Silver

This page is about silver the chemical element. For the color, see silver (color). For other uses, see Silver (disambiguation)

Silver (IPA: /ˈsɪlvə(ɹ)/) is a chemical element with the symbol Ag (Latin: argentum). A soft white lustrous transition metal, it has the highest electrical and thermal conductivity for a metal, and occurs as a free metal, and in minerals such as argentite and chlorargyrite. Most silver is produced as a by-product of copper, gold, lead, and zinc mining.

Silver has been known since antiquity. It has long been valued as a precious metal and used in currency, ornaments and jewelry, as well as utensils (hence the term silverware). Today, it is also used in photographic film, electrical contacts, and mirrors. Elemental silver is also used to catalyze chemical reactions.

Silver has certain antimicrobial activity. In the past, dilute solutions of silver nitrate were used as disinfectants, though this has been supplanted by other treatments. In alternative medicine, there has been increasing interest in the use of colloidal silver as remedies for a wide range of ailments, though these claims are disputed. The consumption of large amounts of silver can lead to a darkening of the skin known as argyria.

Notable characteristics

Silver is a very ductile and malleable (slightly harder than gold) univalent 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 tarnishability has prevented it from being widely used in place of copper for electrical purposes, though it was used in the electromagnets used for enriching uranium during World War II (mainly because of the wartime shortage of copper).

Pure silver has the highest thermal conductivity, whitest color, the highest optical reflectivity, although aluminium slightly outdoes it in parts of the visible spectrum, and 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 the effect of light upon them. This metal is stable in pure air and water, but does tarnish when it is exposed to ozone, hydrogen sulfide, or air containing sulfur. The most common oxidation state of silver is +1 (for example, silver nitrate, AgNO₃); a few +2 (for example, silver(II) fluoride; AgF₂) and +3 compounds (for example, silver(III) persulfate; Ag₂(SO₅)₃) are also known.

History

Silver Anglo-Saxon seolfor; compare Old High German silabar; Ag is from the Latin argentum) has been known since ancient times. It is mentioned in the book of Genesis, and slag heaps found in Asia Minor and on the islands of the Aegean Sea indicate that silver was being separated from lead as early as the 4th millennium BC. Silver has been used for thousands of years for ornaments and utensils, for trade, and as the basis for monetary systems. As a precious metal, it was long considered second only to gold in value. In Ancient Egypt and Medieval Europe, it was often more valuable than gold.

The metal mercury was thought of as a kind of silver, though the two elements are chemically dissimlar (belonging to distinct, although adjacent, groups of the periodic table). The false belief is reflected in the Latin and English names, hydrargyrum ("watery silver") and quicksilver.

Because it was folklorically associated with the moon, as well as with various moon goddesses, alchemists referred to silver by the Latin word for moon, luna; and one of the alchemical symbols for silver is a crescent moon with the open part on the left (see picture, left).

Occurrence and extraction

Silver ore (Lincoln cent is shown for scale)

Silver is found in native form, combined with sulfur, arsenic, antimony, or chlorine and in various ores such as argentite (Ag₂S) and horn silver (AgCl). Another ore it is found in is pyrargyrite. The principal sources of silver are copper, copper-nickel, gold, lead and lead-zinc ores obtained from Canada, Mexico (historically Batopilas), Peru, Australia and the United States.

This metal can also be produced during the electrolytic refining of copper 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 silver and purities greater than 99.999% are available. Mexico is the world's largest silver producer. According to the Secretary of Economics of Mexico, it produced 80,120,000 troy ounces (2492 metric tons) in 2000, about 15% of the annual production of the world.

Applications

A major use of silver is as a precious metal. Sterling silver is 92.5 % silver, alloyed usually with copper. Jewelry and silverware are traditionally made from this. Silver is used in medals, denoting second place. Many high end musical instruments are made with silver, which benefit from a higher tone quality.^{[citation needed]}

The name of United Kingdom monetary unit 'Pound' originally had the value of one troy pound of sterling silver. Silver has been coined to produce money since 700 BC by the Lydians, in the form of electrum. Later, silver was refined and coined in its pure form. The words for "silver" and "money" are the same in at least 14 languages.

The largest single end use^{[citation needed]} of silver is photography, in the form of silver nitrate and silver halides are widely used in photography — 30 % of US production is used here.^[1]

Some electrical and electronic products use silver for its superior conductivity, even when tarnished. For example, printed circuits are made using silver paints,^[1] and computer keyboards use silver electrical contacts. Silver cadmium oxide is used in high voltage contacts because it can minimize arcing. Silver is also used to make solder and brazing alloys, electrical contacts, and high capacity silver-zinc and silver-cadmium batteries. Silver in a thin layer of on top of a bearing material can provide a significant increase in galling resistance and reduce wear under heavy load, particularly against steel.

