Platinum

From Wikipedia, the free encyclopedia

Platinum,  ₇₈Pt

General properties
Name, symbol	platinum, Pt
Pronunciation	/ˈplætɨnəm/
Appearance	silvery white
Platinum in the periodic table
Pd ↑ Pt ↓ Ds iridium ← platinum → gold
Atomic number	78
Standard atomic weight (±)	195.084(9)[1]
Element category	transition metal
Group, block	group 10, d-block
Period	period 6
Electron configuration	[Xe] 4f14 5d9 6s1
per shell	2, 8, 18, 32, 17, 1
Physical properties
Phase	solid
Melting point	2041.4 K ​(1768.3 °C, ​3214.9 °F)
Boiling point	4098 K ​(3825 °C, ​6917 °F)
Density near r.t.	21.45 g·cm−3
when liquid, at m.p.	19.77 g·cm−3
Heat of fusion	22.17 kJ·mol−1
Heat of vaporization	510 kJ·mol−1
Molar heat capacity	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 ​(a mildly basic oxide)
Electronegativity	Pauling scale: 2.28
Ionization energies	1st: 870 kJ·mol−1 2nd: 1791 kJ·mol−1
Atomic radius	empirical: 139 pm
Covalent radius	136±5 pm
Van der Waals radius	175 pm
Miscellanea
Crystal structure	​face-centered cubic (fcc)
Speed of sound thin rod	2800 m·s−1 (at r.t.)
Thermal expansion	8.8 µm·m−1·K−1 (at 25 °C)
Thermal conductivity	71.6 W·m−1·K−1
Electrical resistivity	105 nΩ·m (at 20 °C)
Magnetic ordering	paramagnetic
Tensile strength	125–240 MPa
Young's modulus	168 GPa
Shear modulus	61 GPa
Bulk modulus	230 GPa
Poisson ratio	0.38
Mohs hardness	3.5
Vickers hardness	400–550 MPa
Brinell hardness	300–500 MPa
CAS Registry Number	7440-06-4
History
Discovery and first isolation	Antonio de Ulloa (1735)
Most stable isotopes
Main article: Isotopes of platinum
iso NA half-life DM DE (MeV) DP 190Pt 0.014% 6.5×1011 y α 3.252 186Os 192Pt 0.782% >4.7×1016 y (α) 2.4181 188Os 193Pt syn 50 y ε – 193Ir 194Pt 32.967% – (α) 1.5045 190Os 195Pt 33.832% – (α) 1.1581 191Os 196Pt 25.242% – (α) 0.7942 192Os 198Pt 7.163% >3.2×1014 y (α) 0.0870 194Os (β−β−) 1.0472 198Hg
Decay modes in parentheses are predicted, but have not yet been observed

Platinum is a chemical element with symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, gray-white transition metal. Its name is derived from the Spanish term platina, which is literally translated into "little silver".^[2][3]

Platinum is a member of the platinum group of elements and group 10 of the periodic table of elements. It has six naturally occurring isotopes. It is one of the rarer elements in Earth's crust with an average abundance of approximately 5 μg/kg. It occurs in some nickel and copper ores along with some native deposits, mostly in South Africa, which accounts for 80% of the world production. Because of its scarcity in Earth's crust, only a few hundred tonnes are produced annually, so, given its important uses, it is highly valuable and is a major precious metal commodity.

Platinum is the least reactive metal. It has remarkable resistance to corrosion, even at high temperatures, and is therefore considered a noble metal. Consequently, platinum is often found chemically uncombined as native platinum. Because it occurs naturally in the alluvial sands of various rivers, it was first used by pre-Columbian South American natives to produce artifacts. It was referenced in European writings as early as 16th century, but it was not until Antonio de Ulloa published a report on a new metal of Colombian origin in 1748 that it became investigated by scientists.

Platinum is used in catalytic converters, laboratory equipment, electrical contacts and electrodes, platinum resistance thermometers, dentistry equipment, and jewellery. Being a heavy metal, it leads to health issues upon exposure to its salts, but due to its corrosion resistance, it is not as toxic as some metals.^[4] Compounds containing platinum, most notably cisplatin, are applied in chemotherapy against certain types of cancer.^[5]

§Characteristics

§Physical

Pure platinum is a lustrous, ductile, and malleable, silver-white metal.^[6] Platinum is more ductile than gold, silver or copper, thus being the most ductile of pure metals, but it is less malleable than gold.^[7][8] The metal has excellent resistance to corrosion, is stable at high temperatures and has stable electrical properties. 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 hot aqua regia to form chloroplatinic acid, H₂PtCl₆.^[9]

Its physical characteristics and chemical stability make it useful for industrial applications.^[10] Its resistance to wear and tarnish is well suited to use in fine jewelry.

