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Platinum

[plat-n-uhm, plat-nuhm]
Platinum is a chemical element with the atomic symbol Pt and an atomic number of 78. It is in group 10 of the Periodic Table of Elements. A heavy, malleable, ductile, precious, gray-white transition metal, platinum is resistant to corrosion and occurs in some nickel and copper ores along with some native deposits. Platinum is used in jewelry, laboratory equipment, electrical contacts, dentistry, and automobile emissions control devices. Platinum bullion has the ISO currency code of XPT. As of October 2nd, 2008, Platinum was worth $1,038.00 per troy ounce (approximately $33 per gram).

Characteristics

As a pure metal, platinum is silvery-white in appearance, lustrous, ductile, and malleable. It does not oxidize at any temperature, though it is corroded by halogens, cyanides, sulfur, and caustic alkalis. Platinum is insoluble in hydrochloric and nitric acid, though it does dissolve in aqua regia to form chloroplatinic acid, H2PtCl6.

Platinum's wear- and tarnish-resistance characteristics are well suited for making fine jewelry. Platinum is more precious than gold. The price of platinum changes along with its supply and demand. During periods of sustained economic stability and growth, the price of platinum tends to be as much as twice as the price of gold, whereas, during periods of economic uncertainty, the price of platinum tends to decrease due to reduced demand, falling below the price of gold, partly due to increased gold prices. In the 18th century, platinum's rarity made King Louis XV of France declare it the only metal fit for a king. Platinum possesses high resistance to chemical attack, excellent high-temperature characteristics, and stable electrical properties. All these properties have been exploited for industrial applications.

Common oxidation states of platinum include +2, and +4. The +1 and +3 oxidation states are less common, and are often stabilized by metal bonding in bimetallic (or polymetallic) species.

Applications

The most common use of platinum is as catalyst in chemical reactions. It has been employed in this application since the early 1800s, when platinum powder was used to catalyze the ignition of hydrogen. The most important application of platinum is in automobiles as a catalytic converter, which converts harmful emissions from the engine into less harmful gases. Platinum is also used in the petroleum cracking process, which breaks down crude oil molecules into smaller organic molecules, such as gasoline. PtO2, also known as Adams' catalyst, is used as a hydrogenation catalyst, specifically for liquid vegetable oils. Platinum metal also strongly catalyzes the decomposition of hydrogen peroxide into water and oxygen gas.

Platinum is used as an alloying agent for various metal products, including fine wires, noncorrosive laboratory containers, medical instruments, jewelry, dental equipment, electrical contacts, and thermocouples. Platinum-cobalt, an alloy comprised of roughly 3 parts platinum and 1 part cobalt, is used to make extremely strong permanent magnets. Platinum-based anodes are used in ships, pipelines, and steel piers.

Chloroplatinic acid (H2PtCl6), the result of platinum dissolving in aqua regia, has various applications. It is used in photography, zinc etchings, indelible ink, plating, mirrors, porcelain coloring, and as a catalyst.

Certain platinum-containing compounds are capable of crosslinking DNA and kill cells by similar pathways to alkylating chemotherapeutic agents. Cisplatin, carboplatin and oxaliplatin are licensed examples of this class of drugs.

In watchmaking, Vacheron Constantin, Patek Philippe, Rolex, Breitling and other companies use platinum for producing their limited edition watch series. Watchmakers highly appreciate the unique properties of platinum as it neither tarnishes nor wears out.

Of the 239 tonnes of platinum sold in 2006, 130 tonnes were used for automobile emissions control devices, 49 tonnes were used for jewelry, 13.3 tonnes were used in electronics, and 11.2 tonnes were used by the chemical industry as a catalyst. The remaining 35.5 tonnes produced were used in various other minor applications, such as electrodes, anticancer drugs, oxygen sensors, spark plugs and turbine engines.

History

Platinum occurs naturally in the alluvial sands of various rivers, though there is little evidence of its use by ancient peoples. However, the metal was used by pre-Columbian Native Americans near modern-day Esmeraldas, Ecuador to produce artifacts of a white gold-platinum alloy. The first European reference to platinum appears in 1557 in the writings of the Italian humanist Julius Caesar Scaliger as a description of an unknown noble metal found between Darién and Mexico, "which no fire nor any Spanish artifice has yet been able to liquefy."

In 1741, Charles Wood, a British metallurgist, found various samples of Columbian platinum in Jamaica, which he sent to William Brownrigg for further investigation. Antonio de Ulloa, also credited with the discovery of platinum, returned to Spain from the French Geodesic Mission in 1746. His historical account of the expedition included a description of platinum as being neither separable nor calcinable. Ulloa also anticipated the discovery of platinum mines. After publishing the report in 1748, Ulloa did not continue to investigate the new metal. In 1758, he was sent to superintend mercury mining operations in Huancavelica.

