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Gold Part 9 |
Compounds
Although gold is a noble metal, it forms many and diverse compounds. The oxidation state of gold in its compound ranges from -1 to +5 but Au(I) and Au(III) dominate. Gold(I), referred to as the aurous ion, is the most common oxidation state with "soft" ligands such as thioethers, thiolates, and tertiary phosphines. Au(I) compounds are typically linear. A good example is Au(CN)2, which is the soluble form of gold encountered in mining. Curiously, aurous complexes of water are rare. The binary gold halides, such as AuCl, form zig-zag polymeric chains, again featuring linear coordination at Au. Most drugs based on gold are Au(I) derivatives
Gold(III) ("auric") is a common oxidation state and is illustrated by gold(III) chloride, AuCl3. Its derivative is chloroauric acid, HAuCl4, which forms when Au dissolves in aqua regia. Au(III) complexes, like other d8 compounds, are typically square planar.
Less common oxidation states: Au(-I), Au(II), and Au(V)
Compounds containing the Au- anion are called aurides. Caesium auride, CsAu which crystallizes in the caesium chloride motif. Other aurides include those of Rb+, K+, and tetramethylammonium (CH3)4N+. Gold(II) compounds are usually diamagnetic with Au-Au bonds such as [Au(CH2)2P(C6H5)2]2Cl2. A noteworthy, legitimate Au(II) complex contains xenon as a ligand, [AuXe4](Sb2F11)2. Gold pentafluoride is the sole example of Au(V), the highest verified oxidation state.
Some gold compounds exhibit aurophilic bonding, which describes the tendency of gold ions to interact at distances that are too long to be a conventional Au-Au bond but shorter that van der Waals bonding. The interaction is estimated to be comparable in strength to that of a hydrogen bond.
Mixed valence compounds
Well-defined cluster compounds are numerous. In such cases, gold has a fractional oxidation state. A representative example is the octahedral species {Au(P(C6H5)3)}62+. Gold chalcogenides, e.g. "AuS" feature equal amounts of Au(I) and Au(III).
Isotopes
There is one stable isotope of gold, and 18 radioisotopes with 195Au being the most stable with a half-life of 186 days.
Gold has been proposed as a "salting" material for nuclear weapons (cobalt is another, better-known salting material, as per the proposed cobalt bomb). A jacket of natural gold, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope Au-198 with a half-life of 2.697 days and produce approximately .411 MeV of gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several days. Such a weapon is not known to have ever been built, tested, or used. |
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Gold Part 8 |
Price
Gold is a precious metal and only three parts out of every billion in earth’s crust is gold. The high price of gold is due to its rare occurrence. Gold, like other metals, is measured by troy weight or by grams. When gold is alloyed with other metals the term carat or karat is used to indicate the amount of gold present. 24 karats is said to be the pure gold.
In the old age, gold was used to back currency, a certain weight of gold was given the name of a unit of currency. For a long period, the US government set the value of the US dollar so that one troy ounce was equal to $20.67 ($664.56/kg), but in 1934 the dollar was revalued to $35.00 per troy ounce ($1125.27/kg). Later, it was becoming hard to maintain the price because of which the US and European banks agreed to manipulate the market to prevent further currency devaluation against increased gold demand.
In 1968, due to economic circumstances the gold pool was collapsed and a two-tiered pricing scheme was introduced. In this scheme, gold was still used to settle international accounts at the old value of $35.00 per troy ounce ($1.13/g) and the price of gold on private market was allowed to fluctuate. This pricing system was abandoned in 1975 and price of gold was left to find its free-market level.
The price of gold is determined on the open market. In London, a procedure called as the Gold Fixing provides a daily benchmark figure to the industry. The afternoon fixing appeared in 1968 to fix a price when US markets are open
In 2005 the World Gold Council estimated total global gold supply to be 3,859 tonnes and demand to be 3,754 tonnes. The largest gold depository in the world is that of the U.S. Federal Reserve Bank in New York, which holds about 3% of the gold ever mined, as does the similarly-laden U.S. Bullion Depository at Fort Knox
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Gold Part 7 |
Gold Occurence
In nature, gold usually occurs alloyed with 8 to 10% silver. Gold Ores consist of grains or microscopic particles of metallic gold embedded in rock, in association with quartz or sulfide minerals like pyrite. These are called "lode" deposits. Native gold is also found in the form of free flakes, grains or larger nuggets that come from rocks and end up in alluvial deposits (also known as placer deposits). Gold sometimes occurs in minerals in chemical composition with other elements, especially in association with tellurium. Examples are calaverite, sylvanite, nagyagite, petzite and krennerite. Gold rarely occurs as a mercury-gold amalgam, and in very low concentrations in seawater.
