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Silver is a metallic chemical element with the chemical symbol Ag ( ) and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal. The metal occurs naturally in its pure, free form (native silver), as an alloy with gold and other metals, and in minerals such as argentite and chlorargyrite. Most silver is produced as a by-product of copper, gold, lead, and zinc refining.

Silver has long been valued as a precious metal, and it is used to make ornaments, jewelry, high-value tableware, utensils (hence the term silverware), and currency coins. Today, silver metal is also used in electrical contacts and conductors, in mirrors and in catalysis of chemical reactions. Its compounds are used in photographic film and dilute silver nitrate solutions and other silver compounds are used as disinfectants and microbiocides. While many medical antimicrobial uses of silver have been supplanted by antibiotics, further research into clinical potential continues.

Characteristics

Silver is a very ductile and malleable (slightly harder than gold) monovalent coinage metal with a brilliant white metallic luster that can take a high degree of polish. It has the highest electrical conductivity of all metals, even higher than copper, but its greater cost and tendency to tarnish have prevented it from being widely used in place of copper for electrical purposes. Despite this, 13,540 tons were used in the electromagnets used for enriching uranium during World War II (mainly because of the wartime shortage of copper). Another notable exception is in high-end audio cables.

Among metals, pure silver has the highest thermal conductivity (the non-metal diamond and superfluid helium II are higher) and one of the highest optical reflectivity. (Aluminium slightly outdoes silver in parts of the visible spectrum, and silver is a poor reflector of ultraviolet light). Silver also has the lowest contact resistance of any metal. Silver halides are photosensitive and are remarkable for their ability to record a latent image that can later be developed chemically. Silver is stable in pure air and water, but tarnishes when it is exposed to air or water containing ozone or hydrogen sulfide to form a black layer of silver sulfide which can be cleaned off with dilute hydrochloric acid. The most common oxidation state of silver is +1 (for example, silver nitrate: AgNO3); in addition, +2 compounds (for example, silver fluoride: AgF2) and the less common +3 compounds (for example, potassium tetrafluoroargentate: K[AgF4] ) are known.

Isotopes

Naturally occurring silver is composed of two stable isotopes, 107Ag and 109Ag, with 107Ag being the most abundant (51.839% natural abundance). Silver's atomic weight is 107.8682(2) g/mol.Twenty-eight radioisotopes have been characterized, the most stable being 105Ag with a half-life of 41.29 days, 111Ag with a half-life of 7.45 days, and 112Ag with a half-life of 3.13 hours. This element has numerous meta states, the most stable being 108mAg (t1/2 = 418 years), 110mAg (t1/2 = 249.79 days) and 106mAg (t1/2 = 8.28 days). All of the remaining radioactive isotopes have half-lives that are less than an hour, and the majority of these have half-lives that are less than 3 minutes.

Isotopes of silver range in relative atomic mass from 93.943 (94Ag) to 126.936 (127Ag). The primary decay mode before the most abundant stable isotope, 107Ag, is electron capture and the primary mode after is beta decay. The primary decay products before 107Ag are palladium (element 46) isotopes, and the primary products after are cadmium (element 48) isotopes.

The palladium isotope 107Pd decays by beta emission to 107Ag with a half-life of 6.5 million years. Iron meteorites are the only objects with a high-enough palladium-to-silver ratio to yield measurable variations in 107Ag abundance. Radiogenic 107Ag was first discovered in the Santa Claramarker meteorite in 1978. The discoverers suggest that the coalescence and differentiation of iron-cored small planets may have occurred 10 million years after a nucleosynthetic event. 107Pd–107Ag correlations observed in bodies that have clearly been melted since the accretion of the solar system must reflect the presence of unstable nuclides in the early solar system.

Compounds

Silver metal dissolves readily in nitric acid ( ) to produce silver nitrate ( ), a transparent crystalline solid that is photosensitive and readily soluble in water. Silver nitrate is used as the starting point for the synthesis of many other silver compounds, as an antiseptic, and as a yellow stain for glass in stained glass. Silver metal does not react with sulfuric acid, which is used in jewellery-making to clean and remove copper oxide firescale from silver articles after silver soldering or annealing. However, silver reacts readily with sulfur or hydrogen sulfide to produce silver sulfide, a dark-colored compound familiar as the tarnish on silver coins and other objects. Silver sulfide also forms silver whiskers when silver electrical contacts are used in an atmosphere rich in hydrogen sulfide.



