Cerium
In 1803, Jöns Jakob Berzelius and Wilhelm Hisinger stood in a laboratory in Bastnäs, Sweden. They had just isolated a new substance from the heavy gangue rock known as cerite. This mineral sat in Hisinger's mine, which he owned and controlled for years. The two men named their discovery after Ceres, an asteroid found only two years prior. At that time, astronomers considered Ceres to be a planet rather than a dwarf world. Martin Heinrich Klaproth in Germany made the same discovery independently later that year. Carl Gustaf Mosander would not separate pure cerium(III) oxide until 1839. Wilhelm Hisinger was a wealthy mine-owner who sponsored Berzelius throughout this work. Mosander lived with Berzelius for many years and was persuaded to investigate the material further. By 1875, William Francis Hillebrand became the first person to isolate the metal itself.
A volume change of about 10% occurs when cerium is subjected to high pressures or low temperatures. The energy of the 4f electron is nearly the same as that of the outer 5d and 6s electrons. Only a small amount of energy is required to change the relative occupancy of these electronic levels. In its high pressure phase, alpha-cerium, the 4f electrons are delocalized and itinerate. This contrasts sharply with the localized 4f electrons found in the gamma-cerium phase at lower pressures. Four allotropic forms exist at standard pressure and carry labels from alpha to delta. The stable form below 726 degrees Celsius is gamma-cerium with an fcc crystal structure. A double hexagonal close-packed form called beta-cerium exists approximately from room temperature down to minus 150 degrees Celsius. Cooling below minus 160 degrees Celsius starts the formation of alpha-cerium but only from remaining gamma-cerium. Transformation temperatures are subject to substantial hysteresis and values quoted here are approximate.
Naturally occurring cerium consists of four isotopes: Ce-136, Ce-138, Ce-140, and Ce-142. These make up 0.19%, 0.25%, 88.45%, and 11.11% respectively. All are observationally stable though light isotopes are theoretically expected to undergo double electron capture. The heaviest isotope 142Ce is expected to undergo double beta decay or alpha decay. Thus 140Ce remains the only theoretically stable isotope. None of these decay modes have yet been observed in experiments. Current experimental limits for their half-lives reach about 1 times 10 to the power of 17 years. All other cerium isotopes are synthetic and radioactive. The most stable among them is 144Ce with a half-life of 284.9 days. Isotopes between 140Ce and 144Ce inclusive occur as fission products of uranium. Some isotopes of neodymium can alpha decay or are predicted to decay to isotopes of cerium.
Cerium exists in two main oxidation states, Ce(III) and Ce(IV). This pair dominates several aspects of the element's chemistry. Ceric ammonium nitrate serves as the most common compound encountered in laboratories. Six nitrate ligands bind as bidentate ligands to form a 12-coordinate complex. Aqueous cerium(IV) ions appear orange-yellow due to ligand-to-metal charge transfer. In the Belousov, Zhabotinsky reaction, cerium oscillates between +4 and +3 oxidation states. Halides include all four trihalides CeX where X equals fluorine, chlorine, bromine, or iodine. Unlike most lanthanides, cerium forms a tetrafluoride which appears as a white solid. It also forms a bronze-colored diiodide that possesses metallic properties. Many nonstoichiometric chalcogenides exist alongside trivalent compounds like CeS, CeSe, and CeTe. The monochalcogenides conduct electricity and would better be formulated as CeZ with delocalized electrons.
Bastnäsite is usually lacking in thorium and heavy lanthanides beyond samarium and europium. First, the ore is purified using dilute hydrochloric acid to remove calcium carbonate impurities. The material is then roasted in air to oxidize it into lanthanide oxides. While most lanthanides become sesquioxides, cerium becomes dioxide CeO2. This compound is insoluble in water and can be leached out with 0.5 molar hydrochloric acid. Monazite requires more involved procedures due to its magnetic properties. Repeated electromagnetic separation isolates the mineral from other components. Hot concentrated sulfuric acid produces water-soluble sulfates of rare earths. Thorium precipitates out as hydroxide and is removed before further processing. Care must be taken when handling residues containing radium-228 which emits strong gamma radiation. Production of extremely pure cerium commenced at Ames Laboratory in mid-1944 and continued until August 1945.
Carl Auer von Welsbach invented gas mantles in 1885 after experimenting with magnesium and lanthanum mixtures. Six years later he discovered that mixing thorium oxide with cerium dioxide produced bright white light. Cerium dioxide acts as a catalyst for the combustion of thorium oxide. Mischmetal contains 50% cerium and 25% lanthanum plus other lanthanides. Iron is added to form ferrocerium used widely for lighter flints. The industrial application of ceria focuses on chemical-mechanical planarization polishing techniques. CeO2 decolorizes glass by converting green-tinted ferrous impurities into nearly colorless ferric oxides. Cerium(III)-doped yttrium aluminium garnet emits yellow light between 530 and 540 nanometers. This phosphor behaves as a scintillator and converts blue light from LEDs into white light. Catalytic converters use cerium oxide to increase efficiency of oxidation during low-oxygen exhaust conditions.
Methylacidiphilum fumariolicum bacteria living in volcanic mudpots utilize lanthanum, cerium, praseodymium, and neodymium equally. Cerium does not accumulate in the food chain to any appreciable extent. Bones are primarily calcium phosphate so cerium can accumulate there in small amounts. Large doses of cerium nitrate lead to methemoglobinemia though it treats third-degree burns effectively. A strong reducing agent ignites spontaneously in air at temperatures between 65 and 80 degrees Celsius. Fumes from cerium fires are toxic to human lungs. Workers exposed to cerium have experienced itching, sensitivity to heat, and skin lesions. Animals injected with large doses die due to cardiovascular collapse. Cerium damages cell membranes in aquatic organisms causing environmental contamination. In Russia its occupational exposure limit is set at 5 milligrams per cubic meter.
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Common questions
Who discovered cerium and when was it isolated?
Jöns Jakob Berzelius and Wilhelm Hisinger isolated a new substance from the mineral cerite in 1803. Martin Heinrich Klaproth made the same discovery independently later that year.
What are the four allotropic forms of cerium and their temperature ranges?
Four allotropic forms exist at standard pressure labeled alpha to delta. Gamma-cerium is stable below 726 degrees Celsius, beta-cerium exists from room temperature down to minus 150 degrees Celsius, and alpha-cerium forms below minus 160 degrees Celsius.
Which isotopes of cerium occur naturally and what are their abundances?
Naturally occurring cerium consists of four isotopes: Ce-136, Ce-138, Ce-140, and Ce-142. These make up 0.19%, 0.25%, 88.45%, and 11.11% respectively.
How does cerium dioxide function in industrial applications like glass polishing?
Cerium dioxide acts as a catalyst for the combustion of thorium oxide in gas mantles invented by Carl Auer von Welsbach in 1885. It decolorizes glass by converting green-tinted ferrous impurities into nearly colorless ferric oxides and serves chemical-mechanical planarization polishing techniques.
What are the health risks associated with exposure to cerium compounds?
Large doses of cerium nitrate lead to methemoglobinemia though it treats third-degree burns effectively. Fumes from cerium fires are toxic to human lungs and workers exposed to cerium have experienced itching, sensitivity to heat, and skin lesions.