# Gold adulteration with iridium



## jimdoc (Jan 11, 2010)

This is an older story, about a year old. But it is the first I have heard about this.
http://timesofindia.indiatimes.com/Gold_adulteration_with_iridium_rampant_in_India/articleshow/4049562.cms


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## lala14 (Jan 12, 2010)

is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second densest element (after osmium) and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can even be flammable. The most important iridium compounds in terms of use are the salts and acids it forms with chlorine, though iridium also forms a number of organometallic compounds used in catalysis and in research. 191Ir and 193Ir are the only two naturally occurring isotopes of iridium as well as the only stable isotopes; the latter is the more abundant of the two.

Iridium was discovered in 1803 by Smithson Tennant in London, England, among insoluble impurities in natural platinum from South America. Although it is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes, it has a number of specialized industrial and scientific applications. Iridium is employed when high corrosion resistance at high temperatures is needed, as in spark plugs, crucibles for recrystallization of semiconductors at high temperatures, electrodes for the production of chlorine in the chloralkali process, and radioisotope thermoelectric generators used in unmanned spacecraft. Iridium compounds also find applications as catalysts for the production of acetic acid.

An unusually high abundance of iridium in a clay layer of the K–T geologic boundary was a crucial clue that led to the theory that the extinction of dinosaurs and many other species 65 million years ago was caused by the impact of a massive extraterrestrial object—the so-called Alvarez hypothesis. Iridium is found in meteorites with an abundance much higher than its average abundance in the Earth's crust. It is thought that the amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but because of the high density and tendency of iridium to bond with iron, most iridium descended below the crust and into the Earth's core when the planet was young and still molten.


1 troy ounce (31 g) of arc-melted iridiumA member of the platinum group metals, iridium is white, resembling platinum, but with a slight yellowish cast. Due to its hardness, brittleness, and very high melting point (the tenth highest of all elements), solid iridium is difficult to machine, form, or work, and thus powder metallurgy is commonly employed instead.[2] It is the only metal to maintain good mechanical properties in air at temperatures above 1600 °C.[3] Iridium has a very high boiling point (11th among all elements) and becomes a superconductor under 0.14 K.[4]

Iridium's modulus of elasticity is the second highest among the metals, only being surpassed by osmium.[3] This, together with a high modulus of rigidity and a very low figure for Poisson's ratio (the relationship of longitudinal to lateral strain), indicate the high degree of stiffness and resistance to deformation that have rendered its fabrication into useful components a matter of great difficulty. Despite these limitations and iridium's high cost, a number of applications have developed where mechanical strength is an essential factor in some of the extremely severe conditions encountered in modern technology.[3]

The measured density of iridium is only slightly lower (by about 0.1%) than that of osmium, the densest element known.[5][6] There had been some ambiguity regarding which of the two elements was denser, due to the small size of the difference in density and difficulties in measuring it accurately,[7] but, with increased accuracy in factors used for calculating density X-ray crystallographic data yielded densities of 22.56 g/cm3 for iridium and 22.59 g/cm3 for osmium.[8]

Iridium is the most corrosion-resistant metal known:[9] it is not attacked by any acid, by aqua regia, by any molten metals, or by silicates at high temperatures. It can, however, be attacked by some molten salts, such as sodium cyanide and potassium cyanide,[9] as well as oxygen and the halogens (particularly fluorine)[10] at higher temperatures.[11]

The discovery of iridium is intertwined with that of platinum and the other metals of the platinum group. Native platinum used by ancient Ethiopians[24] and by South American cultures[25] always contained a small amount of the other platinum group metals, including iridium. Platinum reached Europe as platina ("small silver"), found in the 17th century by the Spanish conquerors in a region today known as the department of Chocó in Colombia.[26] The discovery that this metal was not an alloy of known elements, but instead a distinct new element, did not occur until 1748.[27]

Chemists who studied platinum dissolved it in aqua regia (a mixture of hydrochloric and nitric acids) to create soluble salts. They always observed a small amount of a dark, insoluble residue.[3] Joseph Louis Proust thought that the residue was graphite.[3] The French chemists Victor Collet-Descotils, Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also observed the black residue in 1803, but did not obtain enough for further experiments.[3]

In 1803, British scientist Smithson Tennant (1761–1815) analyzed the insoluble residue and concluded that it must contain a new metal. Vauquelin treated the powder alternatively with alkali and acids[9] and obtained a volatile new oxide, which he believed to be of this new metal—which he named ptene, from the Greek word πτηνος (ptènos) for winged.[28][29] Tennant, who had the advantage of a much greater amount of residue, continued his research and identified the two previously undiscovered elements in the black residue, iridium and osmium.[3][9] He obtained dark red crystals (probably of Na2[IrCl6]·nH2O) by a sequence of reactions with sodium hydroxide and hydrochloric acid.[29] He named iridium after Iris (Ιρις), the Greek winged goddess of the rainbow and the messenger of the Olympian gods, because many of the salts he obtained were strongly colored.[note 2][30] Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.[3][31]