Silver chloride ( ) is precipitated from solutions of silver nitrate in the presence of chloride ions, and the other silver halides used in the manufacture of photographic emulsions are made in the same way using bromide or iodide salts. Silver chloride is used in glass electrodes for pH testing and potentiometric measurement, and as a transparent cement for glass. Silver iodide has been used in attempts to seed clouds to produce rain. Silver halides are highly insoluble in aqueous solutions and are used in gravimetric analytical methods.

Silver oxide ( ) produced when silver nitrate solutions are treated with a base, is used as a positive electrode (cathode) in watch (battery) batteries. Silver carbonate ( ) is precipitated when silver nitrate is treated with sodium carbonate ( ).

Silver fulminate ( ), a powerful, touch-sensitive explosive used in percussion caps, is made by reaction of silver metal with nitric acid in the presence of ethanol ( ). Another dangerously explosive silver compound is silver azide ( ), formed by reaction of silver nitrate with sodium azide ( ).

Latent images formed in silver halide crystals are developed by treatment with alkaline solutions of reducing agents such as hydroquinone, metol (4-(methylamino)phenol sulfate) or ascorbate which reduce the exposed halide to silver metal. Alkaline solutions of silver nitrate can be reduced to silver metal by reducing sugars such as glucose, and this reaction is used to silver glass mirrors and the interior of glass Christmas ornaments. Silver halides are soluble in solutions of sodium thiosulfate ( ) which is used as a photographic fixer, to remove excess silver halide from photographic emulsions after image development.

Silver metal is attacked by strong oxidizers such as potassium permanganate ( ) and potassium dichromate ( ), and in the presence of potassium bromide ( ), these compounds are used in photography to bleach silver images, converting them to silver halides that can either be fixed with thiosulfate or re-developed to intensify the original image. Silver forms cyanide complexes (silver cyanide) that are soluble in water in the presence of an excess of cyanide ions. Silver cyanide solutions are used in electroplating of silver.

Applications

Many well known uses of silver involve its precious metal properties, including currency, decorative items and mirrors. The contrast between the appearance of its bright white color in contrast with other media makes it very useful to the visual arts. It has also long been used to confer high monetary value as objects (such as silver coins and investment bars) or make objects symbolic of high social or political rank.

Currency

Silver, in the form of electrum (a gold-silver alloy), was coined to produce money in around 700 BC by the Lydians. Later, silver was refined and coined in its pure form. Many nations used silver as the basic unit of monetary value. In the modern world, silver bullion has the ISO currency code XAG. The name of the United Kingdom monetary unit "pound" (£) reflects the fact that it originally represented the value of one troy pound of sterling silver. In the 1800s, many nations, such as the United Statesmarker and Great Britainmarker, switched from silver to a gold standard of monetary value, then in the 20th century to fiat currency.

Jewellery and silverware



Jewellery and silverware are traditionally made from sterling silver (standard silver), an alloy of 92.5% silver with 7.5% copper. In the US, only an alloy consisting of at least 92.5% fine silver can be marketed as "silver". Sterling silver is harder than pure silver, and has a lower melting point (893 °C) than either pure silver or pure copper. Britannia silver is an alternative hallmark-quality standard containing 95.8% silver, often used to make silver tableware and wrought plate. With the addition of germanium, the patented modified alloy Argentium Sterling Silver is formed, with improved properties including resistance to firescale.

Sterling silver jewelry is often plated with a thin coat of .999 fine silver to give the item a shiny finish. This process is called "flashing". Silver jewelry can also be plated with rhodium (for a bright, shiny look) or gold.

Silver is a constituent of almost all colored carat gold alloys and carat gold solders, giving the alloys paler color and greater hardness. White 9 carat gold contains 62.5% silver and 37.5% gold, while 22 carat gold contains up to 8.4% silver or 8.4% copper.

Historically the training and guild organization of goldsmiths included silversmiths as well, and the two crafts remain largely overlapping. Unlike blacksmiths, silversmiths do not shape the metal while it is red-hot but instead, work it at room temperature with gentle and carefully placed hammerblows. The essence of silversmithing is to take a flat piece of metal and by means of different hammers, stakes and other simple tools, to transform it into a useful object.

