Chemical Properties
grey metal ingots (in mineral oil)
Uses
In metallurgy as stabilizers in alloys and as an alternative to thorium oxide in welding electrodes. In glass as polishing agent, decolorizer to stabilize impurities, to render glass opaque to near uv radiation, to resist discoloration from strong light or high energy electron bombardment (as in television screens). In ceramics as an opacifying and strengthening agent. Catalysts to impart high cracking activity for crude oil processing, in automotive exhaust control devices, as combustion additive, polymerization initiator, paint drier, polymer stabilizer. As phosphor in fluorescent lamps, cathode ray tubes and thorium dioxide gas mantles.
General Description
Cerium is a gray colored, ductile solid. This form of cerium is slabs, ingots or rods. When heated to high temperatures CERIUM, SLABS, INGOTS OR RODS(7440-45-1) will burn readily and may be difficult to extinguish. CERIUM, SLABS, INGOTS OR RODS(7440-45-1) is used to make signaling devices.
Reactivity Profile
CERIUM is a strong reducing agent. Resembles aluminum in its chemical properties. [Lewis]. Reactivity is enhanced by a state of high physical subdivision, as in TURNINGS OR GRITTY POWDER. Attacked by dilute and concentrated mineral acids and alkalis with the generation of flammable gases. Readily oxidized by moist air at room temperature. Reacts with zinc with explosively violence. Gives very exothermic reactions with antimony or bismuth. Reacts violently with phosphorus at 400-500°C [Mellor 8, Supp. 3:347 1971].
Air & Water Reactions
Finely divided metal powder is pyrophoric [Bretherick 1979 p. 170-171]. This material will react vigorously if exposed to water or moist air and will generate flammable and/or toxic fumes.
Hazard
May ignite on heating to 300F (148.9C).
Strong reducing agent.
Health Hazard
Oxides from metallic fires are a severe health hazard. Inhalation or contact with substance or decomposition products may cause severe injury or death. Fire may produce irritating, corrosive and/or toxic gases. Runoff from fire control or dilution water may cause pollution.
Fire Hazard
May react violently or explosively on contact with water. Some are transported in flammable liquids. May be ignited by friction, heat, sparks or flames. Some of these materials will burn with intense heat. Dusts or fumes may form explosive mixtures in air. Containers may explode when heated. May re-ignite after fire is extinguished.
Description
Cerium is a rare earth metal and the most abundant member of
the lanthanide series discovered by Jons J. Berzelius and W. von
Hisinger in 1803 in Sweden. Berzelius and Hisinger discovered
the new element in a rare reddish-brown mineral now known
as cerite, a cerium–lanthanide silicate. Although they could not
isolate the pure metal, they found that cerium had two
oxidation states: trivalent state (Ce3+, cerous, usually orangered)
and the tetravalent state (Ce4+, ceric, usually colorless).
Cerium is the only material known to have a solid-state critical
point.
Physical properties
Cerium is a grayish/iron-colored, very reactive metallic element that is attacked by bothacids and alkalies. Pure cerium will ignite if scratched with a knife, but it can be combinedsafely with many other elements and materials. It is relatively soft and both malleable andductile.
Its melting point is 798°C, its boiling point is 3,443°C, and its density is 6.770g/cm3.
Isotopes
There are 44 isotopes of cerium, four of which are considered stable. Ce-140accounts for most of the cerium (88.450%) found in the Earth’s crust, and Ce-138makes up just 0.251% of the element in the crust. There are two isotopes with half-liveslong enough to be considered stable: Ce-136 (0.185%), with a half-life of 0.7×10+14years, and Ce-142 (11.14%), with a half-life of 5×10+16 years. All the other isotopes areradioactive with half-lives ranging from 150 nanoseconds to 137.641 days. All are madeartificially.
Origin of Name
Named for the asteroid Ceres, which was discovered two years before
the element.
