|
|
| Appearance |
silvery gray metallic
 |
| General properties |
| Name, symbol, number |
cadmium, Cd, 48 |
| Element category |
transition metal |
| Category notes |
Alternatively considered a post-transition metal |
| Group, period, block |
12, 5, d |
| Standard atomic weight |
112.411 g·mol−1 |
| Electron configuration |
[Kr] 5s2 4d10 |
| Electrons per shell |
2, 8, 18, 18, 2 (Image) |
| Physical properties |
| Phase |
solid |
| Density (near r.t.) |
8.65 g·cm−3 |
| Liquid density at m.p. |
7.996 g·cm−3 |
| Melting point |
594.22 K, 321.07 °C, 609.93 °F |
| Boiling point |
1040 K, 767 °C, 1413 °F |
| Heat of fusion |
6.21 kJ·mol−1 |
| Heat of vaporization |
99.87 kJ·mol−1 |
| Specific heat capacity |
(25 °C) 26.020 J·mol−1·K−1 |
| Vapor pressure |
| P/Pa |
1 |
10 |
100 |
1 k |
10 k |
100 k |
| at T/K |
530 |
583 |
654 |
745 |
867 |
1040 |
|
| Atomic properties |
| Oxidation states |
2, 1 (mildly basic oxide) |
| Electronegativity |
1.69 (Pauling scale) |
| Ionization energies |
1st: 867.8 kJ·mol−1 |
| 2nd: 1631.4 kJ·mol−1 |
| 3rd: 3616 kJ·mol−1 |
| Atomic radius |
151 pm |
| Covalent radius |
144±9 pm |
| Van der Waals radius |
158 pm |
| Miscellanea |
| Crystal structure |
hexagonal |
| Magnetic ordering |
diamagnetic[1] |
| Electrical resistivity |
(22 °C) 72.7 nΩ·m |
| Thermal conductivity |
(300 K) 96.6 W·m−1·K−1 |
| Thermal expansion |
(25 °C) 30.8 µm·m−1·K−1 |
| Speed of sound (thin rod) |
(20 °C) 2310 m/s |
| Young's modulus |
50 GPa |
| Shear modulus |
19 GPa |
| Bulk modulus |
42 GPa |
| Poisson ratio |
0.30 |
| Mohs hardness |
2.0 |
| Brinell hardness |
203 MPa |
| CAS registry number |
7440-43-9 |
| Most stable isotopes |
| Main article: Isotopes of cadmium |
|
|
|
|
Cadmium (pronounced /ˈkædmiəm/, KAD-mee-əm) is a chemical element with the symbol Cd and atomic number 48. The soft, bluish-white transition metal is chemically similar to the two other metals in group 12, zinc and mercury. Similar to zinc it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low melting point for a transition metal. Cadmium is a relatively abundant element. Cadmium was discovered in 1817 by Friedrich Stromeyer as an impurity in zinc carbonate.
Cadmium occurs as a minor component in most zinc ores and therefore is a by-product of zinc production. Cadmium was for a long time used as pigment and for corrosion resistant plating on steel. Cadmium compounds were used to stabilize plastic. With the exception of its use in nickel-cadmium batteries, the use of cadmium is generally decreasing in all other applications. This decrease is due to the high toxicity and carcinogenicity of cadmium and the associated health and environmental concerns. Although cadmium is toxic, one enzyme, a carbonic anhydrase with a cadmium as reactive centre has been discovered.
Characteristics
Cadmium is a soft, malleable, ductile, toxic, bluish-white bivalent metal. It is similar in many respects to zinc but forms more complex compounds.
Chemical
The most common oxidation state of cadmium is +2, though rare examples of +1 can be found. Cadmium burns in air to form brown amorphous cadmium oxide (CdO). The crystalline form of the same compound is dark red and changes colour when heated, similar to zinc oxide. Hydrochloric acid, sulfuric acid and nitric acid dissolve cadmium by forming cadmium chloride (CdCl2) cadmium sulfate (CdSO4) or cadmium nitrate (Cd(NO3)2). The oxidation state +1 can be reached by dissolving cadmium in a mixture of cadmium chloride and aluminium chloride, forming the Cd22+ cation, which is similar to the Hg22+ cation in mercury(I) chloride.[2]
- Cd + CdCl2 + 2 AlCl3 → Cd2[AlCl4]2
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 decay is not observed, due to extremely long half-life times. The two natural radioactive isotopes are 113Cd (beta decay, half-life is 7.7 × 1015 years) and 116Cd (two-neutrino double beta decay, half-life is 2.9 × 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 stable. Among the isotopes absent in 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 (t½ 14.1 years), 115mCd (t½ 44.6 days), and 117mCd (t½ 3.36 hours).