While silversmiths specialize in, and principally work, silver, they also work with other metals such as gold, copper, steel, and brass. They make jewellery, silverware, armour, vases, and other artistic items. Because silver is such a malleable metal, silversmiths have a large range of choices with how they prefer to work the metal. Historically, silversmiths are mostly referred to as goldsmiths, which was usually the same guild. In the western Canadian silversmith tradition, guilds do not exist; however, mentoring through colleagues becomes a method of professional learning within a community of craftspeople.

Silver is cheaper than gold, though still valuable, and so is very popular with jewelers who are just starting out and cannot afford to make pieces in gold, or as a practicing material for goldsmith apprentices. Silver has also become very fashionable, and is used frequently in more artistic jewellery pieces.

Traditionally silversmiths mostly made "silverware" (cutlery, table flatware, bowls, candlesticks and such). Only in more recent times has silversmithing become mainly work in jewellery, as much less solid silver tableware is now handmade.

Silver is used for awards medals, denoting second place. Some high-end musical instruments are made from sterling silver, such as the flute.

Dentistry

Silver can be alloyed with mercury, tin and other metals at room temperature to make amalgams that are widely used for dental fillings. To make dental amalgam, a mixture of powdered silver and other metals is mixed with mercury to make a stiff paste that can be adapted to the shape of a cavity. The dental amalgam achieves initial hardness within minutes but sets hard in a few hours.

Photography and electronics

Photography used 30.98% of the silver consumed in 1998 in the form of silver nitrate and silver halides. In 2001, 23.47% was used for photography, while 20.03% was used in jewelry, 38.51% for industrial uses, and only 3.5% for coins and medals. The use of silver in photography has rapidly declined, due to the lower demand for consumer color film from the advent of digital technology, since in 2007 of the 894.5 million ounces of silver in supply, just 128.3 million ounces (14.3%) were consumed by the photographic sector, and the total amount of silver consumed in 2007 by the photographic sector compared to 1998 is just 50%.

Some electrical and electronic products use silver for its superior conductivity, even when tarnished. For example, printed circuits can be made using silver paints, and computer keyboards use silver electrical contacts. Some high-end audio hardware (DACs, preamplifiers, etc.) are fully silver-wired, which is believed to cause the least loss of quality in the signal. Silver cadmium oxide is used in high voltage contacts because it can withstand arcing.

During World War II the short supply of copper brought about the government's use of silver from the Treasury vaults for conductors at Oak Ridge National Laboratorymarker. (After the war ended the silver was returned to the vaults.)

Small devices such as hearing aids and watches commonly use Silver oxide batteries due to their long life and high energy/weight ratio. Another usage are high-capacity silver-zinc and silver-cadmium batteries.

Mirrors and optics

Mirrors which need superior reflectivity for visible light are made with silver as the reflecting material in a process called silvering, though common mirrors are backed with aluminium. Using a process called sputtering, silver (and sometimes gold) can be applied to glass at various thicknesses, allowing different amounts of light to penetrate. Silver is usually reserved for coatings of specialized optics, and the silvering most often seen in architectural glass and tinted windows on vehicles is produced by sputtered aluminium, which is cheaper and less susceptible to tarnishing and corrosion.

Other industrial and commercial applications

Silver's catalytic properties make it ideal for use as a catalyst in oxidation reactions, for example, the production of formaldehyde from methanol and air by means of silver screens or crystallites containing a minimum 99.95 weight-percent silver. Silver (upon some suitable support) is probably the only catalyst available today to convert ethylene to ethylene oxide (later hydrolyzed to ethylene glycol, used for making polyesters)— an important industrial reaction. Because silver readily absorbs free neutrons, it is commonly used to make control rods that regulate the fission chain reaction in pressurized water nuclear reactors, generally in the form of an alloy containing 80% silver, 15% indium, and 5% cadmium. Silver is used to make solder and brazing alloys, and as a thin layer on bearing surfaces can provide a significant increase in galling resistance and reduce wear under heavy load, particularly against steel.

Medicinal



Silver ions and silver compounds show a toxic effect on some bacteria, viruses, algae and fungi, typical for heavy metals like lead or mercury, but without the high toxicity to humans that are normally associated with these other metals. Its germicidal effects kill many microbial organisms in vitro, but testing and standardization of silver products is difficult.