The Greek goddess Iris, after whom iridium was named.British scientist John George Children was the first to melt a sample of iridium in 1813 with the aid of "the greatest galvanic battery that has ever been constructed" (at that time).[3] The first to obtain high purity iridium was Robert Hare in 1842. He found that it had a density of around 21.8 g/cm3 and noted that the metal is nearly unmalleable and very hard. The first melting in appreciable quantity was done by Henri Sainte-Claire Deville and Jules Henri Debray in 1860. They required burning more than 300 L of pure O2 and H2 for each kilogram of iridium.[3]

These extreme difficulties in melting the metal limited the possibilities for handling iridium. John Isaac Hawkins was looking to obtain a fine and hard point for fountain pen nibs and in 1834 managed to create an iridium-pointed gold pen. In 1880 John Holland and William Lofland Dudley were able to melt iridium by adding phosphorus and patented the process in the United States; British company Johnson Matthey later stated that they had been using a similar process since 1837 and had already presented fused iridium at a number of World Fairs.[3] The first use of an alloy of iridium with ruthenium in thermocouples was made by Otto Feussner in 1933. These allowed for the measurement of high temperatures in air up to 2000 °C.[3]

In 1957 Rudolf Mössbauer, in what has been called one of the "landmark experiments in twentieth century physics",[32] discovered the resonant and recoil-free emission and absorption of gamma rays by atoms in a solid metal sample containing only 191Ir.[33] This phenomenon, known as the Mössbauer effect (which has since been observed for other nuclei, such as 57Fe), and developed as Mössbauer spectroscopy, has made important contributions to research in physics, chemistry, biochemistry, metallurgy, and mineralogy.[23] Mössbauer received the Nobel Prize in Physics in 1961, just three years after he published his discovery.[34]

:shock: Iridium is one of the least abundant elements in the Earth's crust, having an average mass fraction of 0.001 ppm in crustal rock; gold is 4 times more abundant, platinum is 10 times more abundant, and silver and mercury are 80 times more abundant.[2] Tellurium is about as abundant as iridium, and only three naturally occurring elements are less abundant: rhenium, ruthenium, and rhodium, iridium being 10 times more abundant than the last two.[2] In contrast to its low abundance in crustal rock, iridium is relatively common in meteorites, with concentrations of 0.5 ppm or more.[36] It is thought that the overall concentration of iridium on Earth is much higher than what is observed in crustal rocks, but because of the density and siderophilic ("iron-loving") character of iridium, it descended below the crust and into the Earth's core when the planet was still molten.[16]

Iridium is found in nature as an uncombined element or in natural alloys; especially the iridium–osmium alloys, osmiridium (osmium rich), and iridiosmium (iridium rich).[9] In the nickel and copper deposits the platinum group metals occur as sulfides (i.e. (Pt,Pd)S)), tellurides (i.e. PtBiTe), antimonides (PdSb), and arsenides (i.e. PtAs2). In all of these compounds platinum is exchanged by a small amount of iridium and osmium. As with all of the platinum group metals, iridium can be found naturally in alloys with raw nickel or raw copper.[37]

Within the Earth's crust, iridium is found at highest concentrations in three types of geologic structure: igneous deposits (crustal intrusions from below), impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in the Bushveld igneous complex in South Africa,[38] though the large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin in Canada are also significant sources of iridium. Smaller reserves are found in the United States.[38] Iridium is also found in secondary deposits, combined with platinum and other platinum group metals in alluvial deposits. The alluvial deposits used by pre-Columbian people in the Chocó Department of Colombia are still a source for platinum-group metals. As of 2003 the world reserves had not been estimated.[9] :mrgreen: :mrgreen: :mrgreen:


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## lala14 (Jan 12, 2010)

The global demand for iridium in 2007 was 119,000 troy ounces (3,700 kg), out of which 25,000 ozt (780 kg) were used for electrical applications such as spark plugs; 34,000 ozt (1,100 kg) for electrochemical applications such as electrodes for the chloralkali process; 24,000 ozt (750 kg) for catalysis; and 36,000 ozt (1,100 kg) for other uses.[51]

[edit] Industrial and medical :twisted: 

Molecular structure of Ir(mppy)3The high melting point, hardness and corrosion resistance of iridium and its alloys determine most of its applications. Iridium and especially iridium–platinum alloys or osmium–iridium alloys have a low wear and are used, for example, for multi-pored spinnerets, through which a plastic polymer melt is extruded to form fibers, such as rayon.[52] Osmium–iridium is used for compass bearings and for balances.[9]

Corrosion and heat resistance makes iridium an important alloying agent. Certain long-life aircraft engine parts are made of an iridium alloy and an iridium–titanium alloy is used for deep-water pipes because of its corrosion resistance.[9] Iridium is also used as a hardening agent in platinum alloys. The Vickers hardness of pure platinum is 56 HV while platinum with 50% of iridium can reach over 500 HV.[53][54]