In 1750, after studying the platinum sent to him by Wood, Brownrigg presented a detailed account of the metal to the Royal Society, mentioning that he had seen no mention of it in any previous accounts of known minerals. Brownrigg also made note of platinum's extremely high melting point and refractoriness toward borax. Other chemists across Europe soon began studying platinum, including Torbern Bergman, Jöns Jakob Berzelius, William Lewis, and Pierre Macquer. In 1752, Henric Scheffer published a detailed scientific description of the metal, which he referred to as "white gold", including an account of how he succeeded in fusing platinum ore with the aid of arsenic. Scheffer described platinum as being less pliable than gold, but with similar resistance to corrosion.

Carl von Sickingen researched platinum extensively in 1772. He succeeded in making malleable platinum by alloying it with gold, dissolving the alloy in aqua regia, precipitating the platinum with ammonium chloride, igniting the ammonium chloroplatinate, and hammering the resulting finely-divided platinum to make it cohere. Franz Karl Achard made the first platinum crucible in 1784. He worked with the platinum by fusing it with arsenic, then later volatilizing the arsenic.

In 1786, Charles III of Spain provided a library and laboratory to Pierre-François Chabaneau to aid in his research of platinum. Chabaneau succeeded in removing various impurities from the ore, including gold, mercury, lead, copper, and iron. This led him to believe that he was working with a single metal, but in truth the ore still contained the yet-undiscovered platinum group metals. This led to inconsistent results in his experiments. At times the platinum seemed malleable, but when it was alloyed with iridium, it would be much more brittle. Sometimes the metal was entirely incombustible, but when alloyed with osmium, it would volatilize. After several months, Chabaneau succeeded in producing 23 kilograms of pure, malleable platinum by hammering and compressing the sponge form while white-hot. Chabeneau realized that the infusibility of platinum would lend value to objects made of it, and so started a business with Joaquín Cabezas producing platinum ingots and utensils. This started what is known as the "platinum age" in Spain.

From 1875 to 1960 the SI unit of length (the standard metre) was defined as the distance between two lines on a standard bar of an alloy of ninety percent platinum and ten percent iridium, measured at 0 degrees Celsius.

In 2007 Gerhard Ertl won the Nobel Prize in Chemistry for determining the detailed molecular mechanisms of the catalytic oxidation of carbon monoxide over platinum (catalytic converter).

Occurrence

Platinum is an extremely rare metal, occurring as only 0.003 ppb in the Earth's crust, and is 30 times rarer than gold. It is sometimes mistaken for silver (Ag) but platinum is whiter in appearance.

Platinum is often found chemically uncombined as native platinum and alloyed with iridium as platiniridium. Most often the native platinum is found in secondary deposits, platinum is combined with the other platinum group metals in alluvial deposits. The alluvial deposits used by pre-Columbian people in the Chocó Department, Colombia are still a source for platinum group metals. Another large alluvial deposit was found in the Ural mountains, Russia, which is still mined.

In the nickel and copper deposits the platinum group metals occur as sulphides (i.e. (Pt,Pd)S)), tellurides (i.e. PtBiTe), antimonides (PdSb), and arsenides (i.e. PtAs2), and as end alloys with raw nickel or raw copper. The platinum arsenide, sperrylite (PtAs2), is a major source of platinum associated with nickel ores in the Sudbury Basin deposit in Ontario, Canada. The rare sulfide mineral cooperite, (Pt,Pd,Ni)S, contains platinum along with palladium and nickel. Cooperite occurs in the Merensky Reef within the Bushveld complex, Gauteng, South Africa.

The largest known primary reserves are in the Bushveld complex in South Africa, the large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin, Canada with its large ore deposits are the two other large deposits. In the Sudbury Basin the huge quantities of nickel ore processed makes up for the fact that platinum is present as only 0.5 ppm in the ore. Smaller reserves can be found in the United States, for example the and in the Absaroka Range in Montana. This is also shown in the production of 2005. In 2005, South Africa was the top producer of platinum with an almost 80% share followed by Russia and Canada.

Platinum exists in relatively higher abundances on the Moon and in meteorites. Correspondingly, platinum is found in slightly higher abundances at sites of bolide impact on the Earth that are associated with resulting post-impact volcanism, and can be mined economically; the Sudbury Basin is one such example.