Gold Production
In the 1880s, South Africa was the source for a large proportion of the world’s gold supply. In 1970, South Africa produced about 1000 tonnes of gold which accounted for 79% of the world gold supply. However by 2007 production was just 272 tonnes. This sharp decline was due to the increasing difficulty of extraction, changing economic factors affecting the industry, and tightened safety auditing. In 2007 China produced about 276 tonnes of gold and overtook South Africa as the world's largest gold producer.
The city of Johannesburg located in South Africa was founded as a result of the Witwatersrand Gold Rush which resulted in the discovery of some of the largest gold deposits the world has ever seen. The Savuka and TauTona mines are currently the world's deepest gold mine at 3,777 m. The Second Boer War of 1899–1901 was at least partly over possession of the gold wealth in South Africa and the rights of miners.
Other major gold producers are United States, Australia, China, Russia and Peru. Two-thirds of gold used in the United States is supplied from South Dakota and Nevada mines. Today about one-quarter of the world gold output is estimated to originate from artisanal or small scale mining.
After initial production, gold is often subsequently refined industrially by the Wohlwill process or the Miller process. Other methods of and purifying smaller amounts of gold include parting and inquartation as well as cuppelation, or refining methods based on the dissolution of gold in aqua regia. The average gold mining and extraction costs are $238 per troy ounce but these can vary widely depending on mining type and ore quality.
The world's oceans hold a vast amount of gold, but in very low concentrations. Fritz Haber attempted commercial extraction of gold from sea water. Unfortunately, his assessment of the concentration of gold in sea water was unduly high, probably due to sample contamination. The effort produced little gold and cost the German government far more than the commercial value of the gold recovered. No commercially viable mechanism for performing gold extraction from sea water has yet been identified. Gold synthesis is not economically viable and is unlikely to become so in future. In 2001, global mine production amounted to 67% of total gold demand in that year, that is approximately 2,604 tonnes. At the end of 2006, it was estimated that all the gold ever mined totaled 158,000 tonnes. At current consumption rates, the supply of gold is believed to last 45 years. |
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Gold Part 6 |
History
Gold has been considered the most desirable of precious metals, and its value has been used as the standard for many currencies. Gold has been used as a symbol for purity, value and royalty.
Gold was considered as a sign of wealth and prestige in many parts of the world. But gold was made fun of by Thomas More in his essay Utopia. On that imaginary island, gold was in so much abundance that it is used to make chains for slaves, tableware and lavatory-seats. When ambassadors from other countries arrive, dressed in precious gold jewels, the Utopians mistook them for menial servants, paying homage instead to the most modestly-dressed of their party.
Biting gold was one way to test its authenticity in the olden days. Although this is certainly not a professional way of testing gold, the bite test should score the gold since gold is considered a soft metal according to the Mohs' scale of mineral hardness. The purer the gold the easier it should be to mark it. Lead is softer than gold, hence painted lead can cheat this test.
Gold in old age was relatively easy to obtain geologically; however, 75% of all gold ever produced has been extracted since 1910. One main goal of the chemists was to produce gold from other substances, such as lead - and interaction with philosopher's stone. Although they never succeeded in this attempt, the chemists promoted an interest in what can be done with substances, and this laid a basis for today's chemistry.
Their symbol for gold was the circle with a point at its center, which was also the astrological symbol, the Egyptian hieroglyph and the ancient Chinese character for the Sun.
During the 19th century, gold rushes occurred whenever large gold deposits were discovered. The first major gold strike in the United States occurred in a small north Georgia town called Dahlonega. Further gold rushes occurred in California, Colorado, Otago, Australia, Witwatersrand, Black Hills, and Klondike. Because of its historically high value, much of the gold mined throughout history is still in circulation in one form or another. |
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Gold Part 5 |
History
The symbol for the Sun has been used since ancient times to represent gold. Gold has been known and highly-valued since prehistoric times. It may have been the first metal used by humans and was valued for ornamentation and rituals. Egyptian hieroglyphs from as early as 2600 BC describe gold, which king Tushratta of the Mitanni claimed was "more plentiful than dirt" in Egypt.[7] Egypt and especially Nubia had the resources to make them major gold-producing areas for much of history. The earliest known map is known as the Turin papyrus and shows the plan of a gold mine in Nubia together with indications of the local geology. The primitive working methods are described by Strabo and included fire-setting. Large mines also occurred across the Red Sea in what is now Saudi Arabia.
