Platinum is a chemical element; it has symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
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. Because of its scarcity in Earth's crust, barely a few hundred metric tonnes are produced annually, and given its critical and important uses, it is highly valuable as well as a major precious metal commodity.
Platinum does not corrode, 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 the 16th century, but it was not until Antonio de Ulloa published a report on a new metal of Colombian origin in 1748 that it began to be understood by scientists.
Platinum is a key component in catalytic converters, laboratory equipment, electrical contacts and electrodes, platinum resistance thermometers, dentistry equipment, and jewelry. Platinum is also used in the glass industry to manipulate molten glass, which does not "wet" platinum. Compounds containing platinum, such as cisplatin, oxaliplatin and carboplatin, are applied in chemotherapy as treatment for certain types of cancer.
Characteristics
Physical Platinum is a lustrous, ductile, and malleable, silver-white metal. Platinum is more ductile than gold, silver or copper, thus being the most ductile of pure metals.
Its physical characteristics and chemical stability make it useful for industrial applications. Its resistance to wear and tarnish is well suited to use in fine jewelry.
Chemical
Platinum does not corrode, and bulk platinum does not oxidize in air at any temperature, but heated metal wires lose weight faster in air or oxygen than it does in a vacuum. The suggestion is that Pt forms a thin surface film of that decomposes when heated above 500 °C.
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. Tetracoordinate platinum(II) compounds tend to adopt 16-electron square planar geometries. Although elemental platinum is generally unreactive, it is attacked by chlorine, bromine, iodine, and sulfur. It reacts vigorously with fluorine at to form platinum tetrafluoride. Platinum is insoluble in hydrochloric and nitric acid, but dissolves in hot aqua regia (a mixture of nitric and hydrochloric acids), to form aqueous chloroplatinic acid, :
As a soft acid, the ion has a great affinity for sulfide and sulfur ligands. Numerous DMSO complexes have been reported and care is taken in the choosing of reaction solvents.
In 2007, the German scientist 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).
Isotopes
Platinum has six naturally occurring isotopes: , , , , , and . The most abundant of these is , comprising 33.83 % of all platinum; it is the only stable isotope with a non-zero spin, of ½, and it is favorable for use in NMR. Due to its spin and large abundance, satellite peaks are also often observed in and NMR spectroscopy (e.g., for Pt-phosphine and Pt-alkyl complexes). The radioactive is the least abundant of these at only 0.012 %; it undergoes alpha decay with a half-life of 4.83 years, causing the very low activity of 16.8 Bq/kg of natural platinum. The decay of this isotope has some use in isotope geology, though not directly for dating.
The other natural isotopes are theoretically capable of alpha decay also, but this has never been observed, and therefore they are considered stable. Platinum also has 38 synthetic isotopes ranging in atomic mass from 165 to 208, making the total number of known isotopes 44. The most stable of these radioisotopes is , with a half-life of 50 years. Most platinum isotopes decay by some combination of beta decay and (on the proton-rich side) alpha decay. , , and decay only by electron capture (besides the very small alpha branch of the first). and are predicted to have energetically favorable double beta decay paths.
Occurrence
Platinum is an extremely rare metal on Earth, occurring at a concentration of only 0.005 ppm in Earth's crust. Platinum is often found chemically uncombined as native platinum and as alloy with the other platinum-group metals mostly. Most often native platinum is found in secondary deposits among 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.
In nickel and copper deposits, platinum-group metals occur as sulfides (e.g., , tellurides (e.g., ), antimonides (PdSb), and arsenides (e.g. ), and as end alloys with nickel or copper. Platinum arsenide, sperrylite (), is a major source of platinum associated with nickel ores in the Sudbury Basin deposit in Ontario, Canada. At Platinum, Alaska, about was mined between 1927 and 1975. The mine ceased operations in 1990. The rare sulfide mineral cooperite, , contains platinum along with palladium and nickel. Cooperite occurs in the Merensky Reef within the Bushveld complex, Gauteng, South Africa.
In 1865, chromites were identified in the Bushveld region of South Africa, followed by the discovery of platinum in 1906.Dan Oancea Platinum In South Africa . MINING.com. September 2008 In 1924, the geologist Hans Merensky discovered a large supply of platinum in the Bushveld Igneous Complex in South Africa. The specific layer he found, named the Merensky Reef, contains around 75 % of the world's known platinum. 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, for example in the Absaroka Range in Montana. 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 .
Advanced techniques to finding platinum deposits by studying ground water found some evidence of new deposits in the state of Tamil Nadu, India.
Platinum exists in somewhat higher quantity 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.
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.
Treatment of hexachloroplatinic acid with an ammonium salt, such as ammonium chloride, gives ammonium hexachloroplatinate, 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.
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:
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, , also known as "Adams' catalyst", is a black powder that is soluble in potassium hydroxide (KOH) solutions and concentrated acids. and the less common PtO both decompose upon heating. Platinum(II,IV) oxide, , is formed in the following reaction:
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. The use of platinum(II) acetylacetonate has also been reported.
Unlike nickel but like palladium, the homoleptic carbonyl complex is not stable.
Platinum exhibits negative oxidation states at surfaces reduced electrochemically, and several "platinides" have been synthesized in which platinum exhibits oxidation states ranging from −1 to −2. The negative oxidation states exhibited by platinum are unusual for metallic elements, and they are attributed to the relativistic stabilization of the 6s orbitals. Barium platinides include BaPt, , and . Caesium platinide, , a dark-red transparent crystalline compound has been shown to contain Pt anions. The "platinum Grignard" conjecturally contains as well.
It is predicted that even the cation in which platinum exists in the +10 oxidation state may be achievable.
Zeise's salt, containing an ethylene ligand, was one of the first organometallic compounds discovered. 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.
Cisplatin, or is the first of a series of square planar platinum(II)-containing chemotherapy drugs. Others include carboplatin and oxaliplatin. These compounds are capable of crosslinking DNA, and kill cells by similar pathways to alkylating chemotherapeutic agents. (Side effects of cisplatin include nausea and vomiting, hair loss, tinnitus, hearing loss, and nephrotoxicity.)
Organoplatinum compounds such as the above anti-tumor agents, as well as soluble inorganic platinum complexes, are routinely characterized using nuclear magnetic resonance spectroscopy.
Non-reactivity with animal life Platinum exposure has shown no adverse effects with animal life as it is one of the least reactive metals.
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 that platinum could enter the body and cause adverse effects has been studied. The Food and Drug Administration and other institutions have reviewed the issue and found no evidence to suggest toxicity in vivo.
Platinum salts 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.
History
Early uses Archaeologists have discovered traces of platinum in the gold used in ancient Egyptian burials. For example, a small box from burial of Shepenupet II was found to be decorated with gold-platinum hieroglyphs. However, the extent of early Egyptians' knowledge of the metal is unclear. It is possible they did not recognize there was platinum in their gold.