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History

Cadmium (Latin cadmia, Greek kadmeia meaning "calamine") was discovered in Germany in 1817 by 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 Codex from 1907 states that cadmium iodide was used as a medicine to treat "enlarged joints, scrofulous glands, and chilblains".

In 1927, the International Conference on Weights and Measures redefined the meter in terms of a red cadmium spectral line (1m = 1,553,164.13 wavelengths). This definition has since been changed.

Characteristics

Cadmium is a soft, malleable, ductile, bluish-white bivalent metal which can be easily cut with a knife.  It is similar in many respects to zinc but reacts to form more complex compounds.

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

Occurrence

Cadmium-containing ores are rare and when found they occur in small quantities.   Greenockite (CdS), the only cadmium mineral of importance, is nearly always associated with sphalerite (ZnS).  Consequently, cadmium is produced mainly as a byproduct from mining, smelting, and refining sulfide ores of zinc, and to a lesser degree, lead and copper.  Small amounts of cadmium, about 10% of consumption, are produced from secondary sources, mainly from dust generated by recycling iron and steel scrap.  Production in the United States began in 1907 but it was not until after World War I that cadmium came into wide use.

A role of cadmium in biology has been recently discovered.  A cadmium-dependent carbonic anhydrase has been found in marine diatoms.  Cadmium does the same job as zinc in other anhydrases, but the diatoms live in environments with very low zinc concentrations, thus biology has taken cadmium rather than zinc, and made it work.   The discovery was made using x-ray absorption fluoresence spectroscopy (XAFS), and cadmium was characterised by noting the energy of the x-rays which were absorbed.

Applications

About three-quarters of cadmium is used in batteries (especially Ni-Cd batteries) and most of the remaining quarter is used mainly for pigment, coatings and plating, and as stabilizers for platics.  Other uses include:

• Used in some of the lowest melting alloys. 
• Due to a low coefficient of friction and very good fatigue resistance, it is used in bearing alloys.
• 6% of cadmium finds use in electroplating.
• Many kinds of solder contain this metal.
• As a barrier to control nuclear fission. 
• Compounds containing cadmium are used in black and white television phosphors and also in the blue and green phosphors for color television picture tubes.
• Cadmium forms various salts, with cadium sulfide being the most common.  This sulfide is used as a yellow pigment.  Cadmium selenide can be used as red pigment, commonly called cadmium red.  To painters who work with the pigment, cadmium yellows, oranges and reds are the most potent colours to use . In fact, during production these colors are significantly toned down before they are ground with oils and binders, or blended into watercolours, gouaches, casesin, acrylics and other paint and pigment formulations.  These pigments are toxic and it is recommended to use a barrier cream on your hands to prevent absorption through the skin when working with them.   There is no such thing as cadmium blue, green or violet.
• Used in some semiconductors such as cadmium sulfide, cadmium selenide, and cadmium telluride, which can be used for light detection or solar cells.  HgCdTe is sensitive to infrared.
• Some cadmium compounds are employed in PVC as stabilizers.
• Used in the first neutrino detector.
• Used to block voltage-dependent calcium channels from fluxing calcium ions in molecular biology.

Hydrated cadmium sulfate (3CdSO₄·5H₂O), one of cadmium's compounds, is used in a device called a Weston cell, a type of battery that produces a precise voltage used to calibrate medical and laboratory equipment.  Cadmium sulfide (CdS), another cadmium compound, is a yellow powder that is used as a pigment.  Other cadmium compounds are used in the phosphors of black and white television sets and in the blue and green phosphors in color television sets.

Extraction

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 presence 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.

Compounds

Isotopes

Naturally occurring cadmium is composed of 8 isotopes.  For two of them, natural radioactivity was observed, and three others are predicted to be radioactive but their decays were never observed, due to extremely long half-life times. The two natural radioactive isotopes are ¹¹³Cd (beta decay, half-life is 7.7 X 10¹⁵ years) and ¹¹⁶Cd (two-neutrino double beta decay, half-life is 2.9 X 10¹⁹ years).  The other three are ¹⁰⁶Cd, ¹⁰⁸Cd (double electron capture), and ¹¹⁴Cd (double beta decay); only lower limits on their half-life times have been set.  At least 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 radioactive isotopes have half-lives that are less than 2.5 hours and the majority of these have half-lives that are less than 5 minutes.  This element also has 8 known meta states with 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 mass from 94.950 u (⁹⁵Cd) to 131.946 u (¹³²Cd).  The primary decay mode before the second most abundant stable isotope, ¹¹²Cd, is electron capture and the primary modes after are beta emission and electron capture.  The primary decay product before ¹¹²Cd is element 47 (silver) and the primary product after is element 49 (indium).

Precautions

While working with cadmium it is important to do so under a fume hood or with the use of an appropriate respirator to protect against dangerous fumes.  Solder, for example, which may contain cadmium, should be handled with care.

Cadmium is an occupational hazard associated with industrial processes such as metal plating and the production of nickel-cadmium batteries, pigments, plastics and other synthetics.  The primary route of exposure in industrial settings is inhalation.   Inhalation of cadmium-containing fumes can result initially in metal fume fever but may progress to chemical pneumonitis, pulmonary edema, and death.

Cadmium is also a potential environmental hazard.  Human exposures to environmental cadmium are primarily the result of the burning of fossil fuels and municipal wastes.  However, there have been notable instances of toxicity as the result of long-term exposure to cadmium in contaminated food and water.  In the decades following World War II, Japanese mining operations contaminated the Jinzu River with cadmium and traces of other toxic metals.  Consequently, cadmium accumulated in the rice crops growing along the riverbanks downstream of the mines.  The local agricultural communities consuming the contaminated rice developed Itai-itai disease and renal abnormalities, including proteinuria and glucosuria.

Cadmium and several cadmium-containing compounds are known carcinogens and can induce many types of cancer.

The mechanism of cadmium toxicity has not been established. 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 magnesium and calcium in a similar fashion.