in some of the lowest melting alloys.
• Due to a low coefficient of frictionand very good fatigue resistance, it is used in bearing alloys.
• 6% of cadmium finds use in electroplating.
• Many kinds of soldercontain this metal.
• As a barrier to control nuclear fission.
• Compounds containing cadmium are used in black and white televisionphosphors and also in the blue and green phosphors for color television picture tubes.
• Cadmium forms various salts, with cadmium sulfidebeing the most common. This sulfide is used as a yellow pigment. Cadmium selenidecan be used as red pigment, commonly called cadmium red.
• Used in some semiconductorssuch as cadmium sulfide, cadmium selenide, and cadmium telluride, which can be used for lightdetection or solar cells. HgCdTeis sensitive to infrared.
• Some cadmium compounds are employed in PVCas stabilizers.
• Used in the first neutrinodetector.
• Used to block voltage-dependent calcium channels from fluxing calcium ions in molecular biology.

History

Cadmium (Latincadmia, Greekkadmeia meaning "calamine") was discoveredin Germanyin 1817by Friedrich Strohmeyer. Strohmeyer found the new element within an impurity in zinc carbonate(calamine) and for 100 years Germany remained the only important producer of the metal. The metal was named after the Latin word for calamine since the metal was found in this zinc compound. Strohmeyer noted that some impure samples of calamine changed color when heated but pure calamine did not.

Even though cadmium and its compounds are highly toxic, the British Pharmaceutical Codexfrom 1907 states that cadmium iodidewas used as a medicineto treat "enlarged joints, scrofulous glands, and chilblains".

In 1927, the International Conference on Weights and Measuresredefined the meterin terms of a red cadmium spectral line (1m = 1,553,164.13 wavelengths). This definition has since been changed (see krypton).

Occurrence

Image:CadmiumMetalUSGOV.jpg Cadmium-containing ores are rare and when found they occur in small quantities. Greenockite(CdS), the only cadmium mineralof importance, is nearly always associated with sphalerite(ZnS). Consequently, cadmium is produced mainly as a byproduct from mining, smelting, and refining sulfideores of zinc, and to a lesser degree, leadand copper. Small amounts of cadmium, about 10% of consumption, are produced from secondary sources, mainly from dust generated by recycling ironand steelscrap. Production in the United Statesbegan in 1907but it was not until after World War Ithat cadmium came into wide use.

see also category:Cadmium minerals

Isotopes

Naturally occurring cadmium is composed of 8 isotopes. For two of them, natural radioactivitywas observed, and other three are predicted to be radioactivebut their decays were never observed, due to extremely long half-lifetimes. The two natural radioactive isotopes are ¹¹³Cd (beta decay, half-lifeis 7.7 X 10¹⁵ years) and ¹¹⁶Cd (two-neutrino double beta decay, half-lifeis 2.9 X 10¹⁹ years). Other three ones are ¹⁰⁶Cd, ¹⁰⁸Cd (double electron capture), and ¹¹⁴Cd (double beta decay); only lower limits on their half-lifetimes have been set. At last, three isotopes - ¹¹⁰Cd, ¹¹¹Cd, and ¹¹²Cd - are absolutely stable. Among the isotopes absent in the natural cadmium, the most long-lived are ¹⁰⁹Cd with a half-life of 462.6 days, and ¹¹⁵Cd with a half-life of 53.46 hours. All of the remaining radioactiveisotopes have half-lifes that are less than 2.5 hours and the majority of these have half-lifes that are less than 5 minutes. This element also has 8 known meta stateswith the most stable being ^113mCd (t_½ 14.1 years), ^115mCd (t_½ 44.6 days) and ^117mCd (t_½ 3.36 hours).

The known isotopes of cadmium range in atomic weightfrom 96.935 u(⁹⁷Cd) to 129.934 amu (¹³⁸Cd). The primary decay modebefore the second most abundant stable isotope, ¹¹²Cd, is electron captureand the primary modes after are beta emissionand electron capture. The primary decay productbefore ¹¹²Cd is element 47 (silver) and the primary product after is element 49 (indium).

Precautions

Cadmium has no constructive purpose in the humanbody. This element and solutions of its compounds are extremely toxiceven in low concentrations, and will bioaccumulatein organisms and ecosystems. One possible reason for its toxicity is that it interferes with the action of zinc-containing enzymes. Zinc is an important element in biological systems, but cadmium, although similar to zinc chemically in many ways, apparently does not substitute or "stand in" for it well at all. Cadmium may also interfere with biological processes containing magnesiumand calciumin a similar fashion.

Inhaling cadmium laden dust quickly leads to respiratory tractand kidneyproblems which can be fatal (often from renal failure). Ingestion of any significant amount of cadmium causes immediate poisoning and damage to the liverand the kidneys. Compounds containing cadmium are also carcinogenic. Cadmium poisoning is the cause of the itai-itai disease, which literally means "pain pain" in Japanese. In addition to kidney damage, patients suffered from osteoporosisand osteomalacia.

While working with cadmium it is important to do so under a fume hoodto protect against dangerous fumes. Silversolder, for example, which contains cadmium, should be handled with care. Serious toxicity problems have resulted from long-term exposure to cadmium plating baths.

See also: Cadmium Poisoning

References

1. http://www.webelements.com, see 'cadmium'

• Los Alamos National Laboratory – Cadmium

External links

• IARC Monograph "Cadmium and Cadmium Compounds"
• National Pollutant Inventory - Cadmium and compounds
• WebElements.com – Cadmium

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It uses material from the http://en.wikipedia.org/wiki/Cadmium Wikipedia article Cadmium.