The cadmium-113 total cross section clearly showing the cadmium cutoff.
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. The primary decay product before 112Cd is element 47 (silver), and the primary product after is element 49 (indium).
One isotope of cadmium, 113Cd, absorbs neutrons with very high probability if they have an energy below the cadmium cut-off and transmits them readily otherwise. The cadmium cut-off is about 0.5 eV.[3] Neutrons with energy below the cutoff are deemed slow neutrons, distinguishing them from intermediate and fast neutrons.
Applications
Batteries
About three-quarters of all the cadmium used is in batteries, predominantly in rechargeable nickel-cadmium batteries. Nickel-cadmium cells have a nominal cell potential of 1.2 V. The cell consists of a positive nickel hydroxide electrode and a negative cadmium electrode plate separated by an alkaline electrolyte (potassium hydroxide). More recent nickel-metal hydride batteries reduce the use of Ni-Cd batteries. The European Union banned the use of cadmium in electronics in 2004 with several exceptions but reduced the allowed content of cadmium in electronics to 0.002%.[4]
Other uses
Most of the remaining quarter is used mainly for cadmium pigments, coatings and plating, and as stabilizers for plastics. Other uses include:
Train painted with cadmium yellow
- In some of the lowest-melting alloys, such as Wood's metal.
- In bearing alloys, due to a low coefficient of friction and very good fatigue resistance.[5]
- In electroplating (6% cadmium).[5] Cadmium electroplating is widely used in aircraft industry due to the excellent corrosion resistance of cadmium-plated steel components. Cadmium provides cathodic protection to low-alloyed steels, since it is positioned lower in the galvanic series. The coating is usually passivated by chromate salts. A significant limitation of cadmium plating is hydrogen embrittlement of high-strength steels caused by the electroplating process. Therefore, steel parts heat-treated to tensile strength above 1300 MPa (200 ksi) should be coated by an alternative method (such as special low-embrittlement cadmium electroplating processes or physical vapor deposition).
- In many kinds of solder.[5]
- As a barrier to control neutrons in nuclear fission.[5]
- The pressurized water reactor designed by Westinghouse Electric Company uses an alloy consisting of 80% silver, 15% indium, and 5% cadmium.[5]
- Cadmium oxide in black and white television phosphors and in the blue and green phosphors for colour television picture tubes.[6]
- Cadmium sulfide (CdS) as a photoconductive surface coating for photocopier drums.
- In paint pigments, cadmium forms various salts, with CdS 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 colours are significantly toned down before they are ground with oils and binders, or blended into watercolours, gouaches, acrylics, and other paint and pigment formulations. These pigments are toxic, and it is recommended to use a barrier cream on the hands to prevent absorption through the skin when working with them.
- 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.
- In PVC as stabilizers.
- In molecular biology, it is used to block voltage-dependent calcium channels from fluxing calcium ions.
- See also Category:Cadmium compounds.
History
Cadmium (Latin cadmia, Greek καδμεία meaning "calamine", a cadmium-bearing mixture of minerals, which was named after the Greek mythological character, Κάδμος Cadmus) was discovered in Germany in 1817 by Friedrich Strohmeyer.[7] Strohmeyer found the new element as 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 colour when heated but pure calamine did not. He was persistent in studying these results and eventually isolated cadmium metal by roasting and reduction of the sulfide.
Even though cadmium and its compounds are highly toxic, the British Pharmaceutical Codex from 1907 states that cadmium iodide was used as a medication to treat "enlarged joints, scrofulous glands,[8] and chilblains".
In 1927, the International Conference on Weights and Measures redefined the metre in terms of a red cadmium spectral line (1m = 1,553,164.13 wavelengths).[9] This definition has since been changed (see krypton).