Devices that must withstand extremely high temperatures are often made from iridium. For example, high-temperature crucibles made of iridium are used in the Czochralski process to produce oxide single-crystals (such as sapphires) for use in computer memory devices and in solid state lasers.[55][56] The crystals, such as gadolinium gallium garnet and yttrium gallium garnet, are grown by melting pre-sintered charges of mixed oxides under oxidizing conditions at temperatures up to 2100 °C.[3] Its resistance to arc erosion makes iridium alloys ideal for electrical contacts for spark plugs.[56][57]

Iridium compounds are used as catalysts in the Cativa process for carbonylation of methanol to produce acetic acid.[58] Iridium itself is used as a catalyst in a type of automobile engine introduced in 1996 called the direct-ignition engine.[9]

The radioisotope iridium-192 is one of the two most important sources of energy for use in industrial γ-radiography for non-destructive testing of metals.[59][60] Additionally, 192Ir is used as a source of gamma radiation for the treatment of cancer using brachytherapy, a form of radiotherapy where a sealed radioactive source is placed inside or next to the area requiring treatment. Specific treatments include high dose rate prostate brachytherapy, bilary duct brachytherapy, and intracavitary cervix brachytherapy.[9]

[edit] Scientific :shock: :idea: 

International Prototype Meter barAn alloy of 90% platinum and 10% iridium was used in 1889 to construct the International Prototype Meter and kilogram mass, kept by the International Bureau of Weights and Measures near Paris.[9] The meter bar was replaced as the definition of the fundamental unit of length in 1960 by a line in the atomic spectrum of krypton,[note 4][61] but the kilogram prototype is still the international standard of mass.[62]

Iridium has been used in the radioisotope thermoelectric generators of unmanned spacecraft such as the Voyager, Viking, Pioneer, Cassini, Galileo, and New Horizons. Iridium was chosen to encapsulate the plutonium-238 fuel in the generator because it can withstand the operating temperatures of up to 2000 °C and for its great strength.[3]

Another use concerns X-ray optics, especially X-ray telescopes.[63] The mirrors of the Chandra X-ray Observatory are coated with a layer of iridium 60 nm thick. Iridium proved to be the best choice for reflecting X-rays after nickel, gold, and platinum were tested. The iridium layer, which had to be smooth to within a few atoms, was applied by depositing iridium vapor under high vacuum on a base layer of chromium.[64]

Iridium is used in particle physics for the production of antiprotons, a form of antimatter. Antiprotons are made by shooting a high-intensity proton beam at a conversion target, which needs to be made from a very high density material. Although tungsten may be used instead, iridium has the advantage of better stability under the shock waves induced by the temperature rise due to the incident beam.[65]


Oxidative addition to hydrocarbons in organoiridium chemistry.[66][67]Carbon–hydrogen bond activation (C–H activation) is an area of research on reactions that cleave carbon–hydrogen bonds, which were traditionally regarded as unreactive. The first reported successes at activating C–H bonds in saturated hydrocarbons, published in 1982, used organometallic iridium complexes that undergo an oxidative addition with the hydrocarbon.[67][66]

Iridium complexes are being investigated as catalysts for asymmetric hydrogenation. These catalysts have been used in the synthesis of natural products and able to hydrogenate certain difficult substrates, such as unfunctionalized alkenes, enantioselectively (generating only one of the two possible enantiomers).[68][69]

Iridium forms a variety of complexes of fundamental interest in triplet harvesting.[70][71][72]

[edit] Historical  :mrgreen: 

Fountain pen nib labeled Iridium PointIridium–osmium alloys were used to tip fountain pen nibs. The first major use of iridium was in 1834 in nibs mounted on gold.[3] Since 1944, the famous Parker 51 fountain pen was fitted with a nib tipped by a ruthenium and iridium alloy (with 3.8% iridium). The tip material in modern fountain pens is still conventionally called "iridium," although there is seldom any iridium in it; other metals such as tungsten have taken its place.[73]

An iridium–platinum alloy was used for the touch holes or vent pieces of cannons. According to a report of the Paris Exhibition of 1867, one of the pieces being exhibited by Johnson and Matthey "has been used in a Withworth gun for more than 3000 rounds, and scarcely shows signs of wear yet. Those who know the constant trouble and expense which are occasioned by the wearing of the vent-pieces of cannon when in active service, will appreciate this important adaptation".[74]

The pigment iridium black, which consists of very finely divided iridium, is used for painting porcelain an intense black; it was said that "all other porcelain black colors appear grey by the side of it".[75] :lol: :lol: :lol:


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## Lou (Jan 13, 2010)

Did you copy and paste the whole wikipedia article?


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## lazersteve (Jan 13, 2010)

It's best to link to outside articles that you intend to be read in full. Quoting a section of the wiki article is usually good enough to point folks here in the right direction. 


Steve


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## eaglewings35 (Jun 12, 2018)

So, what chemical process will dissolve Iridium? I have some 14kt and 18kt gold pen nibs that have this on the tips. Since it melts at such high temp, would it be able to be separated from a gold blob?


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## Lino1406 (Jun 12, 2018)

Dissolving the gold, leaves the iridium


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