§Chemical

Platinum dissolves in hot aqua regia

The most common oxidation states of platinum 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 16-electron square planar geometries. Although elemental platinum is generally unreactive, it dissolves in hot aqua regia to give aqueous chloroplatinic acid (H₂PtCl₆):^[11]

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

As a soft acid, platinum has a great affinity for sulfur, such as on dimethyl sulfoxide (DMSO); numerous DMSO complexes have been reported and care should be taken in the choice of reaction solvent.^[12]

§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. It is the only stable isotope with a non-zero spin; with a spin of ¹/₂, ¹⁹⁵Pt satellite peaks are often observed in ¹H and ³¹P NMR spectroscopy (i.e., Pt-phosphine and Pt-alkyl complexes). ¹⁹⁰Pt is the least abundant at only 0.01%. 

Of the naturally occurring isotopes, only ¹⁹⁰Pt is unstable, though it decays with a half-life of 6.5×10¹¹ years. ¹⁹⁸Pt can undergo alpha decay, but its decay had never been observed (the half-life is known to be longer than 3.2×10¹⁴ years), therefore 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, whereas the most stable is ¹⁹³Pt with a half-life of 50 years. Most platinum 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.^[13]

§Occurrence

A native platinum nugget, Kondyor mine, Khabarovsk Krai

Platinum output in 2005

Platinum is an extremely rare metal,^[14] occurring at a concentration of only 0.005 ppm in Earth's crust.^[15][16] It is sometimes mistaken for silver (Ag). Platinum is often found chemically uncombined as native platinum and as alloy with the other platinum-group metals and iron mostly. Most often the native platinum is found in secondary deposits 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 is in the Ural Mountains, Russia, and it is still mined.^[9]

In nickel and copper deposits, platinum-group metals occur as sulfides (e.g. (Pt,Pd)S), tellurides (e.g. PtBiTe), antimonides (PdSb), and arsenides (e.g. 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. At Platinum, Alaska, about 17,000 kg (550,000 ozt) had been mined between 1927 and 1975. The mine ceased operations in 1990.^[17] 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.^[18]

In 1865, chromites were identified in the Bushveld region of South Africa, followed by the discovery of platinum in 1906.^[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 make up for the fact 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] In 2010, South Africa was the top producer of platinum, with an almost 77% share, followed by Russia at 13%; world production in 2010 was 192,000 kg.^[22]

Platinum deposits are present in the state of Tamil Nadu, India.^[23] and a MOU has been signed between Geological Survey of India with TAMIN – Tamil Nadu Minerals Ltd.^[24]

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 Earth that are associated with resulting post-impact volcanism, and can be mined economically; the Sudbury Basin is one such example.^[25]

§Compounds

§Halides

Hexachloroplatinic acid mentioned above is probably the most important platinum compound, as it serves as the precursor for many other platinum compounds. By itself, it has various applications in photography, zinc etchings, indelible ink, plating, mirrors, porcelain coloring, and as a catalyst.^[26]
Treatment of hexachloroplatinic acid with an ammonium salt, such as ammonium chloride, gives ammonium hexachloroplatinate,^[11] which is relatively insoluble in ammonium solutions. Heating this ammonium salt in the presence of hydrogen reduces it to elemental platinum. Potassium hexachloroplatinate is similarly insoluble, and hexachloroplatinic acid has been used in the determination of potassium ions by gravimetry.^[27]

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:^[28]

(H₃O)₂PtCl₆·nH₂O 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 oxidizing oxygen.

§Oxides

Platinum(IV) oxide, PtO₂, also known as Adams' catalyst, is a black powder that is soluble in KOH solutions and concentrated acids.^[29] PtO₂ and the less common PtO both decompose upon heating.^[6] Platinum(II,IV) oxide, Pt₃O₄, is formed in the following reaction:

2 Pt²⁺ + Pt⁴⁺ + 4 O²⁻ → Pt₃O₄

§Other compounds

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.^[30]

Several barium platinides have been synthesized in which platinum exhibits negative oxidation states ranging from −1 to −2. These include BaPt, Ba
3Pt
2, and Ba
2Pt.^[31] Caesium platinide, Cs
2Pt, a dark-red transparent crystalline compound^[32] has been shown to contain Pt2− anions.^[33] Platinum also exhibits negative oxidation states at surfaces reduced electrochemically.^[34] The negative oxidation states exhibited by platinum are unusual for metallic elements, and they are attributed to the relativistic stabilization of the 6s orbitals.^[33]

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.^[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.^[35]

• 

  The hexachloroplatinate ion
• 

  The anion of Zeise's salt
• 

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

  Cisplatin

§History

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".^[36]

This alchemical symbol for platinum was made by joining the symbols of silver and gold.

Antonio de Ulloa is credited with the discovery of platinum.

