platinum

iridium ← platinum → gold Pd ↑ Pt ↓ Ds 78Pt Periodic table
Appearance
grayish white
General properties
Name, symbol, number	platinum, Pt, 78
Element category	transition metal
Group, period, block	10, 6, d
Standard atomic weight	195.084 g·mol−1
Electron configuration	[Xe] 4f14 5d9 6s1
Electrons per shell	2, 8, 18, 32, 17, 1 (Image)
Physical properties
Phase	solid
Density (near r.t.)	21.45 g·cm−3
Liquid density at m.p.	19.77 g·cm−3
Melting point	2041.4 K, 1768.3 °C, 3214.9 °F
Boiling point	4098 K, 3825 °C, 6917 °F
Heat of fusion	22.17 kJ·mol−1
Heat of vaporization	469 kJ·mol−1
Specific heat capacity	(25 °C) 25.86 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k at T/K 2330 (2550) 2815 3143 3556 4094
Atomic properties
Oxidation states	6, 5, 4, 3 , 2, 1, -1, -2 (mildly basic oxide)
Electronegativity	2.28 (Pauling scale)
Ionization energies	1st: 870 kJ·mol−1
	2nd: 1791 kJ·mol−1
Atomic radius	139 pm
Covalent radius	136±5 pm
Van der Waals radius	175 pm
Miscellanea
Crystal structure	face-centered cubic
Magnetic ordering	paramagnetic
Electrical resistivity	(20 °C) 105 nΩ·m
Thermal conductivity	(300 K) 71.6 W·m−1·K−1
Thermal expansion	(25 °C) 8.8 µm·m−1·K−1
Tensile strength	125-240 MPa
Young's modulus	168 GPa
Shear modulus	61 GPa
Bulk modulus	230 GPa
Poisson ratio	0.38
Mohs hardness	4–4.5
Vickers hardness	549 MPa
Brinell hardness	392 MPa
CAS registry number	7440-06-4
Most stable isotopes
Main article: Isotopes of platinum
iso NA half-life DM DE (MeV) DP 190Pt 0.014% 6.5×1011 y α 3.18 186Os 191Pt syn 2.76 d ε  ? 191Ir 192Pt 0.782% 192Pt is stable with 114 neutrons 193Pt syn 50 y ε  ? 193Ir 193mPt syn 4.33 d IT 0.1355e 193Pt 194Pt 32.967% 194Pt is stable with 116 neutrons 195Pt 33.832% 195Pt is stable with 117 neutrons 195mPt syn 4.02 d IT 0.1297e 195Pt 196Pt 25.242% 196Pt is stable with 118 neutrons 197Pt syn 19.8913 h β− 0.719 197Au 197mPt syn 1.59 h IT 0.3465 197Pt 198Pt 7.163% 198Pt is stable with 120 neutrons
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Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River."^[1] It is in Group 10 of the periodic table of elements. A dense, 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 and electrodes, platinum resistance thermometers, dentistry equipment, and catalytic converters. Platinum bullion has the ISO currency code of XPT. Platinum is a commodity with a value that fluctuates according to market forces. As of 30 October 2009 (2009 -10-30)^[update], platinum was worth US$1,324.00 per troy ounce (approximately US$42.57 per gram).^[2]

Characteristics

As a pure metal, platinum is silvery-white in appearance, lustrous, ductile, and malleable.^[3] It does not oxidize at any temperature, although it is corroded by halogens, cyanides, sulfur, and caustic alkalis. Platinum is insoluble in hydrochloric and nitric acid, but dissolves in aqua regia to form chloroplatinic acid, H₂PtCl₆.^[4]

Platinum's wear- and tarnish-resistance characteristics are well suited for making fine jewelry. Platinum is more precious than gold or silver. Platinum possesses high resistance to chemical attack, excellent high-temperature characteristics, and stable electrical properties. All of these properties have been exploited for industrial applications.

