Silver (revised)
SILVER (REVISED)
Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Overview
Chemists classify silver as a transition metal. The transition metals are elements between Groups 2 and 13 in the periodic table. The periodic table is a chart that shows how chemical elements are related to one another. More than 40 elements, all metals, fall within the transition metal range.
Silver is also classified as a precious metal. Precious metals are not very abundant in the Earth's crust. They are attractive and not very chemically active. These properties make the metal desirable in jewelry, coins, and art. About a half dozen metals near silver in the periodic table are also precious metals. These include gold, platinum, palladium, rhodium, and indium.
Silver has been used by humans for thousands of years. It often occurs as a free element in nature. It can also be extracted from its ores fairly easy. These properties made it easy for early humans to learn about silver.
SYMBOL
Ag
ATOMIC NUMBER
47
ATOMIC MASS
107.868
FAMILY
Group 11 (IB)
Transition metal
PRONUNCIATION
SIL-ver
Today, the most important use of silver is in photography. Three silver compounds used in photography are silver chloride (AgCl), silver bromide (AgBr), and silver iodide (AgI). Silver is also used to make electrical equipment, mirrors, medical and dental equipment, and jewelry. It is often used to make alloys with gold for some of these applications. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals.
Discovery and naming
Silver was probably first discovered after gold and copper. Gold and copper often occur as free elements in nature. They have very distinctive colors, which made it easy for early humans to find these metals.
Silver also occurs as a free metal, but much less often than gold or copper. At some point, humans learned to extract silver from its ores. But that discovery must have occurred very early on in human history. Archaeologists (scientists who study ancient civilizations) have found silver objects dating to about 3400 b.c. in Egypt. Drawings on some of the oldest pyramids show men working with metal, probably extracting silver from its ores.
Other early cultures also used silver. Written records from India describe the metal as far back as about 900 b.c. Silver was in common use in the Americas when Europeans first arrived.
The Bible contains many references to silver. The metal was used as a way of paying for objects. It also decorated temples, palaces, and other important buildings. The Bible also contains sections that describe the manufacture of silver.
The word silver goes back to at least the 12th century, a.d. It seems to have come from an old English word used to describe the metal, seolfor. The symbol for silver (Ag), however, comes from its Latin name, argentum. The name may have originated from the Greek word argos, meaning "shiny" or "white."
Physical properties
Silver is a soft, white metal with a shiny surface. It is the most ductile and most malleable metal. Ductile means capable of being drawn into thin wires. Malleable means capable of being hammered into thin sheets. Silver has two other unique properties. It conducts heat and electricity better than any other element. It also reflects light very well.
Silver's melting point is 961.5°C (1,762°F) and its boiling point is about 2,000 to 2,200°C (3,600 to 4,000°F). Its density is 10.49 grams per cubic centimeter.
Drawings on some of the oldest pyramids show men working with metal, probably extracting silver from its ores.
Chemical properties
Silver is a very inactive metal. It does not react with oxygen in the air under normal circumstances. It does react slowly with sulfur compounds in the air, however. The product of this reaction is silver sulfide (Ag2S), a black compound. The tarnish that develops over time on silverware and other silver-plated objects is silver sulfide.
Silver does not react readily with water, acids, or many other compounds. It does not burn except as silver powder.
Occurrence in nature
Silver is a fairly rare element in the Earth's crust. Its abundance is estimated to be about 0.1 parts per million. It is also found in seawater. Its abundance there is thought to be about 0.01 parts per million.
Silver usually occurs in association with other metal ores, especially those of lead . The most common silver ores are argentite (Ag2S); cerargyrite, or "horn silver" (AgCl); proustite (3Ag2S ○ As2S3); and pyrargyrite (Ag2S ○ Sb2S3).
The largest producers of silver in the world are Mexico, Peru, the United States, Canada, Poland, Chile, and Australia. In the United States, silver is produced at about 76 mines in 16 states. The largest state producers are Nevada, Idaho, and Arizona. These three states account for about two-thirds of all the silver mined in the United States.
Isotopes
Two naturally occurring isotopes of silver exist: silver-107 and silver-109. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
About 16 radioactive isotopes of silver are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
None of the radioactive isotopes of silver has any commercial use.