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. This is most often seen in architectural glass and tinted windows on vehicles.

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)—a very important industrial reaction.

Oxygen dissolves in silver relatively easily compared to other gases present in air. Attempts have been made to construct silver membranes of only a few monolayers thickness. Such a membrane could be used to filter pure oxygen from air.

In medicine

Hippocrates, the father of modern medicine, wrote that silver had beneficial healing and anti-disease properties, and the Phoenicians 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. Silver compounds were used successfully to prevent infection in World War I before the advent of antibiotics. Silver nitrate solution was a standard of care but was largely replaced by silver sulfadiazine cream (SSD Cream)^[2] which was generally the "standard of care" for the antibacterial/antibiotic treatment of serious burns until the late 1990's. Now, other options such as silver coated dressings (activated silver dressings) are used in addition to SSD cream, and may present advantages such as pain reduction and capacity for treatment at home.

The widespread use of silver went out of fashion with the development of modern antibiotics. However, recently there has been renewed interest in silver as a broad spectrum antimicrobial. In particular, it is being used with alginate, a naturally occurring biopolymer derived from seaweed, in a range of silver alginate products designed to prevent infections as part of wound management procedures, particularly applicable to burn victims. In addition, Samsung has introduced washing machines with a final rinse containing silver ions to provide several days of antibacterial protection in the clothes.^[3] Additionally, Kohler has introduced a line of toilets that have silver ions embedded in the porcelain to kill germs. A company called Thomson Research Associates has began treating products with Ultra Fresh, an anti-microbial technology involving "proprietary nano-technology to produce the ultra-fine silver particles essential to ease of application and long-term protection."^[4]

The malleability, non-toxicity and beauty of silver make it useful in dental alloys for fittings and fillings.

Health precautions

Silver plays no known natural biological role in humans, and possible health effects of silver are a subject of dispute. 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 harm a person's health, it is disfiguring and usually permanent. Argyria is rare and mild forms are sometimes mistaken for cyanosis.

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 is normally associated with them. Its germicidal effects kill many microbial organisms in vitro (i.e. in a test tube or a petri dish). The exact process by which this is done is still not well understood, although different theories exist. One of these is a process generally known for heavy metals called the oligodynamic effect, which goes a long way explaining the effect on microbial lifeforms but does not explain certain antiviral functions.

Alternative medicine

Today, various kinds of silver compounds, or devices to make solutions or colloids containing silver, are sold as remedies for a wide variety of diseases. Although mostly harmless, some people using these home-made solutions use far too much and develop argyria over a period of months or years, and several have been documented in the last few years in the medical literature, including one possible case of coma associated with a high intake of silver (see medical references). It is strongly advised to notify a doctor when taking silver as a form of self-medication.

In food

In India, foods can be found decorated with a thin layer of silver, known as Varak. Silver as a food additive is given the E number E174 and classed as a food coloring. It is used solely for external decoration, such as on chocolate confectionery, in the covering of dragées and the decoration of sugar-coated flour confectionery. In Australia, it is banned as a food additive.

In clothing

Silver is a natural antimicrobial ingredient that has been used for years by professionals in the field of wound healing, mainly because of its scientifically proven antimicrobial efficacy. Silver inhibits the growth of bacteria and of fungus. It keeps odour to a minimum and reduces the risk of bacterial and fungal infection. In clothing the combination of silver and moisture movement (wicking) is the best combination to reduce the harmful effects of prolonged use in active and humid conditions.

Silver is used in clothing in two main forms: • where silver ions are integrated into the polymer from which yarns are made (nano-technology) • where the silver is physically coated onto the yarns. In both cases the silver prevents the growth of a broad spectrum of bacteria and fungi. At the same time, silver is a very skin-friendly and highly compatible agent to which – unlike many antibiotics – bacteria rarely build up resistance.

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 while storing it, but 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. Today silver containing wound dressings are well established for clinical wound care and have recently been introduced in consumer products such as sticking plasters.

“In comparison to other antiseptics, silver has its advantage of being effective on a broad range of germs even in small concentrations and over a long period of time. And this being non-toxic at the same time." Prof. Dr. med. Heinz-Peter Werner; Microbiologist

Skinlife (tm) is a trademarked nano-technology; Xstatic is a trademarked yarn application. Silver is used in clothing by Rohan Designs in broad travel clothing applications including shoes and by Bridgedale (amongst others) in socks.