After the industrial scale production of cadmium started in the 1930s and 1940s the major application was the coating of steel and copper alloys to prevent corrosion. In 1944 62% and in 1956 59% of the cadmium in the United States was used for this purpose.[10] The second application where red and yellow pigments based on sulfides and selenides of cadmium. In 1956 24% of the cadmium used within the United States was used for this purpose.[10] The stabilizing effect of cadmium containing chemicals on plastics led to a increased use of those compounds in the 1970s and 1980s. The use of Cadmium in all applications mentioned above declined drastically due to environmental and health regulations from 1980 on. In 2006 only 7% of the cadmium is used for plating and coating and only 10% is used for pigments. The decrease in the consumption in other applications was made up by a growing demand of cadmium in nickel-cadmium batteries, which accounted for 81% of the cadmium consumption in the United States in 2006. The overall consumption of cadmium has decreased by more than 10 times from the 1970s till 2009.[11]
Occurrence
Cadmium-containing ores are rare and are found to occur in small quantities. However, traces do naturally occur in phosphate, and have been shown to transmit in food through fertilizer application.[12] Greenockite (CdS), the only cadmium mineral of importance, is nearly always associated with sphalerite (ZnS). As a consequence, 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.
One place where metallic cadmium can be found is the Vilyuy River basin in Siberia.[13]
- See also Category:Cadmium minerals.
In 2001, China was the top producer of cadmium with almost one-sixth world share closely followed by South Korea and Japan, reports the British Geological Survey.
Cadmium is a common impurity in zinc ores, and it is most often isolated during the production of zinc. Some zinc ores concentrates from sulfidic zinc ores contain up to 1,4% of cadmium.[14] In 1970s the output of cadmium was 6.5 pounds per ton of zinc.[14] 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.[15]
Biological role
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 fluorescence spectroscopy (XAFS), and cadmium was characterized by noting the energy of the X-rays that were absorbed.[16][17]
Image of the violet light from a
helium cadmium metal vapor
laser. The highly
monochromatic color arises from the 441.563 nm transition
line of cadmium.
Toxicity
Cadmium poisoning 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.[18]
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.[19] 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 leading up to World War II, Japanese mining operations contaminated the Jinzu River with cadmium and traces of other toxic metals. As a consequence, 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.[20] Cadmium is one of six substances banned by the European Union's Restriction on Hazardous Substances (RoHS) directive, which bans certain hazardous substances in electronics.
Cadmium and several cadmium-containing compounds are known carcinogens and can induce many types of cancer.[21]
Research has found that cadmium toxicity may be carried into the body by zinc binding proteins; in particular, proteins that contain zinc finger protein structures. Zinc and cadmium are in the same group on the periodic table, contain the same common oxidation state (+2), and when ionized are almost the same size. Due to these similarities, cadmium can replace zinc in many biological systems, in particular, systems that contain softer ligands such as sulfur. Cadmium can bind up to ten times more strongly than zinc in certain biological systems, and is notoriously difficult to remove. In addition, cadmium can replace magnesium and calcium in certain biological systems, although these replacements are rare.
Tobacco smoking is the most important single source of cadmium exposure in the general population. It has been estimated that about 10% of the cadmium content of a cigarette is inhaled through smoking. The absorption of cadmium from the lungs is much more effective than that from the gut, and as much as 50% of the cadmium inhaled via cigarette smoke may be absorbed.[22]
On average, smokers have 4-5 times higher blood cadmium concentrations and 2-3 times higher kidney cadmium concentrations than non-smokers. Despite the high cadmium content in cigarette smoke, there seems to be little exposure to cadmium from passive smoking. No significant effect on blood cadmium concentrations could be detected in children exposed to environmental tobacco smoke.[23]
References
- ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81th edition, CRC press.
- ^ Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils; (1985) (in German). Lehrbuch der Anorganischen Chemie (91–100 ed.). Walter de Gruyter. pp. 1056–1057. ISBN 3-11-007511-3.
- ^ Knoll, G.F. (1999). Radiation Detection and Measurement, 3rd edition. Wiley. ISBN 978-0471073383. p505
- ^ "Battery collection;recycling, nature protected". http://www.europarl.europa.eu/sides/getDoc.do?pubRef=-//EP//TEXT+IM-PRESS+20060628BRI09328+FULL-TEXT+DOC+XML+V0//EN. Retrieved 2008-11-04.
- ^ a b c d e Scoullos, Michael J.; Vonkeman, Gerrit H.; Thornton, Iain; Makuch, Zen (2001). Mercury, Cadmium, Lead: Handbook for Sustainable Heavy Metals Policy and Regulation. Springer. ISBN 9781402002243. http://books.google.de/books?id=9yzN-QGag_8C.