The tarnish that develops over time on silverware and other silver-plated objects is silver sulfide.
Extraction
Ores rich in silver disappeared long ago due to mining. Today, silver usually comes from ores that contain very small amounts of the metal. These amounts can range from about a few thousandths of an ounce per ton of ore to 100 ounces per ton. The metal is most commonly produced as a by-product of mining for other metals. After the primary metal has been removed, the waste often contains small amounts of silver. These wastes are treated with chemicals that react with the silver. The silver can then be extracted by electrolysis. Electrolysis is a process by which a compound is broken down by passing an electric current through it.
Uses and compounds
About 10 percent of silver produced in the United States is used in coins, jewelry, and artwork. One way silver is used is in alloys with gold. Gold is highly desired for coins and jewelry. But it is much too soft to use in its pure form. Adding silver to gold, however, makes an alloy that is much stronger and longer lasting. Most "gold" objects today are actually alloys, often alloys of silver and gold.
Other objects use much more of the silver metal, however. About half of the silver produced in the United States goes into photographic film. Pure silver is first converted to a compound: silver chloride, silver bromide, or silver iodide. The compound is then used to make photographic film (see accompanying sidebar).
The second most important use of silver is in electrical and electronic equipment. About 20 percent of all silver produced is used for this purpose. Silver is actually the most desirable of all metals for electrical equipment. Electricity flows through silver more easily than it does through any other metal. In most cases, however, metals such as copper or aluminum are used because they are less expensive.
Silver's important role in film
T aking a photograph depends on a simple chemical idea: Light can cause electrons to move around. Here is what that means:
Silver metal will combine with chlorine, bromine, or iodine to form compounds. As an example:
In this reaction, each silver atom loses one electron to a chlorine atom. The silver atom becomes "one electron short" of what it usually has. The one-electron-short silver atom is called a silver ion.
Photographic film is coated with a thin layer of silver chloride, silver bromide, or silver iodide. That means the film is covered with many silver ions. Silver ions are colorless, so photographic film has no color to it.
What happens when photographic film is exposed to light? Light gives energy to electrons in the photographic film. Some of these electrons find their way back to silver ions, transforming them back to atoms:
But silver atoms are not colorless. They are black. So, a photographic film exposed to light turns black at every point where light strikes a silver ion.
In taking a picture, of course, not all of the film gets equal amounts of light. A picture of a person, for example, will have areas that get much more light than others. So some places on the film become very dark, and other places become less dark.
Additional steps are necessary to "develop" photographic film or to produce a picture from it. But the first step in taking a photograph is changing silver ions back to silver atoms with light.
But sometimes, an electrical device is so important that cost is not a consideration. For example, electrical devices on spacecraft, satellites, and aircraft must work reliably and efficiently. The cost of using silver is not as important as it would be in a home appliance. Thus, silver is used for electrical wiring and connections in these devices.
In some cases, silver plating solves a practical problem where the more expensive silver would work best. Silver plating is the process by which a very thin layer of silver metal is laid down on top of another metal. Silver is so malleable that it can be hammered into sheets thinner than a sheet of paper. Silver this thin can be applied to another metal. Then the other metal takes on some of the properties of the silver coating. For example, it may work very well as a reflector because silver is such a good reflector. It does not matter if the second metal is a good reflector or not. The silver coating serves as the reflecting surface in the combination.
About a fifth of all silver produced is used in a variety of other products. For example, it is often used in dental amalgams. An amalgam is an alloy in which mercury is one of the metals used. Silver amalgams work well for filling decayed teeth. They are non-toxic and do not break down or react with other materials very readily. Silver is also used in specialized batteries, including silver-zinc and silver-cadmium batteries.
Electricity flows through silver more easily than it does through any other metal.
Health effects
Silver is a mildly toxic element. When the metal or its compounds get on the skin, they can cause a bluish appearance known as argyria or argyrosis. Breathing in silver dust can have serious long-term health effects also. The highest recommended exposure for silver dust is 0.1 milligrams per cubic meter of air.