Hippocrates, the "father of medicine", wrote that silver had beneficial healing and anti-disease properties, and the Phoeniciansmarker used to store water, wine, and vinegar in silver bottles to prevent spoiling. In the early 1900s people would put silver dollars in milk bottles to prolong the milk's freshness. Its germicidal effects increased its value in utensils and as jewellery. The exact process of silver's germicidal effect is still not entirely understood, although theories exist. One of these is the oligodynamic effect, which explains the effect on microorganisms but would not explain antiviral effects.

Silver is widely used in topical gels and impregnated into bandages because of its wide-spectrum antimicrobial activity. The anti-microbial properties of silver stem from the chemical properties of its ionized form, Ag+. This ion forms strong molecular bonds with other substances used by bacteria to respire, such as molecules containing sulfur, nitrogen, and oxygen. When the Ag+ ion forms a complex with these molecules, they are rendered unusable by the bacteria, depriving them of necessary compounds and eventually leading to the bacteria's death.

Silver compounds were used to prevent infection in World War I before the advent of antibiotics. Silver nitrate solution use continued, then was largely replaced by silver sulfadiazine cream (SSD Cream), which generally became the "standard of care" for the antibacterial and antibiotic treatment of serious burns until the late 1990s. Now, other options, such as silver-coated dressings (activated silver dressings), are used in addition to SSD cream. However, the evidence for the effectiveness of such silver-treated dressings is mixed and although the evidence is promising it is marred by the poor quality of the trials used to assess these products. Consequently a major systematic review by the Cochrane Collaboration found insufficient evidence to recommend the use of silver-treated dressings to treat infected wounds.

There has been renewed interest in silver as a broad-spectrum antimicrobial agent. One application has silver being used with alginate, a naturally occurring biopolymer derived from seaweed, in a range of products designed to prevent infections as part of wound management procedures, particularly applicable to burn victims. The year 2007, saw the first antibacterial glass introduced to fight hospital-caught infection: it is covered with a thin layer of silver. In addition, the U.S. Food and Drug Administration (FDA) has recently approved an endotracheal breathing tube with a fine coat of silver for use in mechanical ventilation, after studies found it reduced the risk of ventilator-associated pneumonia.

Another example uses the known enhanced antibacterial action of silver by applying an electric field. It was found recently that the antibacterial action of silver electrodes is greatly improved if the electrodes are covered with silver nanorods.

Silver is commonly used in catheters. Silver alloy catheters are more effective than standard catheters for reducing bacteriuria in adults in hospital having short term catheterisation. This meta-analysis clarifies discrepant results among trials of silver-coated urinary catheters by revealing that silver alloy catheters are significantly more effective in preventing urinary tract infections than are silver oxide catheters. Though silver alloy urinary catheters cost about $6 more than standard urinary catheters, they may be worth the extra cost since catheter-related infection is a common cause of nosocomial infection and bacteremia.

Various silver compounds, devices to make homeopathic solutions and colloidal silver suspensions are sold as remedies for numerous conditions. Although most colloidal silver preparations are harmless, there are cases where excessive use led to argyria over a period of months or years. High doses of colloidal silver can result in coma, pleural edema, and hemolysis.

Clothing

Silver inhibits the growth of bacteria and fungi. It keeps odor to a minimum and reduces the risk of bacterial and fungal infection. In clothing, the combination of silver and moisture movement (wicking) may help to reduce the harmful effects of prolonged use in active and humid conditions.

Silver is used in clothing in two main forms:
  • A form in which silver ions are integrated into the polymer from which yarns are made (a form of nanotechnology)
  • A form in which the silver is coated onto the yarns.
In both cases the silver prevents the growth of a broad spectrum of bacteria and fungi.

History

Silver has been used for thousands of years for ornaments and utensils, for trade, and as the basis for many monetary systems. Its value as a precious metal was long considered second only to gold. The word "silver" appears in Anglo-Saxon in various spellings such as seolfor and siolfor. A similar form is seen throughout the Teutonic languages (compare Old High German silabar and silbir). The chemical symbol Ag is from the Latin for "silver", argentum (compare Greek άργυρος, árgyros), from the Indo-European root *arg- meaning "white" or "shining". Silver has been known since ancient times. Mentioned in the book of Genesis, slag heaps found in Asia Minormarker and on the islands of the Aegean Seamarker indicate that silver was being separated from lead as early as the 4th millennium BC using surface mining.

