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解説
Ce．原子番号58の元素．電子配置[Xe]4f 15d16s2の周期表3族ランタノイド元素．希土類元素セリウム族の一つ．原子量140.116(1)．質量数136(0.185(2)％)，138(0.251(2)％)，140(88.450(51)％)，142(11.114(51)％)の4種の安定同位体と，質量数119～157の放射性同位体が知られている．1803年，J.J. Berzelius(ベルセリウス)，M.H. Klaproth，W. Hisingerにより発見された．元素名は1801年に発見された小惑星セレス(Ceres)にちなんで命名された．セレスは農業収穫を司るローマ神話の女神である．
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性質
セリウム,融点799 ℃，沸点3426 ℃．α，β，γ，δ(β→γ，168 ℃)の4変態があり，α，γ，δは立方最密構造，βは六方最密構造．密度α 8.24；β 6.749；γ 6.773；δ 6.70 g cm－3．標準電極電位 Ce3＋/Ce －2.34 V，Ce4＋/Ce3＋1.71 V．第一イオン化エネルギー5.539 eV．酸化数3，4．希土類元素でCeのみが安定な酸化数4の状態をもつ．Ce4＋ の電子配置は[Xe]である．空気中で容易に酸化される．熱水と反応して水素を発生する．CeⅢ塩は無色のものが多いが，CeⅣ塩は黄色～赤色．標準電極電位 CeⅢ/Ce －2.34 V，CeⅣ/CeⅢ1.71 V．
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用途
治金における合金の安定剤、溶接棒の酸化トリウムの代替品、ガラスの研磨剤、テレビ画面のガラスの下塗り剤、セラミックの乳白剤および補強剤、原油処理の触媒、自動車の排気装置における燃焼助剤、重合開始剤、塗料乾燥剤、高分子安定剤、蛍光灯のリン光体、ブラウン管、タバコのライターに使用される発火合金のミッシュメタル
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構造
セリウムの常温常圧での安定結晶構造は面心立方格子構造です。730°C以上で体心立方格子構造を取り、低温で六方最密充填構造になり、-150°C以下では再び面心立方格子構造を取っています。
セリウムの室温付近での密度は6.770g/cm3、融点での液体密度は6.55g/cm3です。電子配置は[Xe] 4f1 5d1 6s2です。
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応用
わが国におけるセリウムの最大の用途は研磨材用で，酸化セリウム(Ⅳ)CeO2(セリア)の形で液晶ガラス研磨や半導体ウェファー研磨に用いられる．このほか，CeO2は自動車排ガス浄化用の3元触媒にアルミナとともに担体・助触媒や，UVフィルターガラス(自動車用ガラス)のUV吸収用添加剤，蛍光灯・CRT用蛍光体にも用いられる．そのほか，セリウムは高温用超合金酸化防止用，アルミニウム合金用，石油精製用触媒に添加される．毒性は高くないが，「セリウム及びその化合物」は大気汚染防止法の有害大気汚染物質に指定されている．
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製法
鉱石を塩酸または硫酸で処理(バストネス石)するか，水酸化ナトリウムで処理(モナズ石)した後，ばい焼して CeⅣにして沈殿法または溶媒抽出法で分離するセリウム．
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製造
汎用研磨剤には、アメリカ産のバストネサイトを酸化・粉砕・粒度分級して使われます。水酸化物を塩酸によって抽出して、酸化セリウムのような化合物が製造されています。
金属カルシウム還元や溶融電解によって、金属セリウムを生成可能です。主にフェロセリウム (英: Ferrocerium) はアメリカで製造されており、鉄鋼添加剤として輸入しています。
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説明
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.
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化学的特性
Elemental cerium has a face-centered cubic crystal structure at 25 °C. Cerium is the most abundant element of the rare-earth group and is 28th in ranking of the naturally occurring elements in the earth’s crust. The element is a silver-gray metal which oxidizes readily at room temperature, particularly in moist air, to form the oxide CeO2, which is of a pale yellowish-green color. Above 300 °C, the element may ignite and burn with a bright red glow. Of the nineteen isotopes of cerium, only four occur in nature, 136Ce, 138Ce, 140Ce, and 142Ce. The thermal neutron-absorption cross section of the element is low. The element has a low toxicity rating.
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物理的性質
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.
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同位体
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.
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名前の由来
Named for the asteroid Ceres, which was discovered two years before
the element.
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天然物の起源
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.
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特性
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.
