Cerium
MELTING POINT:
798°C
BOILING POINT
: 3,257°C
DENSITY
: 6.773 g/cm
3
MOST COMMON IONS
: Ce
3+
, Ce
4+
In 1751 the Swedish chemist Axel F. Cronstedt found, near Bastnäs, Sweden, a mineral that was eventually named cerita (its name related to the planetoid Ceres). Independently, Martin Klaproth, Jöns J. Berzelius, and Wilhelm Hisinger, working with cerita, each isolated a product, ceria (in 1803), from which Carl G. Mosander obtained three different substances, as oxides: cerium, lanthanum, and a mixture of oxides known as didymia.
Cerium is the most abundant member of the lanthanide , or rare earth, elements. It has two stable valence states, Ce 3+ (cerous) and Ce 4+ (ceric). It is found as a trace element in several minerals, but only two, bastnasite, LnFCO 3 , and monazite, (Ln, Th)PO 4 (where Ln = a lanthanide element, such as lanthanum, praseodymium, neodymium, or cerium), which are approximately 30 percent and 22 to 25 percent cerium, respectively, are the principal sources of this element.
To obtain cerium, a bastnasite concentrate is treated with sulfuric acid or hydrochloric acid, yielding the hydrated sulfate or the chloride of cerium. The sulfate is converted to the hydroxide or the carbonate, and then to the fluoride. A monozite concentrate is digested in an autoclave with an excess of caustic soda at 150°C (302°F).
LnTh(PO 4 ) + 3NaOH → Na 3 PO 4 + Ln(OH) 3 + Th(OH) 4
Ln(OH) 3 can be converted to the corresponding hydrated chloride or fluoride by treatment with the appropriate acids. These halide compounds are carefully dehydrated to yield the lanthanide anhydrous salts. Misch metal is a mixture of lanthanide elements (it is approximately 50% cerium), and is obtained via the electrolysis of the fused mixed lanthanide chlorides.
Of Ce(III) compounds: the nitrate, chloride, and bromide are water-soluble; the carbonate, fluoride, hydroxide, oxalate, and phosphate are water-insoluble; the acetate and sulfate are sparingly soluble. These compounds are usually prepared from a reactive precursor such as carbonate, basic carbonate, or oxide using the appropriate acids. They are practically colorless.
Ce(IV) compounds include soluble salts that are orange or red in color. Ceric sulfate crystals are orange, as are some complexes in which Ce(IV) is present in the anion . Diammonium hexanitrocerate (NH 4 ) 2 [Ce(NO 3 ) 6 ], for example, is strongly red.
There are practically no medical applications for cerium. Several metallurgical, glass, ceramic, and others applications exist, including:
- lighter flints (misch metal)
- glass polishing (CeO 2 powders)
- glass decolorization (the oxidation of Fe 2+ to Fe 3+ by Ce 4+ )
- UV absorption (Ce 4+ is opaque to near ultraviolet)
- enamels (CeO 2 )
- luminescent applications
- catalytic (cracking crude oil to gasoline)
- vehicle emission control (CeO 2 )
SEE ALSO Berzelius, JÖns Jakob ; Dysprosium ; Erbium ; Europium ; Gadolinium ; Holmium ; Lanthanum ; Lutetium ; Neodymium ; Praseodymium ; Promethium ; Samarium ; Terbium ; Ytterbium .
Lea B. Zinner
Geraldo Vicentini
Bibliography
Maestro, P. (1998). "From Properties to Industrial Applications." In Rare Earths , ed. R. S. Puche and P. Caro. Madrid: Editorial Complutense.
Moeller, Therald (1975). "The Chemistry of the Lanthanides." In Comprehensive Inorganic Chemistry , ed. J. C. Bailar Jr. Oxford, UK: Pergamon.