オスミウム

解説

オスミウム,元素記号 Os ，原子番号 76，原子量 190.23。天然の安定同位体には，オスミウム 192 (存在比 41.0%) のほか6核種が知られている。周期表8族，鉄族元素の1つ。 1804年 S.テナントにより発見された。白金の鉱石であるオスミリジウム中に含まれる。単体は青白色の光沢ある金属。融点 2700℃。比重 22.48で，物質中最大の比重をもつ。空気中では安定であるが，粉末は常温でも徐々に酸化される。熱濃硫酸，王水，濃硝酸と反応し溶ける。リン，硫黄の蒸気中で燃焼する。2，3，4，6，8価として多様な化合物をつくり，また錯塩をつくりやすい。イリジウムとの合金はペン先や精密ベアリングに使われ，またアンモニア合成用触媒，有機化合物の水素化触媒として使われる。

存在度

1803年イギリスのS. Tennantが白金鉱中に発見した．原子量190.23(3)．質量数184(0.02(1)％)，186(1.59(3)％)，187(1.96(2)％)，188(13.24(8)％)，189(16.15(5)％)，190(26.26(2)％)，192(40.78(19)％)の7種の安定同位体と，質量数162～197，199の放射性同位体が知られている．天然には，白金鉱中にイリジウムとの合金として産出する．矩周期型の昔の周期表では，Os，Ir，PtはⅧ族で性質は類似するため，相互分離は困難である．金属は酸化物を水素で還元すると得られる．オスミウム触媒は担体にオスミウム塩を吸着させ還元すると得られる．加工が困難で，普通は焼結して利用する．

説明

Osmium is silvery white, hard transition metals with a high melting point. osmium is one of the rarest elements on earth.

化学的特性

Osmium is a blue－white metal. It is found in platinum ores and in the naturally occurring alloy osmiridium. Osmium when heated in air or when the finely divided form is exposed to air at room temperature, oxidizes to form the tetroxide (OsO4), osmic acid. Osmium tetraoxide is a colorless, crystalline solid or pale-yellow mass. Unpleasant, acrid, chlorine-like odor. A liquid above 41°C.

物理的性質

One of the important properties of osmium is the formation of gases when the metal isexposed to air. These fumes are extremely toxic, which limits osmium’s usefulness. Osmium is ahard, tough, brittle, bluish-white metal that is difficult to use except in a powder form that oxidizesinto osmium tetroxide (OsO₄), which not only has objectionable odor but also is toxic.
Osmium has a relatively high melting point of 3,054°C and a boiling point of 5,500°C,with a density of 22.61 g/cm³.

同位体

Osmium has 41 isotopes, five of which are stable. Two are naturally radioactiveisotopes with very long half-lives. Following are the stable isotopes and their contributionto the element’s natural existence in the Earth’s crust: Os-187 = 1.6%, Os-188 =13.29%, Os-189 = 16.21%, Os-190 = 26.36%, and Os-192 = 40.93%. The remain ing percentage of the element on Earth is in the form of the two naturally radioactiveisotopes: Os-184 = 0.02% and Os-186 = 1.59%. All the other isotopes of osmium areradioactive and artificially produced in nuclear reactors and particle accelerators.

名前の由来

Its name is derived from the Greek word osme, meaning “odor” or “smell,” because of the element’s objectionable smell when it is first isolated from platinum ores using aqua regia.

天然物の起源

Osmium is the 80th most abundant element on Earth. As a metal, it is not found free innature and is considered a companion metal with iridium. It is also found mixed with platinum-and nickel-bearing ores. It is recovered by treating the concentrated residue of these oreswith aqua regia (a mixture of 75% HCl and 25% HNO). The high cost of refining osmium ismade economically feasible by also recovering marketable amounts of platinum and nickel.
Osmium occurs along with iridium in nature as the mineral iridosmine. It is found inCanada, Russia, and parts of Africa.

特性

Osmium is found in group 8 (VIII) of the periodic table and has some of the same chemical,physical, and historical characteristics as several other elements. This group of similarelements is classed as the platinum group, which includes Ru, Rh, and Pd of the second transitionseries and Os, Ir, and Pt of the third series of transition metals .

使用

Because of its hard brittle nature, the metal osmium has few uses. However, the powderedform can be sintered under high pressure and temperatures to form some useful products,despite its toxicity and malodor. Its main use is as an alloy to manufacture devices that resistwear and stand up to constant use. As an alloy, osmium loses both its foul odor and toxicity.Some of these products are ballpoint and fountain pen tips, needles for record players, andpivot points for compass needles. Osmium alloys are also used for contact points on specialswitches and other devices that require reduced frictional wear.
Another use is as a stain for animal tissues that are to be examined with a microscope toimprove the contrast of the specimen.

調製方法

Osmium is obtained in the reaction of osmium tetroxide (OsO4) reduction with carbon at red heat temperature. Osmium is also obtained from osmiridium. After separation from other metals with aqua regia, osmiridium is distilled in chlorine stream with formation of osmium tetrachloride (OsCl4), from which the metal is recovered by reduction.

