Ceriu

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Cerium (IPA: /ˈsiːriəm, ˈsɪəriəm/) is a chemical element in the periodic table that has the symbol Ce and atomic number 58.

Cuprins

1 Notable characteristics
2 Applications
3 History
4 Occurrence
5 Compounds
6 Isotopes
7 Precautions
8 References
9 External links

[modifică] Notable characteristics

Cerium is a silvery metallic element, belonging to the lanthanide group. It is used in some rare-earth alloys. It resembles iron in color and luster, but is soft, and both malleable and ductile. It tarnishes readily in the air. Only europium is more reactive than cerium among rare earth elements. Alkali solutions and dilute and concentrated acids attack the metal rapidly. The pure metal is likely to ignite if scratched with a knife. Cerium oxidizes slowly in cold water and rapidly in hot water.

Although cerium belongs to chemical elements group called rare earth metals, it is not rare at all. Cerium is available in relatively large quantities (68 ppm in Earth’s crust); in fact it is more common than lead.

Cerium in the +3 oxidation state is referred to as cerous, while the metal in the +4 oxidation state is called ceric.

Cerium(IV) salts are orange red or yellowish, whereas cerium(III) salts are usually white.

[modifică] Applications

Uses of cerium:

In metallurgy:
- Cerium is used in making aluminium alloys.
- Adding cerium to cast irons opposes graphitization and produces a malleable iron.
- In steels, cerium degasifies and can help reduce sulfides and oxides.
- Cerium is used in stainless steel as a precipitation hardening agent.
- 3 to 4% cerium added to magnesium alloys, along with 0.2 to 0.6% zirconium, helps refine the grain and give sound casting of complex shapes. It also adds heat resistance to magnesium castings.
- Cerium is used in alloys that are used to make permanent magnets.
- Cerium is used as an alloying element in tungsten electrodes for gas tungsten arc welding.
- Cerium is a major component of ferrocerium, also known as "lighter flint". Although modern alloys of this type generally use Mischmetal rather than purified cerium, it still is the most prevalent constituent.
- Cerium is used in carbon-arc lighting, especially in the motion picture industry.
Cerium(IV) oxide
- The oxide is used in incandescent gas mantles, such as the Welsbach mantle, where it was combined with Thorium, Lanthanum, Magnesium or Yttrium oxides .
- The oxide is emerging as a hydrocarbon catalyst in self cleaning ovens, incorporated into oven walls.
- Cerium(IV) oxide has largely replaced Rouge in the glass industry as a polishing abrasive.
- Cerium(IV) oxide is finding use as a petroleum cracking catalyst in petroleum refining.
- In glass, cerium(IV) oxide allows for selective absorption of ultraviolet light.
Cerium(IV) sulfate is used extensively as a volumetric oxidizing agent in quantitative analysis.
Cerium compounds are used in the manufacture of glass, both as a component and as a decolorizer.
Cerium compounds are used for the coloring of enamel.
Cerium(III) and cerium(IV) compounds such as cerium(III) chloride have uses as catalysts in organic synthesis.

[modifică] History

Cerium was discovered in Sweden by Jöns Jakob Berzelius and Wilhelm von Hisinger, and independently in Germany by Martin Heinrich Klaproth, both in 1803. Cerium was so named by Berzelius after the asteroid Format:Dp, discovered two years earlier (1801).

[modifică] Occurrence

Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight. It is found in a number of minerals including allanite (also known as orthite)—(Ca, Ce, La, Y)₂(Al, Fe)₃(SiO₄)₃(OH), monazite (Ce, La, Th, Nd, Y)PO₄, bastnasite(Ce, La, Y)CO₃F, hydroxylbastnasite (Ce, La, Nd)CO₃(OH, F), rhabdophane (Ce, La, Nd)PO₄-H₂O, zircon (ZrSiO₄), and synchysite Ca(Ce, La, Nd, Y)(CO₃)₂F. Monazite and bastnasite are presently the two most important sources of cerium.

Cerium is most often prepared via an ion exchange process that uses monazite sands as its cerium source.

Large deposits of monazite, allanite, and bastnasite will supply cerium, thorium, and other rare-earth metals for many years to come.

[modifică] Compounds

Cerium(IV)-sulfate

Cerium has two common oxidation states, +3 and +4. The most common compound of cerium is cerium(IV) oxide (CeO₂), which is used as "jeweller's rouge" as well as in the walls of some self-cleaning ovens. Two common oxidising agents used in titrations are ammonium cerium(IV) sulfate (ceric ammonium sulfate, (NH₄)₂Ce(SO₄)₃) and ammonium cerium(IV) nitrate (ceric ammonium nitrate or CAN, (NH₄)₂Ce(NO₃)₆). Cerium also forms a chloride, CeCl₃ or cerium(III) chloride, used to facilitate reactions at carbonyl groups in organic chemistry. Other compounds include cerium(III) carbonate (Ce₂(CO₃)₃), cerium(III) fluoride (CeF₃), cerium(III) oxide (Ce₂O₃), as well as cerium(IV) sulfate (ceric sulfate, Ce(SO₄)₂) and cerium(III) triflate (Ce(OSO₂CF₃)₃).

[modifică] Isotopes

Naturally occurring cerium is composed of 3 stable isotopes and 1 radioactive isotope; ¹³⁶Ce, ¹³⁸Ce, ¹⁴⁰Ce, and ¹⁴²Ce with ¹⁴⁰Ce being the most abundant (88.48% natural abundance). 27 radioisotopes have been characterized with the most {abundant and/or stable} being ¹⁴²Ce with a half-life of greater than 5×10¹⁶ years, ¹⁴⁴Ce with a half-life of 284.893 days, ¹³⁹Ce with a half-life of 137.640 days, and ¹⁴¹Ce with a half-life of 32.501 days. All of the remaining radioactive isotopes have half-lives that are less than 4 days and the majority of these have half-lives that are less than 10 minutes. This element also has 2 meta states.

The isotopes of cerium range in atomic weight from 123 u (¹²³Ce) to 152 u (¹⁵²Ce).

[modifică] Precautions

Cerium, like all rare earth metals, is of low to moderate toxicity. Cerium is a strong reducing agent and ignites spontaneously in air at 65 to 80 °C. Fumes from cerium fires are toxic. Water should not be used to stop cerium fires, as cerium reacts with water to produce hydrogen gas. Workers exposed to cerium have experienced itching, sensitivity to heat, and skin lesions. Animals injected with large doses of cerium have died due to cardiovascular collapse.

Cerium(IV) oxide is a powerful oxidizing agent at high temperatures and will react with combustible organic materials. While cerium is not radioactive, the impure commercial grade may contain traces of thorium, which is radioactive. Cerium serves no known biological function.