Платина

Описание

Platinum was discovered in Colombia, South America by Ulloa in 1735 and six years later in 1741 by Wood. The metal was isolated from native platinum by Delisle in 1775 and produced in malleable form by Chabaneau in 1786. Wollaston in 1803 developed a method of obtaining pure malleable platinum from crude platinum by extraction with aqua regia. The process led to the discovery of two other platinum group metals, palladium and rhodium, that were found in the aqua regia extract after platinum precipitated. Platinum derived its name from platina originating from the Spanish word plata for silver, because it was thought to be a trivial unwanted material associated with gold in gold mines of Central America.
Platinum occurs in nature as a bright-white cubic crystalline solid with metallic luster associated with other noble metals of its group. Platinum also occurs as the mineral sperrylite, PtAs2, found as tin-white brittle cubic crystals containing 52−57% platinum in certain nickel-bearing deposits. Some other minerals of platinum are cooperite PtS (Pt 80-86%); and braggite(Pt, Pd, Ni)S (Pt 58-60%). The abundance of platinum in the earth’s crust is estimated to be 0.005 mg/kg.

Химические свойства

Platinum is a soft, ductile, malleable, silverwhite metal. It is found in the metallic form and as the arsenide, sperrylite. It forms complex soluble salts, such as Na2PtCl6. It also forms halides. Metallic platinum is insoluble in water. Platinum(IV) chloride is red-brown crystals or powder.

Физические свойства

Platinum is classed by tradition and commercial usefulness as a precious metal that is soft,dense, dull, and silvery-white in color, and it is both malleable and ductile and can be formedinto many shapes. Platinum is considered part of the “precious” metals group that includesgold, silver, iridium, and palladium. It is noncorrosive at room temperature and is not solublein any acid except aqua regia. It does not oxidize in air, which is the reason that it is foundin its elemental metallic form in nature. Its melting point is 1,772°C, its boiling point is3,827°C, and its density is 195.09g/cm³.

Изотопы

There are a total of 43 isotopes for platinum. Five of these are stable, andanother has such a long half-life that it is considered practically stable (Pt-190 with ahalf-life of 6.5×10⁺¹¹ years). Pt-190 contributes just 0.014% to the proportion of platinumfound on Earth. The stable isotopes and their contributions to platinum’s existenceon Earth are as follows: Pt-192 = 0.782%, Pt-194 = 32.967%, Pt-195 = 33.832%, Pt-196 = 25.242%, and Pt-198 = 7.163%. All the other isotopes are radioactive and areproduced artificially. They have half-lives ranging from a few microseconds to minutes tohours, and one has a half-life of 50 years (Pt-193).

Происхождение имени

The name “platinum” is derived from the Spanish word platina, which means “silver.”

Вхождение

Platinum is the 75th most abundant element and, unlike many elements, is found in itspure elemental form in nature, as are deposits of silver and gold. Platinum is widely distributedover the Earth and is mined mainly in the Ural Mountains in Russia and in South Africa,Alaska, the western United States, Columbia in South America, and Ontario in Canada.When found in the mineral sperrylite (PtAs₂), it is dissolved with aqua regia to form a precipitatecalled “sponge” that is then converted into platinum metal. It is also recovered as aby-product of nickel mining, mainly in Ontario, Canada.

Характеристики

Platinum is the main metal in the platinum group, which consists of metals in both period5 and period 6. They are ruthenium (Ru), rhodium (Ro), and palladium (Pd) in period 5 andosmium (Os), iridium (Ir), and platinum (Pt) in period 6. All six of these metals share someof the same physical and chemical properties. Also, the other metals in the group are usuallyfound in platinum ore deposits.
Platinum can absorb great quantities of hydrogen gas, which makes it useful as a catalystin industry to speed up chemical reactions.

Использование

manufacture of apparatus for laboratory and industrial use, thermocouples, platinum resistance thermometers, acidproof containers, electrodes, etc. In dentistry; jewelry; electroplating. As oxidation catalyst in manufacture of acetic acid, nitric acid from ammonia, manufacture of sulfuric acid; control of automotive emissions.

Методы производства

Platinum is obtained mainly from copper and nickel ores, and platinum alloys and by recovery from the catalyst and other waste. The main stages of platinum production include extraction of the precious metal concentrate from the ore followed by separation through a complex refining process, during which the concentrate is dissolved in aqua regia, and the platinum is precipitated in the form of ammonium(IV) hexachloroplatinate. The precipitate is then calcinated at 600–700 °C to give platinum sponge, which is then hardened by melting at high temperatures, such as in the electric arc. The resultant gray platinum sponge contains 99.95–99.9% pure metal. Another method of platinum production involves its reduction to metal from the aqueous platinum salts by zinc, magnesium, iron, or aluminum. Similar procedures are used to recover platinum from the catalytic converters and other waste. The hexachloroplatinic(IV) acid produced by treating platinum with aqua regia is an important chemical platinum compound used to obtain various platinum salts.

Определение

A silvery-white malleable ductile transition metal. It occurs naturally in Australia and Canada, either free or in association with other platinum metals. It is resistant to oxidation and is not attacked by acids (except aqua regia) or alkalis. Platinum is used as a catalyst for ammonia oxidation (to make nitric acid) and in catalytic converters. It is also used in jewelry. Symbol: Pt; m.p. 1772°C; b.p. 3830 ± 100°C; r.d. 21.45 (20°C); p.n. 78; r.a.m. 195.08.