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使用
The compound cerium oxide (either Ce2O3 or CeO2) is used to coat the inside of ovensbecause it was discovered that food cannot stick to oven walls that are coated with ceriumoxide. Cerium compounds are used as electrodes in high-intensity lamps and film projectorsused by the motion picture industry. Cerium is also used in the manufacturing andpolishing of high-refraction lenses for cameras and telescopes and in the manufacture ofincandescent lantern mantles. It additionally acts as a chemical reagent, a misch metal, anda chemical catalyst. Cerium halides are an important component of the textile and photographicindustries, as an additive to other metals, and in automobile catalytic converters.Cerium is also used as an alloy to make special steel for jet engines, solid-state instruments,and rocket propellants.
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定義
Symbol Ce. A silvery metallicelement belonging to the lanthanoids;a.n. 58; r.a.m. 140.12; r.d.6.77 (20°C); m.p. 799°C; b.p. 3426°C.It occurs in allanite, bastnasite,cerite, and monazite. Four isotopesoccur naturally: cerium–136, –138,–140, and –142; fifteen radioisotopeshave been identified. Cerium is usedin mischmetal, a rare-earth metalcontaining 25% cerium, for use inlighter flints. The oxide is used inthe glass industry. It was discoveredby Martin Klaproth (1743–1817) in1803.
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調製方法
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
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一般的な説明
Cerium is a gray colored, ductile solid. This form of cerium is slabs, ingots or rods. When heated to high temperatures CERIUM will burn readily and may be difficult to extinguish. CERIUM is used to make signaling devices.
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空気と水の反応
Finely divided metal powder is pyrophoric [Bretherick 1979 p. 170-171]. CERIUM will react vigorously if exposed to water or moist air and will generate flammable and/or toxic fumes.
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反応プロフィール
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].
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危険性
Most of the compounds of cerium are toxic if ingested or if the fumes are inhaled. Ceriumwill ignite when heated.
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健康ハザード
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.
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火災危険
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.
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同位体
セリウムには、136Ce、138Ce、140Ce、142Ceの4種類の安定同位体と、質量数119～157の放射性同位体があります。これらの中で天然存在比が最も多いのは、88.48%の140Ceです。142Ceは2回のベータ崩壊を起こすと推測されており、半減期は5×1016年以上ですが、観測されたことはありません。
放射性同位体は27種類同定されていて、半減期が284.893日の144Ceが最も安定しています。139Ceの半減期は137.640日で、141Ceの半減期は32.501日です。それ以外の放射性同位体はすべて半減期が4日以内で、ほとんどは10分以内です。2つの核異性体も存在します。
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産出
セリウムの資源は、日本でも少量産出していますが、中国、アメリカ、インドなどの埋蔵量が多いです。鉱物では(Ce,La)(CO3)Fであるバストネサイト(英: Bastnäsite) や(Ce, La, Nd, Th)PO4であるモナザイト (英: Monazite) が主体で、それぞれ酸化セリウムが半数弱含まれています。産出量の約9割が、中国内陸部の磁鉄鉱副産物の複雑鉱石から精製されています。
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使用用途
セリウムは商業的に多くの用途で使われています。セリウムは紫外線を吸収するため、自動車のフロントガラスやUVカットのサングラスに使用可能です。
セリウムの酸化物は、の主要成分であると化学反応を起こすため、ガラスのとして使用されています。鉄との合金は、発火合金としてライター等に使われています。
セリウムは青い蛍光を発する蛍光体としても知られており、LEDやブラウン管に利用可能です。
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工業用途
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.
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安全性プロファイル
Cerium resembles
aluminum in its pharmacological action as
well as in its chemical properties. The
insoluble salts such as the oxalates are stated
to be nontoxic even in large doses. It is used
to prevent vomiting in pregnancy. The
average dose is from 0.05 to 0.5 g.
The effect on the central nervous system
of the rare-earth metals following inhalation
may preclude welding operations with these
materials to any large extent. Cerium is
stated to produce polycythemia but is
useless in the treatment of anemia owing to
its toxic effects. The salts of cerium increase
the blood coagulation rate. See also RARE EARTHS. A strong reducing agent.
Moderate fire hazard; ignites spontaneously
in air at 150-180'. Moderate explosion
hazard in the form of dust when exposed to
flame. The metal or its alloys spark with
friction. Many alloys are pyrophoric in air.
See also IRON DUST. Explosive reaction
with zinc. Very exothermic reaction with
antimony or bismuth. Ignites when heated
in atmospheres of CO2 + N2, Cl2, or Br2.
Violent reaction when heated with
phosphorus (4OO℃), silicon (1400℃).
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環境運命予測
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.