The legend of the golden fleece may refer to the use of fleeces to trap gold dust from placer deposits in the ancient world.Gold is mentioned frequently in the Old Testament, starting with Genesis 2:11 (at Havilah) and is included with the gifts of the magi in the first chapters of Matthew New Testament. The Book of Revelation 21:21 describes the city of New Jerusalem as having streets "made of pure gold, clear as crystal". The south-east corner of the Black Sea was famed for its gold. Exploitation is said to date from the time of Midas, and this gold was important in the establishment of what is probably the world's earliest coinage in Lydia between 643 and 630 BC.
From 6th or 5th century BCE,Chu (state) circulated Ying Yuan,one kind of square gold coin. The Romans developed new methods for extracting gold on a large scale using hydraulic mining methods, especially in Spain from 25 BC onwards and in Romania from 150 AD onwards. One of their largest mines was at Las Medulas in León (Spain), where seven long aqueducts enabled them to sluice most of a large alluvial deposit. The mines a Rosia Montana in Transylvania were also very large, and until very recently, still mined by opencast methods. They also exploited smaller deposits in Wales, such as placer and hard-rock deposits at Dolaucothi. The various methods they used are well described by Pliny the Elder in his encyclopedia Naturalis Historia written towards the end of the first century AD.
The Mali Empire in Africa was famed throughout the old world for its large amounts of gold. Mansa Musa, ruler of the empire (1312-1337) became famous throughout the old world for his great hajj to Mecca in 1324. When he passed through Cairo in July of 1324, he was reportedly accompanied by a camel train that included thousands of people and nearly a hundred camels. He gave away so much gold that it took over a decade for the economy across North Africa to recover, due to the rapid inflation that it initiated.
A contemporary Arab historian remarked
" Gold was at a high price in Egypt until they came in that year. The mithqal did not go below 25 dirhams and was generally above, but from that time its value fell and it cheapened in price and has remained cheap till now. The mithqal does not exceed 22 dirhams or less. This has been the state of affairs for about twelve years until this day by reason of the large amount of gold which they brought into Egypt and spent there [...] "
-Chihab Al-Umari |
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Gold Part 4 |
Some Interesting Facts
In medieval times, gold was often seen as beneficial for the health (even though it was not), in the belief that something that rare and beautiful could not be anything but healthy. Even some modern esotericists and forms of alternative medicine assign metallic gold a healing power. Some gold salts do have anti-inflammatory properties and are used as pharmaceuticals in the treatment of arthritis and other similar conditions. However, only salts and radioisotopes of gold are of pharmacological value, as elemental (metallic) gold is inert to all chemicals it encounters inside the body.
Gold leaf, flake or dust is used on and in some gourmet foodstuffs, notably sweets and drinks as decorative ingredient.Gold flake was used by the nobility in Medieval Europe as a decoration in foodstuffs and drinks, in the form of leaf, flakes or dust, either to demonstrate the host's wealth or in the belief that something that valuable and rare must be beneficial for one's health.
Gold solder is used for joining the components of gold jewelry by high-temperature hard soldering or brazing. If the work is to be of hallmarking quality, gold solder must match the carat weight of the work, and alloy formulas are manufactured in most industry-standard carat weights to color match yellow and white gold. Gold solder is usually made in at least three melting-point ranges referred to as Easy, Medium and Hard. By using the hard, high-melting point solder first, followed by solders with progressively lower melting points, goldsmiths can assemble complex items with several separate soldered joints.
Gold can be used in food and has the E Number 175. Goldwasser (German: "Goldwater") is a traditional herbal liqueur produced in Gdansk, Poland, and Schwabach, Germany, and contains flakes of gold leaf. There are also some expensive (~$1000) cocktails which contain flakes of gold leaf .However, since metallic gold is inert to all body chemistry, it adds no taste nor has it any other nutritional effect and leaves the body unaltered.
