Silver

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

Silver is a chemical element with the symbol Ag (from the traditional abbreviation for the Latin argentum). A soft white lustrous transition metal, it has the highest electrical and thermal conductivity of any metal and occurs in minerals and in free form. This metal is used in coins, jewelry, tableware, photography, and in mirrors.

Notable characteristics

Silver is a very ductile and malleable (slightly harder than gold) univalent coinage metal with a brilliant white metallic lustre 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.

Pure silver also has the highest thermal conductivity, whitest color, the highest optical reflectivity (although it is a poor reflector of ultraviolet light), and 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 with sulfur in it. 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.

Applications

The principal use of silver is as a precious metal. Its salts, especially silver nitrate and silver halides, are also widely used in photography (which is the largest single end use of silver). Some other uses for silver are as follows:

Electrical and electronic products, which need silver's superior conductivity, even when tarnished. For example, printed circuits are made using silver paints, and computer keyboards use silver electrical contacts. Silver is also used in high voltage contacts because it is the only metal that will not arc across contacts, hence it is extremely safe.
Mirrors which need superior reflectivity for visible light are made with silver as the reflecting material in a process called silvering. Common mirrors are backed with aluminium.
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 metal is chosen for its beauty in the manufacture of jewelry and silverware, which are traditionally made from the silver alloy known as Sterling silver, which is 92.5% silver and usually alloyed with copper. The name of United Kingdom monetary unit 'Pound' originally had the value of one troy pound of sterling silver.
Silver is also used in medals and represents second.
The malleability, non-toxicity and beauty of silver make it useful in dental alloys for fittings and fillings.
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.
Used to make solder and brazing alloys, electrical contacts, and high capacity silver-zinc and silver-cadmium batteries.
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 also 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.
Colloidal silver is a possible antibacterial / antibiotic treatment that requires. further clinical testing to support actual efficacy.
Silver nitrate (liquid) and silver sulfadiazine cream (SSD Cream) were the "standard of care" for the antibacterial/antibiotic treatment of serious burns until the late 1990's. Now, Acticoat Burn Dressings (activated silver dressings) have largely replaced those earlier treatments.
Silver is often used in modern horror fiction as a weapon against certain supernatural entities, especially werewolves, who are sometimes described as being burned by silver.

History

Silver (from 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 many monetary systems. Its value as a precious metal was long considered second only to gold. In Ancient Egypt and Medieval Europe, it was often more valuable than gold.

Associated with the moon, as well as with the sea and various lunar goddesses, the metal was referred to by alchemists by the name luna. One of the alchemical symbols for silver is a crescent moon with the open part on the left (see picture, right).

The metal mercury was thought of as a kind of silver, though the two elements are chemically unrelated; its Latin and English names, hydrargyrum ("watery silver") and quicksilver, respectively, reflect this history.

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 been 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.

Europeans found a huge amount of silver in the New World in Zacatecas and Potosí, which triggered a period of inflation in Europe. The conquistador Pizarro was said to have resorted to having his horses shod with silver horseshoes due to the metal's abundance, in contrast to the relative lack of iron in Peru. Silver, which was extremely valuable in China, became a global commodity, contributing to the rise of the Spanish Empire. The rise and fall of its value affected the world market.

The Rio de la Plata was named after silver (in Spanish, plata), and in turn lent the meaning of its name to Argentina.

Silver mining was a driving force in the settlement of western North America, with major booms for silver and associated minerals (lead, mostly) in the galena ore silver is most commonly found in. Notable "silver rushes" were in Colorado, Nevada, Cobalt, Ontario , California and the Kootenay region of British Columbia, notably in the Boundary and "Silvery Slocan". The largest silver ore deposits in the United States were discovered at the Comstock Lode in Virginia City, Nevada, in 1859.

Occurrence

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). The principal sources of silver are copper, copper-nickel, gold, lead and lead-zinc ores obtained from Canada, Cobalt, Ontario, Mexico (historically Batopilas), Peru, Australia and the United States.

This metal is also produced during the electrolytic refining of copper. 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.

Price

Silver is currently about 1/45th the price of gold by weight, and approximately 70 times more valuable than copper. Silver did once trade at 1/6th to 1/7th the price of gold, prior to the Age of Discovery and the finding of large silver deposits in the Americas.

Over the last 100 years the price of silver and the gold/silver 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 $49.45 per troy ounce. By December 2001 the price had fallen to $4.15 per ounce, and in May 2006 it had risen to as high as $15.16 per ounce and is currently $9.64 per ounce. [http://www.kitco.com/market/.

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 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-lifes that are less than an hour and the majority of these have half lifes that are less than 3 minutes. This element also has numerous meta states with the most stable being ^128mAg (t_* 418 years), ^110mAg (t_* 249.79 days) and ^107mAg (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 Pd/Ag 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.

Silver 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.

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.

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 of its 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 kills 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 several 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 India, foods can be found decorated with a thin layer of silver, known as Varak.

Notes

References

Los Alamos National Laboratory – Silver

External links

WebElements.com – Silver
Silver Statistics and Information, USGS publications on the worldwide production of silver
Today's Silver Price Plain and simple site to get the spot price of silver
The Silver Institute A silver industry website
A collection of silver items Samples of silver

Publications on health effects

Chemical elements | Transition metals | Precious metals | Silver

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