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 6.5 × 10¹¹ years. ¹⁹⁸Pt undergoes alpha decay, but because its half-life is estimated as being greater than 3.2 × 10¹⁴ 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.^[5]

Chemistry and compounds

Platinum's most common oxidation states are +2, and +4. The +1 and +3 oxidation states are less common, and are often stabilized by metal bonding in bimetallic (or polymetallic) species. As is expected, tetracoordinate platinum(II) compounds tend to adopt a square planar geometry. While elemental platinum is generally unreactive, it dissolves in aqua regia to give soluble hexachloroplatinic acid ("H₂PtCl₆", formally (H₃O)₂PtCl₆·nH₂O ):^[6]

Pt + 4 HNO₃ + 6 HCl → H₂PtCl₆ + 4 NO₂ + 4 H₂O

This compound has various applications in photography, zinc etchings, indelible ink, plating, mirrors, porcelain coloring, and as a catalyst.^[7]

Treatment of hexachloroplatinic acid with an ammonium salt, such as ammonium chloride, gives ammonium hexachloroplatinate,^[6] which is very insoluble in ammonium solutions. Heating the ammonium salt in the presence of hydrogen reduces it to elemental platinum. Platinum is often isolated from ores and recycled thus.^[8] Potassium hexachloroplatinate is similarly insoluble, such that the acid has been used in the determination of potassium ions by gravimetry.^[9]

When hexachloroplatinic acid is heated, it decomposes through platinum(IV) chloride and platinum(II) chloride to elemental platinum, although the reactions do not occur stepwise, cleanly:^[10]

(H₃O)₂PtCl₆·n H₂O PtCl₄ + 2 HCl + (n + 2) H₂O

PtCl₄ PtCl₂ + Cl₂

PtCl₂ Pt + Cl₂

All three reactions are reversible. Platinum(II) and platinum(IV) bromides are known as well. Platinum hexafluoride is a strong oxidizer capable of oxidising oxygen.

Platinum(IV) oxide, PtO₂, also known as Adams' Catalyst, is a black powder which is soluble in KOH solutions and concentrated acids.^[11] PtO₂ and the less common PtO both decompose upon heating.^[3] Platinum(II,IV) oxide, Pt₃O₄, is formed in the following reaction:

2 Pt²⁺ + Pt⁴⁺ + 4 O²⁻ → Pt₃O₄

Platinum also forms a trioxide, which is actually in the +4 oxidation state.

Unlike palladium acetate, platinum(II) acetate is not commercially available. Where a base is desired, the halides have been used in conjunction with sodium acetate.^[12] The use of platinum(II) acetylacetonate has also been reported.^[13]

Zeise's salt, containing an ethylene ligand, was one of the first organometallic compounds discovered. Dichloro(cycloocta-1,5-diene)platinum(II) is a commercially available olefin complex, which contains easily displaceable cod ligands ("cod" being an abbreviation of 1,5-cyclooctadiene). The cod complex and the halides are convenient starting points to platinum chemistry. As a soft acid, platinum has a great affinity for sulfur, such as on DMSO; numerous DMSO complexes have been reported and care should be taken in the choice of reaction solvent.^[12]

Cisplatin, or cis-diamminedichloroplatinum(II) is the first of a series of square planar platinum(II)-containing chemotherapy drugs, including carboplatin and oxaliplatin. These compounds are capable of crosslinking DNA and kill cells by similar pathways to alkylating chemotherapeutic agents.^[14]

The hexachloroplatinate ion

The anion of Zeise's salt

Dichloro(cycloocta-1,5-diene)platinum(II)

Cisplatin

Several barium platinides have been synthesized, in which platinum exhibits negative oxidation states ranging from −1 to −2. These include BaPt, Ba₃Pt₂, and Ba₂Pt.^[15] Caesium platinide, Cs₂Pt, has been shown to contain Pt²⁻ anions.^[16] Platinum is also shown to exhibit negative oxidation states at surfaces reduced electrochemically.^[17] The negative oxidation states exhibited by platinum, which are unusual for metallic elements, are believed to be due to the relativistic stabilization of the 6s orbitals.^[16]

Occurrence

Platinum ore, US cent included for scale

Platinum output in 2005

Platinum is an extremely rare metal,^[18] occurring as only 0.003 ppb in the Earth's crust. It is sometimes mistaken for silver (Ag).

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 nickel and copper deposits platinum group metals occur as sulfides (i.e. (Pt,Pd)S)), tellurides (i.e. PtBiTe), antimonides (PdSb), and arsenides (i.e. PtAs₂), and as end alloys with nickel or copper. 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.^[19]

The largest known primary reserves are in the Bushveld complex in South Africa.^[20] The large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin, Canada, 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,^[20] for example in the Absaroka Range in Montana.^[21] 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.^[22]

Platinum exists in 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.