In 1741, Charles Wood,^[37] a British metallurgist, found various samples of Colombian 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.^[36]

In 1750, after studying the platinum sent to him by Wood, Brownrigg presented a detailed account of the metal to the Royal Society, stating that he had seen no mention of it in any previous accounts of known minerals.^[38] Brownrigg also made note of platinum's extremely high melting point and refractoriness toward borax.^{[clarification needed]} Other chemists across Europe soon began studying platinum, including Andreas Sigismund Marggraf,^[39] 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.^[36]

Carl von Sickingen researched platinum extensively in 1772. He succeeded in making malleable platinum by alloying it with gold, dissolving the alloy in hot 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.^[36]

Because the other platinum-family members were not discovered yet (platinum was the first in the list), Scheffer and Sickingen made the false assumption that due to its hardness—which is slightly more than for pure iron—platinum would be a relatively non-pliable material, even brittle at times, when in fact its ductility and malleability are close to that of gold. Their assumptions could not be avoided because the platinum they experimented with was highly contaminated with minute amounts of platinum-family elements such as osmium and iridium, amongst others, which embrittled the platinum alloy. Alloying this impure platinum residue called "plyoxen" with gold was the only solution at the time to obtain a pliable compound, but nowadays, very pure platinum is available and extremely long wires can be drawn from pure platinum, very easily, due to its crystalline structure, which is similar to that of many soft metals.^[40]

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 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 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.^[36]

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).^[41]

§Production

1,000 cubic centimeters of 99.9% pure platinum, worth about US$970,600 at 14 July 2012 prices^[42]

Time trend of platinum production^[43]

Platinum, along with the rest of the platinum-group 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.^[44]

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 liquid. Platinum is paramagnetic, whereas nickel and iron are both ferromagnetic. 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, whereas 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.^[45]

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, whereas osmium, iridium, ruthenium and rhodium stay unreacted. The gold is precipitated by the addition of iron(II) chloride and after filtering off the gold, the platinum is precipitated as ammonium chloroplatinate by the addition of ammonium chloride. Ammonium chloroplatinate can be converted to platinum by heating.^[46] Unprecipitated hexachloroplatinate(IV) may be reduced with elemental zinc, and a similar method is suitable for small scale recovery of platinum from laboratory residues.^[47]

§Applications

Cutaway view of a metal-core catalytic converter

Of the 245 tonnes of platinum sold in 2010, 113 tonnes were used for vehicle emissions control devices (46%), 76 tonnes for jewelry (31%). The remaining 35.5 tonnes went to various other minor applications, such as investment, electrodes, anticancer drugs, oxygen sensors, spark plugs and turbine engines.^[44]

§Catalyst

The most common use of platinum is as a catalyst in chemical reactions, often as platinum black. It has been employed as a catalyst since the early 19th century, when platinum powder was used to catalyze the ignition of hydrogen. Its most important application is in automobiles as a catalytic converter, which allows the complete combustion of low concentrations of unburned hydrocarbons 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 that becomes rich in aromatic compounds. PtO₂, also known as Adams' catalyst, is used as a hydrogenation catalyst, specifically for vegetable oils.^[26] Platinum also strongly catalyzes the decomposition of hydrogen peroxide into water and oxygen.^[48]

§Standard

Prototype International 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.^[49]

The standard hydrogen electrode also uses a platinized platinum electrode due to its corrosion resistance, and other attributes.^[50]

§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 jewellery, usually as a 90–95% alloy, due to its inertness but not for its appearance, which compared with gold is relatively dull. It is used for this purpose only for its prestige and inherent bullion value; it is not a decorative metal. Jewellery trade publications advise jewellers to present minute surface scratches (which they term patina) as a desirable feature in attempt to enhance a basically rather unattractive metal.^[51][52]

In watchmaking, Vacheron Constantin, Patek Philippe, Rolex, Breitling, and other companies use platinum for producing their limited edition watch series. Watchmakers appreciate the unique properties of platinum, as it neither tarnishes nor wears out (the latter quality relative to gold).^[53]

Average price of platinum from 1992 to 2012 in US$ per troy ounce (~$20/g)^[54]

The price of platinum, like other industrial commodities, is more volatile than that of gold. In 2008, the price of platinum dropped from $2,252 to $774 per oz,^[55] a loss of nearly 2/3 of its value. By contrast, the price of gold dropped from ~$1,000 to ~$700/oz during the same time frame, a loss of only 1/3 of its value.

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,^[56] 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 (although used in industrial applications) its demand is not so 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.^[57]

§Other uses

In the laboratory, platinum wire is used for electrodes; platinum pans and supports are used in thermogravimetric analysis because of the stringent requirements of chemical inertness upon heating to high temperatures (~1000 °C). 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.^[26] Platinum-based anodes are used in ships, pipelines, and steel piers.^[9]

§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 and credit cards have greater privileges than "gold" cards.^[58] "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".^[59] 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.^[60]

§Health issues

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 2 micrograms per cubic meter of air averaged over an 8-hour work shift.^[61]

Platinum-based antineoplastic agents are used in chemotherapy, and show good activity against some tumors. 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, etc.), the possibility platinum could enter the body and cause adverse effects has merited study. The Food and Drug Administration and other institutions have reviewed the issue and found no evidence to suggest toxicity in vivo.^[62][63]