Isotopes

Platinum has six naturally occurring isotopes: 190Pt, 192Pt, 194Pt, 195Pt, 196Pt, and 198Pt. The most abundant of these is 195Pt, comprising 33.83% of all platinum. 190Pt is the least abundant at only .01%. Of the naturally occurring isotopes, only 190Pt is unstable, though it decays with a half-life of 650 years. 198Pt undergoes alpha decay, but because its half-life is estimated as being greater than 320 years, it is considered stable. Platinum also has 31 synthetic isotopes ranging in atomic mass from 166 to 202, making the total number of known isotopes 37. The least stable of these is 166Pt with a half-life of 300 µs, while the most stable is 193Pt with a half-life of 50 years. Most of platinum's isotopes decay by some combination of beta decay and alpha decay. 188Pt, 191Pt, and 193Pt decay primarily by electron capture. 190Pt and 198Pt have double beta decay paths.

Compounds

Platinum's most common oxidation states are +2 and +4. Platinum(II) chloride, PtCl2, is a green-brown powder which is insoluble in water, alcohol, and ether, though it is soluble in hydrochloric acid, HCl. Platinum(IV) oxide, PtO2, also known as Adams' Catalyst, is a black powder which is soluble in KOH solutions and concentrated acids. PtO2 and the less common PtO both decompose upon heating. Platinum(II,IV) oxide, Pt3O4, is formed in the following reaction:
2Pt2+ + Pt4+ + 4O2- → Pt3O4
Platinum also forms an oxide in the +6 oxidation state, PtO3.

Production

Platinum together with the rest of the platinum metals is obtained commercially as a by-product from nickel and copper mining and processing. During electrorefining of copper, noble metals such as silver, gold and the platinum group metals as well as selenium and tellurium settle to the bottom of the cell as anode mud, which forms the starting point for the extraction of the platinum group metals.

If pure platinum is found in placer deposits or other ores it is isolated from them by various methods of subtracting impurities. Because platinum is significantly denser than many of its impurities, the lighter impurities can be removed by simply floating them away in a water bath. Platinum is also non-magnetic, while nickel and iron are both magnetic. These two impurities are thus removed by running an electromagnet over the mixture. Because platinum has a higher melting point than most other substances, many impurities can be burned or melted away without melting the platinum. Finally, platinum is resistant to hydrochloric and sulfuric acids, while other substances are readily attacked by them. Metal impurities can be removed by stirring the mixture in either of the two acids and recovering the remaining platinum.

One suitable method for purification for the raw platinum, which contains platinum, gold, and the other platinum group metals, is to process it with aqua regia, in which palladium, gold and the platinum are disolved, while osmium, iridium, ruthenium and rhodium stay unreacted. The gold is precipitated by the addition of iron(III) chloride and after filtering of the gold, the platinum is precipitated by the addition of ammonium chloride as ammonium chloroplatinate. Ammonium chloroplatinate can be converted to the metal by heating.

Precautions

According to the Centers for Disease Control and Prevention, short-term exposure to platinum salts "may cause irritation of the eyes, nose, and throat" and long-term exposure "may cause both respiratory and skin allergies." The current OSHA standard is 0.002 milligram per cubic meter of air averaged over an 8-hour work shift.

Certain platinum complexes are used in chemotherapy and show good anti-tumor activity for some tumours. Cisplatin is particularly effective against testicular cancer; cure rate was improved from 10% to 85%. However, the side effects are severe. Cisplatin causes cumulative, irreversible kidney damage and deafness.. As with other ototoxic agents, deafness may be secondary to interactions with melanin in the stria vascularis.

As platinum is a catalyst in the manufacture of the silicone rubber and gel components of several types of medical implants (breast implants, joint replacement prosthetics, artificial lumbar discs, vascular access ports), the possibility that platinum free radicals could enter the body and cause adverse effects has merited study. The FDA and other countries have reviewed the issue and found no evidence to suggest toxicity in vivo.

Symbol of prestige

Platinum's rarity as a metal has caused advertisers to associate it with exclusivity and wealth. "Platinum" debit cards have greater privileges than do "gold" ones. "Platinum awards" are the second highest possible, ranking above "gold", "silver" and "bronze", but below Diamond. For example, in the United States a musical album that has sold more than 1,000,000 copies, will be credited as "platinum", whereas an album that sold more than 10,000,000 copies will be certified as “diamond”. And some products, such as blenders and vehicles, with a silvery-white color are identified as "platinum". Platinum is considered a precious metal, although its use is not as common as the use of gold or silver. The frame of the Crown of Queen Elizabeth the Queen Mother, manufactured for her Coronation as Consort of King George VI, is made of platinum. It was the first British crown to be made of this particular metal.

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