|
|
| General |
| Name, Symbol,
Number |
cerium, Ce, 58 |
| Chemical series |
lanthanides |
| Group, Period,
Block |
n/a, 6,
f |
| Appearance |
silvery white
 |
| Standard atomic weight |
140.116(1)
g·mol−1 |
| Electron configuration |
[Xe] 4f1 5d1 6s2 |
| Electrons per shell |
2, 8, 18, 19, 9, 2 |
| Physical properties |
| Phase |
solid |
| Density (near r.t.) |
6.770 g·cm−3 |
| Liquid density at m.p. |
6.55 g·cm−3 |
| Melting point |
1068 K
(795 °C, 1463 °F) |
| Boiling point |
3716 K
(3443 °C, 6229 °F) |
| Heat of fusion |
5.46 kJ·mol−1 |
| Heat of vaporization |
398 kJ·mol−1 |
| Heat capacity |
(25 °C) 26.94 J·mol−1·K−1 |
Vapor pressure
| P(Pa) |
1 |
10 |
100 |
1 k |
10 k |
100 k |
| at T(K) |
1992 |
2194 |
2442 |
2754 |
3159 |
3705 |
|
| Atomic properties |
| Crystal structure |
cubic face centered |
| Oxidation states |
3, 4
(mildly basic oxide) |
| Electronegativity |
1.12 (scale Pauling) |
Ionization energies
(more) |
1st: 534.4 kJ·mol−1 |
| 2nd: 1050 kJ·mol−1 |
| 3rd: 1949 kJ·mol−1 |
| Atomic radius |
185 pm |
| Miscellaneous |
| Magnetic ordering |
no data |
| Electrical resistivity |
(r.t.) (β, poly) 828 nΩ·m |
| Thermal conductivity |
(300 K) 11.3 W·m−1·K−1 |
| Thermal expansion |
(r.t.) (γ, poly)
6.3 µm/(m·K) |
| Speed of sound (thin rod) |
(20 °C) 2100 m/s |
| Young's modulus |
(γ form) 33.6 GPa |
| Shear modulus |
(γ form) 13.5 GPa |
| Bulk modulus |
(γ form) 21.5 GPa |
| Poisson ratio |
(γ form) 0.24 |
| Mohs hardness |
2.5 |
| Vickers hardness |
270 MPa |
| Brinell hardness |
412 MPa |
| CAS registry number |
7440-45-1 |
| Selected isotopes |
|
|
| References |
Cerium (IPA: /ˈsiːriəm, ˈsɪəriəm/) is a chemical element with the symbol Ce and
atomic number 58.
Notable characteristics
Cerium is a silvery metal, belonging to the lanthanide group. It resembles iron in color
and luster, but is soft, and both malleable and ductile. Cerium has the longest liquid range of any non-radioactive element:
2648°C (795°C to 3443°C). (Thorium has a longer liquid range.)
Although cerium belongs to chemical elements group called rare earth metals, it is
in fact more common than lead. Cerium is available in relatively large quantities (68 ppm in
Earth’s crust). It is used in some rare-earth alloys.
Among rare earth elements only europium is more reactive. It tarnishes readily in the air.
Alkali solutions and dilute and concentrated acids attack the metal rapidly. Cerium oxidizes slowly in cold water and rapidly in
hot water. The pure metal can ignite if scratched.
Cerium(IV) (ceric) salts are orange red or yellowish, whereas cerium(III) (cerous) salts are usually white or colorless. Both
oxidation states absorb ultraviolet light strongly. Cerium(III) can be used to make glasses that are colorless, yet absorb
ultraviolet light almost completely. Cerium can be readily detected in rare earth mixtures by a very sensitive qualitative test:
addition of ammonia and hydrogen peroxide to an aqueous solution of lanthanides produces a characteristic dark brown color if
cerium is present.
Applications
Uses of cerium:
- In metallurgy:
- 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.
- Cerium(IV) additives to diesel fuel cause that to burn more cleanly, with less resulting air-pollution.
- 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.
- Ceric ammonium nitrate is a useful one-electron oxidant in organic chemistry, used to oxidatively etch electronic components,
and as a primary standard for 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.