Production

1000 cubic centimeters of 99.9% pure platinum

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.^[23][24]

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.^[25]

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 platinum are dissolved, 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.^[26]

Applications

Cross section of a metal-core converter

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.^[27]

Catalysis

The most common use of platinum is as a 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 allows the complete combustion of low concentrations of unburned hydrocarbon from the exhaust into carbon dioxide and water vapor. Platinum is also used in the petroleum industry as a catalyst in a number of separate processes, but especially in catalytic reforming of straight run naphthas into higher-octane gasoline which becomes rich in aromatic compounds. PtO₂, also known as Adams' catalyst, is used as a hydrogenation catalyst, specifically for vegetable oils.^[7] Platinum metal also strongly catalyzes the decomposition of hydrogen peroxide into water and oxygen gas.^[28]

Standard

International Prototype Meter bar

From 1889 to 1960, the meter was defined as the length of a platinum-iridium (90:10) alloy bar, known as the International Prototype Meter bar. The previous bar was made of platinum in 1799. The International Prototype Kilogram remains defined by a cylinder of the same platinum-iridium alloy made in 1879.

The standard hydrogen electrode also utilizes a platinized platinum electrode due to its corrosion resistance, and other attributes.

Precious metal

Platinum Eagle

Platinum is a precious metal commodity; its bullion has the ISO currency code of XPT. Coins, bars, and ingots are traded or collected. Platinum finds use in jewelry, usually as a 90-95% alloy, due to its inertness and shine. 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.^[29]

Price

Average price of platinum from 1991 to 2007 in US$ per troy ounce (~$40/g).^[30]

The price of platinum, like other industrial commodities, is more volatile than that of gold. In 2008 the price of platinum ranged from $774 to $2,252 per oz.^[31]

During periods of sustained economic stability and growth, the price of platinum tends to be as much as twice the price of gold, whereas during periods of economic uncertainty^[32], the price of platinum tends to decrease due to reduced industrial demand, falling below the price of gold. Gold prices are more stable in slow economic times, as gold is considered a safe haven and gold demand is not driven by industrial uses. In the 18th century, platinum's rarity made King Louis XV of France declare it the only metal fit for a king.^[33]

Other uses

In the laboratory, platinum wire is used for electrodes; platinum pans are used in thermogravimetric analysis. Platinum is used as an alloying agent for various metal products, including fine wires, noncorrosive laboratory containers, medical instruments, dental prostheses, electrical contacts, and thermocouples. Platinum-cobalt, an alloy of roughly three parts platinum and one part cobalt, is used to make relatively strong permanent magnets.^[7] Platinum-based anodes are used in ships, pipelines, and steel piers.^[4]

Symbol of prestige

An assortment of native platinum nuggets

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 million copies, will be credited as "platinum", whereas an album that sold more than 10 million copies will be certified as “diamond”. 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.

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 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."^[34]

The alchemical symbol for platinum (shown above) was made by joining the symbols of silver and gold.

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 after having been there for eight years. 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.^[34]

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, Henrik 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.^[34]

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.^[34]

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.^[34]

From 1875 to 1960 the SI unit of length (the standard meter) 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).

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.^[35]

Certain platinum complexes are used in chemotherapy and show good anti-tumor activity for some tumors. Cisplatin is particularly effective against testicular cancer; cure rate was improved from 10% to 85%.^[36] However, the side effects are severe. Cisplatin causes cumulative, irreversible kidney damage and deafness.^[37] 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.^[38]

See also

Notes

References

• Nuclides and Isotopes Fourteenth Edition: Chart of the Nuclides, General Electric Company, 1989.
• Jefferson Lab — The Element Platinum

External links

Wikimedia Commons has media related to: Platinum

Look up platinum in Wiktionary, the free dictionary.

• The Platinum Group Metals Database
• A balanced historical account of the sequence of discoveries of platinum; illustrated.
• WebElements.com — Platinum
• Platinum Metals Review E-Journal
• Platinum Guild International
• United States Geological Survey Platinum-Group Metals Statistics and Information
• picture of a 999.5 fine platinum ingot in the element collection from Heinrich Pniok