Общее описание

Silvery, whitish-gray, malleable, ductile metal. Mp: 1772°C; bp: 2187°C. Density: 21.45 g cm-3 at room conditions (very dense). Also shipped as a finely divided powder (Platinum black), as a sponge, and as particles deposited on a supporting material such as alumina. Has strong catalytic activity in these forms; finely divided Platinum can be dangerous to handle in the vicinity of other chemicals on this account. Used Platinum catalysts are particularly dangerous and can be explosive.

Профиль реактивности

Massive Platinum (lump, ingot, etc.) is generally inert. Dissolves readily in aqua regia (mixture of concentrated hydrochloride and concentrated nitric acids). Reacts rapidly with molten alkali metal oxides and peroxides. Reacts with F2 and Cl2 at red heat. Absorbs large volumes of hydrogen when hot. Catalyzes the exothermic oxidation of ammonia by air. Finely divided Platinum is incompatible with aluminum, acetone, arsenic, ethane, hydrazine, hydrogen peroxide, lithium, phosphorus, selenium, tellurium and many fluorides. Explosion can occur upon contact with hydrogen peroxide. Platinum black, sponge and supported catalysts have strong catalytic activity; can be dangerous to handle in the vicinity of other chemicals on this account. Used Platinum catalysts are particularly dangerous and can cause explosions. Ethanol or methanol can ignite on contact with a Platinum-black catalyst. (Urben 1794).

Опасность

Fine platinum powder may explode if near an open flame. Because platinum is rather inertin its elemental metallic form, it is not poisonous to humans, but some of its compounds,particularly its soluble salts, are toxic if inhaled or ingested.

Угроза здоровью

Exposure to the complex salts of platinum, especially ammonium hexachloroplatinate and ammonium tetrachloroplatinate, but not elemental platinum, may cause skin sensitization and a progressive allergic reaction that may lead to pronounced asthmatic symptoms.
The signs and symptoms of hypersensitivity include urticaria, contact dermatitis of the skin, and respiratory disorders ranging from sneezing, shortness of breath, and cyanosis to severe asthma. The latency period from the first contact with platinum to the occurrence of the first symptoms varies from a few weeks to several years.

Профиль безопасности

Questionable carcinogen with experimental tumorigenic data by implant route. Finely divided platinum is a powerful catalyst and can be dangerous to handle. Used catalysts are especially dangerous and may be explosive. May undergo hazardous reactions with aluminum, acetone, arsenic, carbon + methanol, nitrosyl chloride, dioxygen difluoride, ethanol, hydrazine, hydrogen + air, hydrogen peroxide, lithium, methyl hydroperoxide, ozonides, peroxpmonosulfuric acid, phosphorus, selenium, tellurium, vanadium dichloride + water. See also PLATINUM COMPOUNDS.

Возможный контакт

Platinum and its alloys have high electrical conductivity and excellent catalytic properties. They are used in relays, contacts and tubes in electronic equipment, in spark plug electrodes for aircraft; and windings in high-temperature electrical furnaces. Platinum alloys are used for standards for weight, length, and temperature measurement. Platinum and platinum catalysts, for example, hexachloroplatinic acid and H2PtCl6, are widely used in the chemical industry in persulfuric, nitric, and sulfuric acid production, in the synthesis of organic compounds and vitamins, and for producing higher octane gasoline. They are coming into use in catalyst systems for control of exhaust pollutants from automobiles. They are used in the equipment for handling molten glass and manufacturing fibrous glass; in laboratory, medical, and dental apparatus; in electroplating; in photography; in jewelry; and in X-ray fluorescent screens. Because platinum complexes are used as antitumor agents, the potential for carcinogenic activity is present; tests to clarify this aspect should be conducted. While low levels of emissions of platinum particulate have been observed from some catalyst-equipped automobiles, the major potential source of Pt is from the disposal of spent catalysts.

Экологическая судьба

Inhalation of industrial platinum compounds may be a problem. The general population is exposed to platinum by the dermal route, especially from jewelry. The oral route is not significant because the absorption is very poor.
Platinum can enter the environment through automobile emissions from the platinum-containing catalytic converter.Relatively high levels of platinum can be found along congested roadways. A number of chemotherapeutic agents contain platinum and thus their disposal can lead to environmental contamination. In industrialized regions, relatively high concentrations can be found in waterway sediments. Organic matter binds to the metal. In soil, mobility depends on pH, redox potential, and chloride concentration. Platinum will likely only mobilize under highly acidic conditions or in soil water with high chloride content. Some platinum (IV) complexes, in the presence of platinum (II), may undergo methylation by microorganisms.

Перевозки

UN3089 Metal powders, flammable, n.o.s., Hazard Class: 4.1; Labels: 4.1-Flammable solid.

Несовместимости

Dust or powder may form explosive mixture with air. Platinum metal is incompatible with aluminum; acetone, arsenic, ethane, hydrazine, hydrogen peroxide; lithium, phosphorus, selenium, tellurium, various fluorides.

Утилизация отходов

Catalyst disposal is expected to be the largest contributor of Pt to the environment. The value of the metal would help to offset the cost of reclaiming the Pt from discarded catalysts. If direct vehicular emissions of Pt are found to be significant, particulate taps, which are available at reasonable cost, may provide a technological solution. In any event, recovery and recycling is the preferred technique for both health and economic reasons. Details of platinum recovery and recycling from plating wastes, platinum metal refinery effluents; spent catalysts and precious metals scrap have been published.