- ^ Lee, Ching-Hwa; Hs, Chi-Shiung (2002). "Recycling of Scrap Cathode Ray Tubes". Environmental Science and Technology 36 (1): 69–75. doi:10.1021/es010517q S0013-936X(01)00517-X.
- ^ Hermann; Strohmeyer F. (1818). "Noch ein schreiben über das neue Metall(Another letter about the new metal)". Annalen der Physik 59: 113.
- ^ Dunglison, Robley (1866). Medical Lexicon: A Dictionary of Medical Science. Henry C. Lea. pp. 159. http://books.google.com/books?id=PmohO5jV2YsC.
- ^ Burdun, G. D. (1958). "On the new determination of the meter" (pdf). Measurement Techniques 1 (3): 259–264. doi:10.1007/BF00974680. http://www.springerlink.com/content/tk70442064438147/fulltext.pdf?page=1.
- ^ a b Lansche, Arnold M.. "Minerals Yearbook 1956: Cadmium". United States Geological Survey. http://digicoll.library.wisc.edu/cgi-bin/EcoNatRes/EcoNatRes-idx?type=turn&entity=EcoNatRes.MinYB1956v1.p0289&id=EcoNatRes.MinYB1956v1&isize=XL&q1=cadmium. Retrieved 2008-04-21.
- ^ "USGS Commodity Report cadmium". United States Geological Survey. http://minerals.usgs.gov/minerals/pubs/commodity/cadmium/. Retrieved 2009-08-08.
- ^ Jiao, You; Grant, Cynthia A.; Bailey, Loraine D.. "Effects of phosphorus and zinc fertilizer on cadmium uptake and distribution in flax and durum wheat". Journal of the Science of Food and Agriculture 84 (8): 777–785. doi:10.1002/jsfa.1648.
- ^ Fleischer, Michael; Cabri, Louis J.; Chao, Georg Y.; Papst, Adolf (1980). "New Mineral Names". American Mineralogist 65: 1065–1070. http://www.minsocam.org/ammin/AM65/AM65_1065.pdf.
- ^ a b National Research Council (U.S.), Panel on Cadmium, Committee on Technical Aspects of Critical and Strategic Material (1969). Trends in Usage of Cadmium: Report. National Research Council, National Academy of Sciences-National Academy of Engineering. pp. 1–3. http://books.google.de/books?id=okArAAAAYAAJ.
- ^ Cadmium at WebElements.com
- ^ Lane, Todd W.; Morel, François M. M. (2000). "A biological function for cadmium in marine diatoms". Proc. Natl. Acad. Sci. 97 (9): 4627–4631. doi:10.1073/pnas.090091397. http://www.pnas.org/content/97/9/4627.full.pdf+html.
- ^ Lane, Todd W. ,; Saito, Mak A.; George, Graham N.; Pickering, Ingrid J.; Prince, Roger C.; Morel, François M. M. (2005). "A cadmium enzyme from a marine diatom". Nature 435 (42). doi:10.1038/435042a. http://www.whoi.edu/cms/files/msaito/2005/5/LaneSaitoMorel_CdCA_Nature2005_2944.pdf.
- ^ Hayes, Andrew Wallace (2007). Principles and Methods of Toxicology. Philadelphia: CRC Press. http://books.google.de/books?id=vgHXTId8rnYC&dq.
- ^ "EPA summary on cadmium". U.S. Environmental Protection Agency. http://www.epa.gov/ttn/atw/hlthef/cadmium.html. Retrieved 2008-04-21.
- ^ Nogawa, Koji; Kobayashi, Etsuko; Okubo, Yasushiand; Suwazono, Yasushi (2004). "Environmental cadmium exposure, adverse effects, and preventative measures in Japan". Biometals 17 (5): 581–587. doi:10.1023/B:BIOM.0000045742.81440.9c. http://www.springerlink.com/content/n0773057mw738u05/.
- ^ "11th Report on Carcinogens". National Toxicology Program. http://ntp.niehs.nih.gov/index.cfm?objectid=32BA9724-F1F6-975E-7FCE50709CB4C932. Retrieved 2008-04-21.
- ^ Friberg, L. (1983). "Cadmium". Annual Review of Public Health 4: 367–367. doi:10.1146/annurev.pu.04.050183.002055.
- ^ Jarup, L.; Berglund, M.; Elinder, C. G.; Nordberg, G.; Vahter, M (1998). "Health effects of cadmium exposure—a review of the literature and a risk estimate.". Scandinavian Journal of Work, Environment and Health 24: 11–51.
See also
External links