Dentistry. Gold alloys are used in restorative dentistry, especially in tooth restorations, such as crowns and permanent bridges. The gold alloys' slight malleability facilitates the creation of a superior molar mating surface with other teeth and produces results that are generally more satisfactory than those produced by the creation of porcelain crowns. The use of gold crowns in more prominent teeth such as incisors is favored in some cultures and discouraged in others.
Gold can be made into thread and used in embroidery. Gold is ductile and malleable, meaning it can be drawn into very thin wire and can be beaten into very thin sheets known as gold leaf. Gold produces a deep, intense red color when used as a coloring agent in cranberry glass.
In photography, Gold toners are used to shift the color of silver bromide black and white prints towards brown or blue tones, or to increase their stability. Used on sepia-toned prints, gold toners produce red tones. Kodak publish formulas for several types of gold toners, which use gold as the chloride (Kodak, 2006).
Electronics. The concentration of free electrons in gold metal is 5.90×1022 cm-3. Gold is highly conductive to electricity, and has been used for electrical wiring in some high energy applications (silver is even more conductive per volume, but gold has the advantage of corrosion resistance). For example, gold electrical wires were used during some of the Manhattan Project's atomic experiments, but large high current silver wires were used in the calutron isotope separator magnets in the project.
Though gold is attacked by free chlorine, its good conductivity and general resistance to oxidation and corrosion in other environments (including resistance to non-chlorinated acids) has led to its widespread industrial use in the electronic era as a thin layer coating electrical connectors of all kinds, thereby ensuring good connection. For example, gold is used in the connectors of the more expensive electronics cables, such as audio, video and USB cables. The benefit of using gold over other connector metals such as tin in these applications, is highly debated. Gold connectors are often criticized by audio-visual experts as unnecessary for most consumers and seen as simply a marketing ploy. However, the use of gold in other applications in electronic sliding contacts in highly humid or corrosive atmospheres, and in use for contacts with a very high failure cost (certain computers, communications equipment, spacecraft, jet aircraft engines) remains very common, and is unlikely to be replaced in the near future by any other metal.
Besides sliding electrical contacts, gold is also used in electrical contacts because of its resistance to corrosion, electrical conductivity, ductility and lack of toxicity.[4] Switch contacts are generally subjected to more intense corrosion stress than are sliding contacts.
Colloidal gold (Colloidal sols of gold nanoparticles) in water are intensely red-colored, and can be made with tightly-controlled particle sizes up to a few tens of nm across by reduction of gold chloride with citrate or ascorbate ions. Colloidal gold is used in research applications in medicine, biology and materials science. The technique of immunogold labeling exploits the ability of the gold particles to adsorb protein molecules onto their surfaces. Colloidal gold particles coated with specific antibodies can be used as probes for the presence and position of antigens on the surfaces of cells (Faulk and Taylor 1979). In ultrathin sections of tissues viewed by electron microscopy, the immunogold labels appear as extremely dense round spots at the position of the antigen (Roth et al. 1980). Colloidal gold is also the form of gold used as gold paint on ceramics prior to firing.
Gold, or alloys of gold and palladium, are applied as conductive coating to biological specimens and other non-conducting materials such as plastics and glass to be viewed in a scanning electron microscope. The coating, which is usually applied by sputtering with an argon plasma, has a triple role in this application. Gold's very high electrical conductivity drains electrical charge to earth, and its very high density provides stopping power for electrons in the SEM's electron beam, helping to limit the depth to which the electron beam penetrates the specimen. This improves definition of the position and topography of the specimen surface and increases the spatial resolution of the image. Gold also produces a high output of secondary electrons when irradiated by an electron beam, and these low-energy electrons are the most commonly-used signal source used in the scanning electron microscope.
Many competitions, and honors, such as the Olympics and the Nobel Prize, award a gold medal to the winner. As gold is a good reflector of electromagnetic radiation such as infrared and visible light as well as radio waves, it is used for the protective coatings on many artificial satellites, in infrared protective faceplates in thermal protection suits and astronauts' helmets and in electronic warfare planes like the EA-6B Prowler.
Gold is used as the reflective layer on some high-end CDs. The isotope gold-198, (half-life: 2.7 days) is used in some cancer treatments and for treating other diseases. Automobiles may use gold for heat insulation. McLaren F1 uses gold foil in the engine compartment. |
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