Silver
Silver
Background
Silver was one of the earliest metals known to humans, and it has been considered a precious metal since ancient times. Silver has been used as a form of currency by more people throughout history than any other metal, even gold. Although it is usually found in ores with less rare metals, such as copper, lead, and zinc, silver was apparently discovered in nugget form, called native silver, about 4000 b.c. Silver utensils and ornaments have been found in ancient tombs of Chaldea, Mesopotamia, Egypt, China, Persia, and Greece. In more recent times, the principal uses for silver were coinage and silverware.
In 1993, worldwide production of silver from mines totaled 548.2 million ounces (15.5 billion grams). During that year, Mexico was the world's largest producer of silver, with a total production of 75.7 million ounces (2.1 billion grams). The United States was the second leading producer, followed by Canada, Australia, Spain, Peru, and Russia. The vast majority of the world's silver is used in industrial applications, and the United States is the leading consumer. Other top consumers include Japan, India, and eastern European countries.
Silver mining in North America dates back to the eighteenth century. Around 1800, production began in the United States on the east coast and then moved west. The mining of silver was instrumental in the settlement of the state of Nevada. In 1994, Nevada was the largest producer of silver in the United States; Nevada mines produced 22.8 million troy ounces (709 million grams) of silver. Arizona, California, and Nevada are known for large-tonnage, low-grade silver deposits.
Physical Characteristics and Uses of Silver
Silver is the whitest metallic element. It is rare, strong, corrosion resistant, and unaffected by moisture, vegetable acids, or alkalis. Silver is also resonant, moldable, malleable, and possesses the highest thermal and electric conductivity of any substance. The chemical symbol for silver is Ag, from the Latin argentum, which means white and shining. Although silver does not react to many chemicals, it does react with sulfur, which is always present in the air, even in trace amounts. The reaction causes silver to tarnish, therefore, it must be polished periodically to retain its luster.
Silver possesses many special physical characteristics and qualities that make it useful in a variety of industries. The photography industry is the biggest user of silver compounds. Silver forms the most light-sensitive salts, or halides, which are essential to developing high-quality photography. Silver has the highest electrical conductivity per unit volume of any metal, including copper, so it is used extensively in electronics. Specialized uses include switch and relay contacts for automobile controls and accessories, automotive window heating, and in electrodes for electrocardiograms.
Silver is one of the strongest oxidants, making it an essential catalyst for the chemical process industry. It is used in the production of adhesives, dinnerware, mylar recording tape, and many other products. Silver is the most reflective of all metals, and is used to coat glass in mirrors. It is also used in x-ray vacuum tubes and as material for bearings. With the highest level of thermal conductivity among metals and resistance to combustion and sparks, silver is a valuable material for a range of other industrial processes. The most common consumer application of silver is its use in jewelry. Pure silver, which would be too soft to be durable, is mixed with 5-20% copper in an alloy known as sterling silver.
Today, a very small percentage of the world's silver is used in coinage, though silver coins were a popular form of currency until the recent past. As industrialized nations began to produce large numbers of silver coins in the twentieth century, silver became less available, and therefore more expensive. The United States Treasury, which until then had been minting 90% silver coins, changed their minting by a 1965 act of Congress. The Johnson Silver Coinage Act completely demonetized silver, and with the exception of bicentennial coins, all newly-minted United States coins are now made of an alloy of copper and nickel.
The Manufacturing
Process
Silver was first obtained in sixteenth-century Mexico by a method called the patio process. It involved mixing silver ore, salt, copper sulphide, and water. The resultant silver chloride was then picked up by adding mercury. This inefficient method was superseded by the von Patera process. In this process, ore was heated with rock salt, producing silver chloride, which was leached out with sodium hyposulfite. Today, there are several processes used to extract silver from ores.
A method called the cyanide, or heap leach, process has gained acceptance within the mining industry because it is a low-cost way of processing lower-grade silver ores. However, the ores used in this method must have certain characteristics: the silver particles must be small; the silver must react with cyanide solutions; the silver ores must be relatively free of other mineral contaminants and/or foreign substances that might interfere with the cyanidation process; and the silver must be free from sulfide minerals. The idea for cyanidation actually dates back to the eighteenth century, when Spanish miners percolated acid solutions through large heaps of copper oxide ore. The process developed into its present form during the late nineteenth century. The cyanide process is described here.