定義

A transition metal that is found associated with platinum. Osmium is the most dense of all metals. It has a characteristic smell resulting from the production of osmium(VIII) oxide (osmium tetroxide, OsO₄). The metal is used in catalysts and in alloys for pen nibs, pivots, and electrical contacts. Symbol: Os; m.p. 3054°C; b.p. 5027°C; r.d. 22.59 (20°C); p.n. 76; r.a.m. 190.23.

性質

融点3054 ℃，沸点5027 ℃，第一イオン化エネルギー8.28 eV．密度22.59 g cm－3(20 ℃)で，物質中最大の密度をもつ．モース硬さ7.5．通常の酸化数は2(d6)，3(d5)であるが，1を除いて0から8の化合物が知られている．白金族元素のなかでもっとも酸化されやすく，高温ではフッ素，塩素とも反応する．一酸化炭素と反応してOs0 (CO)5からOs8(CO)23までのカルボニル化合物を生じる．熱濃硫酸，濃硝酸，次亜塩素酸ナトリウム，融解硫酸水素アルカリに可溶．融点の高いことを利用してフィラメントに，耐食性，耐摩耗性を利用してペン先に用いる．水素添加，アンモニア合成(ハーバー-ボッシュ法)にも当初は触媒として用いられた．OsO4はオレフィンの酸化触媒として使われる．

危険性

Most of the oxides of osmium are not noxious, but are toxic if inhaled or ingested. Thecompound OsO₄ is extremely poisonous. It is a powerful oxidizing agent that is soluble inwater and will produce serious burns in skin as it oxidizes the various layers of tissues.

工業用途

A platinum-group metal, osmium is noted for its high hardness, about 400 Brinell. The heaviest known metal, it has a high specific gravity, 22.65, and a high melting point, 2698 C. The boiling point is about 5468 C. Osmium has a close-packed hexagonal crystal structure, and forms solid-solution alloys with platinum, having more than double the hardening power of iridium in platinum. However, it is seldom used to replace iridium as a hardener except for fountain-pen tips where the alloy is called osmiridium.
Osmium is not affected by the common acids, and is not dissolved by aqua regia. It is practically unworkable, and its chief use is as a catalyst.
Osmium tetraoxide, a commercially available yellow solid (melting point 40 C) is used commercially as a stain for tissue in microscopy. It is poisonous and attacks the eyes. Osmium metal is catalytically active, but it is not commonly used for this purpose because of its high price. Osmium and its alloys are hard and resistant to corrosion and wear (particularly to rubbing wear). Alloyed with other platinum metals, osmium has been used in needles for record players, fountain-pen tips, and mechanical parts.

安全性プロファイル

Poison by intravenous route. An irritant to eyes and mucous membranes. The principal effects of exposure are ocular disturbances and an asthmatic condition caused by inhalation. Furthermore, it causes dermatitis and ulceration of the skin upon contact. When osmium is heated, it gives off a pungent, poisonous fume of osmium tetroxide. One case of osmium poisoning reported in the literature resulted from the inhalation of osmium, which gave rise to a capillary bronchitis and dermatitis. The tetroxide vapor has a pronounced and nauseating odor that should be taken as a warning of a possibly toxic concentration in the atmosphere, and personnel should immedately move to an area of fresh air. The metal itself is not highly toxic. Flammable in the form of dust when exposed to heat or flame. Slight explosion hazard in the form of dust when exposed to heat or flame. Violent reaction or ignition with chlorine trichloride or oxygen dfluoride. Ignites when heated to l00℃ with fluorine. Incandescent reaction in phosphorus vapor. When heated to decomposition it emits toxic fumes of 0s04. See also OSMIUM TETROXIDE

職業ばく露

Osmium may be alloyed with platinum metals, iron, cobalt, and nickel; and it forms compounds withtin and zinc. The alloy with iridium is used in the manufacture of fountain pen points, engraving tool; record player needles; electrical contacts; compass needles; fine machine bearings; and parts for watch and lock mechanisms. The metal is a catalyst in the synthesis of ammonia; and in the dehydrogenation of organic compounds. It is also used as a stain for histological examination of tissues. Osmium tetroxide is used as an oxidizing agent, catalyst, and as a fixative for tissues in electron microscopy. Other osmium compounds find use in photography. Osmium no longer is used in incandescent lights or in fingerprinting.

不和合性

Osmium tetroxide is a strong oxidizer. Reacts with combustibles and reducing materials. Reacts with hydrochloric acid to form toxic chlorine gas. Forms unstable compounds with alkalis.

廃棄物の処理

Consult with environmental regulatory agencies for guidance on acceptable disposal practices. Generators of waste containing this contaminant (≥100 kg/mo) must conform with EPA regulations governing storage, transportation, treatment, and waste disposal.