History
Cerium was discovered in Bastnäs in Sweden by
Jöns Jakob Berzelius and Wilhelm
Hisinger, and independently in Germany by Martin Heinrich Klaproth, both in 1803. Cerium was so named by
Berzelius after the dwarf planet Ceres, discovered two years earlier
(1801). As originally isolated, cerium was in the form of its oxide, and was named "ceria", a term
that is still used. The metal itself was too electopositive to be isolated by then-current smelting technology, a characteristic
of "earths" in general. But the development of electrochemistry by Humphry Davy was only five years into the future, and then the earths were well on their way to yielding up
their contained metals. Ceria, as isolated in 1803, contained all of the lanthanides present in the cerite ore from Bastnaes,
Sweden, and thus only contained about 45% of what is now known to be pure ceria. It was not until Mosander succeeded in removing
lanthana and "didymia" in the late 1830s that ceria was obtained pure. As an historical aside: Wilhelm Hisinger was a wealthy
mine owner and amateur scientist, and sponsor of Berzelius. He owned or controlled the mine at Bastnaes, and had been trying for
years to find out the composition of the abundant heavy gangue rock (the "Tungstein of Bastnaes") now known as cerite that he had
in his mine. Mosander and his family lived for many years in the same house as Berzelius, and was undoubtedly persuaded by the
latter to investigate ceria further. When the rare earths were first discovered, since they were strong bases like the oxides of
calcium or magnesium, they were thought to be divalent. Thus, "ceric" cerium was thought to be trivalent, and the oxidation state
ratio was therefore thought to be 1.5. Berzelius was extremely annoyed, to keep on getting the ratio 1.33. He was after all one
of the finest analytical chemists in Europe. But he was a better analyst than he thought, since 1.33 was the correct answer!
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.
See also Category:Lanthanide minerals
Compounds
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).
The two oxidation states of cerium differ enormously in basicity: cerium(III) is a strong base, comparable to the other
trivalent lanthanides, but cerium(IV) is weak. This difference has always allowed cerium to be by far the most readily isolated
and purified of all the lanthanides, otherwise a notoriously difficult group of elements to separate. A wide range of procedures
have been devised over the years to exploit the difference. Among the better ones:
- Peaching the mixed hydroxides with dilute nitric acid: the trivalent lanthanides dissolve in cerium-free condition, and
tetravalent cerium remains in the insoluble reside as a concentrate to be further purified by other means. A variation on this
uses hydrochloric acid and the calcined oxides from bastnaesite, but the separation is less sharp.
- Precipitating cerium from a nitrate or chloride solution using potassium permanganate and sodium carbonate in a 1:4 molar
ratio.
- Boiling rare earth nitrate solutions with potassium bromate and marble chips.
Using the classical methods of rare earth separation, there was a considerable advantage to a strategy of removing cerium from
the mixture at the beginning. Cerium typically comprised 45% of the cerite or monazite rare earths, and removing it early greatly
reduced the bulk of what needed to be further processed (or the cost of reagents to be associated with such processing). However,
not all cerium purification methods relied on basicity. Ceric ammonium nitrate [ammonium hexanitratocerate(IV)] crystallization
from nitric acid was one purification method. Cerium(IV) nitrate (hexanitratoceric acid) was more readily extractable into
certain solvents (e.g. tri-n-butyl phosphate) than the trivalent lanthanides. However, modern practice in China seems to be to do
purification of cerium by counter-current solvent extraction, in its trivalent form, just like the other lanthanides.
Cerium(IV) is a strong oxidant under acidic conditions, but stable under alkaline conditions, when it is cerium(III) that
becomes a strong reductant, easily oxidized by molecular dioxygen (or air). This ease of oxidation under alkaline conditions
leads to the occasional geochemical parting of the ways between cerium and the trivalent light lanthanides under supergene
weathering conditions, leading variously to the "negative cerium anomaly" or to the formation of the mineral cerianite.
Air-oxidation of alkaline cerium(III) is the most economical way to get to cerium(IV), which can then be handled in acid
solution.
See also Category:Cerium compounds
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).
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.
References
External links
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