Occurrence
Cerium is the 25th most abundant element on Earth. It is also the most abundant rareearthmetal in the lanthanide series. Its major ores are monazite and bastnasite. Cerium isfound in the Earth’s crust in 46 ppm, which is about 0.0046% of the Earth’s crust. Ceriumis mixed with other elements in its ores, making it difficult to find, isolate, and identify. Itsexistence was unknown until about 1803.
Monazite sands contain most of the rare-earths. The sands of the beaches of Florida andparts of California contain monazite. Monazite is also found in South Africa, India, andBrazil. Bastnasite is found in southern California and New Mexico.
Characteristics
As a pure metal, cerium is unstable and will decompose rapidly in moist air. It also decomposesin hot water to form hydrogen. Its oxide compounds and halides are stable and have anumber of uses.
Cerium is separated from other rare-earth elements by an ion-exchange process in whichit reacts with fluoride. This compound is then reduced with calcium metal (3Ca +2CeF3 →2Ce + 3CaF3). Cerium can also be produced by the electrolysis of molten cerium salts. Themetal ion collects at the cathode, and the chlorine or fluorine gases of the salt compound atthe anode.
Production Methods
Cerium is obtained from its ores by chemical processing and separation. The process involves separation of cerium from other rare-earth metals present in the ore. The ore is crushed, ground, and treated with acid. The extract solution is buffered to pH 3-4 and the element is precipitated selectively as Ce4+ salt. Cerium also may be separated from other metals by an ionexchange process.
Also, the metal may be obtained by high temperature reduction of cerium(III) chloride with calcium:
2CeCl3 + 3Ca → 2Ce + 3CaCl2
Flammability and Explosibility
Flammable
reaction suitability
reagent type: catalyst
core: cerium
Industrial uses
A chemical element, cerium (Ce) is the mostabundant metallic element of the rare earthgroup in the periodic table. Cerium occursmixed with other rare earths in many minerals,particularly monazite and blastnasite, and isfound among the products of the fission of uranium,thorium, plutonium.
Ceric oxide, CeO2, is the oxide usuallyobtained when cerium salts of volatile acids areheated. CeO2 is an almost white powder that isinsoluble in most acids, although it can be dissolvedin H2SO4 or other acids when a reducingagent is present. The metal is an iron-gray colorand it oxidizes readily in air, forming a graycrust of oxide. Misch metal, an alloy of cerium,is used in the manufacture of lighter flints.Cerium has the interesting property that, at verylow temperatures or when subjected to highpressures, it exhibits a face-centered cubicform, which is diamagnetic and 18% denserthan the common form.
Environmental Fate
Cerium resembles aluminum in its biologic and chemical
properties. Cerium and cerium compounds have low to
moderate toxicity unless the associated anions are toxic.
Intratracheally administered nanoparticles tend to accumulate
in liver and cause damage there.
Toxicity evaluation
Although cerium is a rare earth element, it is relatively
abundant in the earth’s crust. It makes up about 0.0046% of
the Earth’s crust by weight and ranks 25th in occurrence at an
average distribution of 20–60 ppm. Cerium is a malleable,
soft, ductile, iron-gray metal, slightly harder than lead. It is
very reactive and readily tarnishes in the air. Cerium oxidizes
slowly in cold water and rapidly in hot water. It dissolves in
acids. Cerium can burn when heated or scratched with
a knife.
Cerium is not expected to exist in elemental form in the
environment since it is a reactive metal. Cerium is dumped in
the environment in many different places, mainly by petrolproducing
industries. It can also enter the environment when
household equipment is thrown away. Cerium compounds
exist solely in particulate form if release into air and not
expected to volatilize. Water-soluble cerium compounds
usually have a pKa of 8.5, which indicates that the hydrated
Ce3+ ion will remain in solution at environmental pHs of 4–9.
The ion is expected to hydrolyze and polymerize at environmental pH and may precipitate out of solution. Thus,
cerium will gradually accumulate in soils and water, which
eventually leads to increasing concentrations in humans,
animals, and soil particles.
Toxics Screening Level
The initial threshold screening level (ITSL) for CERIUM is 6 μg/m3 based on a 24-hour averaging time.