Recorded use of silver to prevent infection dates to ancient Greece and Rome, it was rediscovered in the Middle Ages, where it was used for several purposes, such as to disinfect water and food during storage, and also for the treatment of burns and wounds as wound dressing. In the 19th century, sailors on long ocean voyages would put silver coins in barrels of water and wine to keep the liquid pure. Pioneers in America used the same idea as they made their journey from coast to coast. Silver solutions were approved in the 1920s by the US Food and Drug Administration for use as antibacterial agents.

In the Gospels, Jesus' disciple Judas Iscariot is infamous for having taken a bribe of thirty coins of silver from religious leaders in Jerusalemmarker to turn Jesus Christ over to the Romans.

Set aside certain circumstances, Islam permits Muslim men to wear silver jewelry. Muhammad himself wore a silver signet ring.

Occurrence and extraction

Silver ore with Lincoln penny for scale
Time trend of silver production
Silver is found in native form, as an alloy with gold (see also: electrum), and in ores containing sulfur, arsenic, antimony or chlorine. Ores include argentite (Ag2S), chlorargyrite (AgCl) which includes horn silver , and pyrargyrite (Ag3SbS3). The principal sources of silver are the ores of copper, copper-nickel, lead, and lead-zinc obtained from Perumarker, Mexicomarker, Chinamarker, Australia, Chilemarker, Polandmarker and Serbiamarker. Peru and Mexico have been mining silver since 1546 and are still major world producers. Top silver-producing mines are Proaño / Fresnillo (Mexico), Cannington (Queensland, Australia), Dukat (Russia), Uchucchacua (Peru) and Greens Creek mine (Alaska).

The metal is primarily produced through electrolytic copper refining, gold, nickel and zinc refining, and by application of the Parkes process on lead metal obtained from lead ores that contain small amounts of silver. Commercial-grade fine silver is at least 99.9% pure, and purities greater than 99.999% (five 9s) are available. In 2007, Perumarker was the world's top producer of silver, closely followed by Mexico, according to the British Geological Survey.

Price

Silver output in 2005
As of October 2008 silver is about 1/75th the price of gold by mass. Silver once traded at 1/6th to 1/12th the price of gold, prior to the Age of Discovery and the discovery of great silver deposits in the Americas, including Peru, Mexico and the United States, such as the vast Comstock Lode in Virginia Citymarker, Nevadamarker, US. The glut prompted a prolonged and difficult political debate over US monetary policy and US Mint operations: Free Silver conversion similar to gold minting practice. This remained in public discourse during the latter two decades of the 19h century and into the 20th.

Over the last 100 years the price of silver and the gold/silver price ratio have fluctuated greatly due to competing industrial and store-of-value demands. In 1980 the silver price rose to an all-time high of US$49.45 per troy ounce (T.O.) due to market manipulation. By December 2001 the price had dropped to US$4.15/T.O., and in May 2006 it had risen back as high as US$15.21/T.O. In March 2008 silver reached US$21.34/T.O.

The price of silver is important in Judaic Law. The lowest fiscal amount that a Jewish court, or Beth Din, can convene to adjudicate a case over is a shova pruta (value of a Babylonian pruta coin). This is fixed at 1/8 of a gram of pure, unrefined silver, at market price.

Precautions

Silver plays no known natural biological role in humans, and possible health effects of silver are a disputed subject. Silver itself is not toxic but most silver salts are, and some may be carcinogenic. Silver and compounds containing silver (like colloidal silver) can be absorbed into the circulatory system and become deposited in various body tissues leading to a condition called argyria which results in a blue-grayish pigmentation of the skin, eyes, and mucous membranes. Although this condition does not otherwise harm a person's health, it is disfiguring and usually permanent. Argyria is rare, and mild forms are sometimes mistaken for cyanosis.

See also



References

  1. Useful known and unknown views of the father of modern medicine, Hippocrates and his teacher Democritus., U.S. National Library of Medicine
  2. Hippocrates, Microsoft Encarta Online Encyclopedia 2006. Microsoft Corporation.
  3. Answers.com, Sherman Silver Purchase Act; accessed 2009.04.08.


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