Cadmium

History

(L. cadmia; Gr. kadmeia - ancient name for calamine, zinc carbonate) Discovered by Stromeyer in 1817 from an impurity in zinc carbonate. 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.

Properties

Cadmium is a soft, malleable, ductile, bluish-white bivalent metal which can be easily cut with a knife. It has a low coefficient of friction and very good fatigue resistance. Cadmium and solutions of its compounds are very toxic. It is similar in many respects to zinc but lends itself to more complex compounds. The most common oxidation state of cadmium is +2, though rare examples of +1 can be found.

Sources

Cadmium-containing ores are rare and when found they occur in small quantities. Greenockite, the only cadmium mineral of importance, is nearly always associated with sphalerite. 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.

Uses

Cadmium is a component of some of the lowest melting alloys; it is used in bearing alloys with low coefficients of friction and great resistance to fatigue; it is used extensively in electroplating, which accounts for about 60% of its use. It is also used in many types of solder, for standard E.M.F. cells, for Ni-Cd batteries, and as a barrier to control nuclear fission. Cadmium compounds are used in black and white television phosphors and in blue and green phosphors for color TV tubes. It forms a number of salts, of which the sulfate is most common; the sulfide is used as a yellow pigment. 

Isotopes

Naturally occurring cadmium is composed of 6 stable isotopes. 27 radioisotopes have been characterized with the most stable being Cd-113, Cd-109, and Cd-115. All of the remaining radioactive isotopes 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 meta states with the most stable being Cdm-113, Cdm-115 and Cdm-117.

Hazards

Cadmium is one of the few elements that has no constructive purpose in the human body. This element and solutions of its compounds are extremely toxic even in low concentrations, and will bioaccumulate in organisms and ecosystems.

Inhaling cadmium laden dust quickly leads to respiratory tract and kidney problems which can be fatal. Ingestion of any significant amount of cadmium causes immediate poisoning and damage to the liver and 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 osteoporosis and osteomalacia.

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

Exposure to cadmium dust should not exceed 0.01 mg/m3 (8-hour time-weighted average, 40-hour week). The ceiling concentration (maximum), for a period of 15 min, should not exceed 0.14 mg/m3. Cadmium oxide fume exposure (8-hour, 40-hour week) should not exceed 0.05 mg/m3, and the maximum concentration should not exceed 0.05 mg/m3. These values are presently being restudied and recommendations have been made to reduce the exposure.