Other silver compounds

Silver sulfide, also known as Silver Whiskers, is formed when silver electrical contacts are used in an atmosphere rich in hydrogen sulfide.
Silver fulminate is a powerful explosive.
Silver chloride can be made transparent and is used as a cement for glass.
Silver chloride is a widely used electrode for pH testing and potentiometric measurement.
Silver iodide has been used in attempts to seed clouds to produce rain.
Silver oxide is used as a positive electrode (cathode) in watch batteries.

Price

Silver is currently about 1/50th the price of gold by mass, and approximately 70 times more valuable than copper. Silver did once trade 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, most notably the vast Comstock Lode in Virginia City, Nevada, USA. This then resulted in the debate over cheap Free Silver to benefit the agricultural sector was among the most prolongued and difficult in that country's history and dominated public discourse during the latter decades of the nineteenth century.

Over the last 100 years the price of silver and the gold/silver price ratio has 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. By December 2001 the price had fallen to US$4.15 per ounce, and in May 2006 it had risen back as high as US$15.21 per ounce. As of 2006, current silver prices (and most other metal prices) have been rather volatile, for example quickly dropping from the May high of US$15.21 per ounce to a June low of US$9.60 per ounce before rising back above US$12 per ounce by August.^[5]

Isotopes

Naturally occurring silver is composed of the two stable isotopes ¹⁰⁷Ag and ¹⁰⁹Ag with ¹⁰⁷Ag being the more abundant (51.839% natural abundance). Twenty-eight (28) radioisotopes have been characterised with 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.

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. This element has numerous meta states with the most stable being ^108mAg (t_* 418 years), ^110mAg (t_* 249.79 days) and ^106mAg (t_* 8.28 days).

Isotopes of silver range in atomic weight from 93.943 u (⁹⁴Ag) to 123.929 u (¹²⁴Ag). 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/silver ratio to yield measurable variations in ¹⁰⁷Ag abundance. Radiogenic ¹⁰⁷Ag was first discovered in the Santa Clara 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 versus Ag correlations observed in bodies, which have clearly been melted since the accretion of the solar system, must reflect the presence of live short-lived nuclides in the early solar system.

Folklore and mass culture

Because of the mysticism surrounding silver's lunar associations, as well as the aesthetic qualities of the white, reflective metal that cause it to be associated with purity, silver in European Folklore has long been traditionally believed to be an antidote to various maladies and fictional monsters. Notably, silver was believed to be a repellant against vampires (this primarily originates from its holy connotations; also, mirrors were originally polished silver, and as such, vampires allegedly cannot be seen in them because they are wicked) and it was believed that a werewolf, in his bestial form, could only be killed by a weapon or bullet made of silver, and was equally effective against vampires, as described in Eastern European folklore. This has given rise to the term "silver bullet," which is used to describe things that very effectively deal with one specific problem.

The Lone Ranger of radio serials, comic strips, and some TV programs leaves a silver bullet as a calling card.

In Christian doctrine, Christ's disciple Judas Iscariot is infamous for having, according to the New Testament, taken a bribe of thirty pieces of silver from religious leaders in Jerusalem to turn Jesus Christ over to the Romans.

In heraldry, the argent, in addition to being shown as silver (this has been shown at times with real silver in official representations), can also be shown as white. Occasionally, the word "silver" is used rather than argent; sometimes this is done across-the-board, sometimes to avoid repetition of the word "argent" in blazon.

References

^ ^a ^b Los Alamos National Laboratory, Silver, accessed 12 Feb 2007.
^ Te-Wen Chang and Louis Weinstein, "Prevention of Herpes Keratoconjunctivitis in Rabbits By Silver Sulfadiazine" Antimicrob Agents Chemother. 1975 December; 8(6): 677–678. PMID 429446
^ "Samsung laundry featuring SilverCare Technology". Samsung. Retrieved 2006-05-20.
^ "'Ultra-Fresh technology is based on the power of silver to fight bacteria'". Express Textile.
^ "Silver Cash daily plot". Barchart.com.

External links

WebElements.com – Silver
Silver Statistics and Information, USGS publications on the worldwide production of silver
The Silver Institute A silver industry website
A collection of silver items Samples of silver

Template:Jewellery Materials

[lanl-1] Los Alamos National Laboratory, Silver, accessed 12 Feb 2007.

[2] Te-Wen Chang and Louis Weinstein, "Prevention of Herpes Keratoconjunctivitis in Rabbits By Silver Sulfadiazine" Antimicrob Agents Chemother. 1975 December; 8(6): 677–678. PMID 429446

[3] "Samsung laundry featuring SilverCare Technology". Samsung. Retrieved 2006-05-20.

[4] "'Ultra-Fresh technology is based on the power of silver to fight bacteria'". Express Textile.

[5] "Silver Cash daily plot". Barchart.com.

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