Preparing the ore
- 1 Silver ore is crushed into pieces, usually with 1-1.5 in (2.5-3.75 cm) diameters, to make the material porous. Approximately 3-5 lb (1.4-2.3 kg) of lime per ton of silver ore is added to create an alkaline environment. The ore must be completely oxidized so the precious metal is not confined in sulfide minerals. Where fines or clays exist, the ore is agglomerated to create a uniform leach pile. This process consists of crushing the ore, adding cement, mixing, adding water or a cyanide solution, and curing in dry air for 24-48 hours.
- 2 Broken or crushed ore is stacked on impermeable pads to eliminate the loss of the silver cyanide solution. Pad material may be asphalt, plastic, rubber sheeting, and/or clays. These pads are sloped in two directions to facilitate drainage and the collection of the solutions.
Adding the cyanide solution and curing
- 3 A solution of water and sodium cyanide is added to the ore. Solutions are delivered to the heaps by sprinkler systems or methods of ponding, including ditches, injection, or seepage from capillaries.
Recovering the silver
- 4 Silver is recovered from heap leach solutions in one of several ways. Most common is Merrill-Crowe precipitation, which uses fine zinc dust to precipitate the precious metal from the solution. The silver precipitate is then filtered off, melted, and made into bullion bars.
- 5 Other methods of recovery are activated carbon absorption, where solutions are pumped through tanks or towers containing activated carbon, and the addition of a sodium sulfide solution, which forms a silver precipitate. In another method, the solution is passed through charged resin materials which attract the silver. The recovery method is generally decided based on economic factors.
Silver is rarely found alone, but mostly in ores which also contain lead, copper, gold, and other metals which may be commercially valuable. Silver emerges as a byproduct of processing these metals. To recover silver from zinc-bearing ores, the Parkes process is used. In this method, the ore is heated until it becomes molten. As the mixture of metals is allowed to cool, a crust of zinc and silver forms on the surface. The crust is removed, and the metals undergo a distillation process to remove the zinc from the silver.
To extract silver from copper-containing ores, an electrolytic refining process is used. The ore is placed in an electrolytic cell, which contains a positive electrode, or anode, and a negative electrode, or cathode, in an electrolyte solution. When electricity is passed through the solution, silver, with other metals, accumulates as a slime at the anode while copper is deposited on the cathode. The slimes are collected, then roasted, leached, and smelted to remove impurities. The metals are formed into blocks which are used as anodes in another round of electrolysis. As electricity is sent through a solution of silver nitrate, pure silver is deposited onto the cathode.
The Future
How much silver will be produced in the future depends on many factors, including the rate of production of other metals and future uses of silver. Industrial demand for silver appears to be steady overall. Because silver naturally occurs with other metals, future production is linked to the production of copper, lead, gold, and zinc.
In the future, silver will likely continue to be used for special industrial applications, as well as for consumer items, such as jewelry and silverware. In addition to these traditional uses, the value of silver will also depend on new uses for the metal. For example, using silver as a sanitizing agent is currently under development. Manufacturers have hustled in response to studies by the Atlanta-based Center for Disease Control that many viruses, including those linked to Acquired Immunodeficiency Syndrome (AIDS), will survive briefly outside an individual in fluids deposited on surfaces of plastic products, such as telephones. Matsushita Electric Industrial Co., Ltd. in Osaka, Japan, completed a project at the Research Institute for Microbial Diseases, Osaka University, to produce a surface treatment that provides long-lasting sanitization for its plastic products. Research revealed the most effective system to be a compound based on silver thiosulfate.
Currently marketed under the name Amenitop, the system consists of silica gel microspheres that contain a silver thiosulfate complex. The silica gel coating allows a gradual release of the silver compound onto the surface, which provides long-lasting sanitization. Studies suggest that Amenitop kills bacteria and viruses by destroying the cell's membranes.
Where to Learn More
Books
Coombs, Charles. Gold and Other Precious Metals. William Morrow and Company, 1981.
Robbins, Peter and Douglass Lee. Guide to Precious Metals and Their Markets. Nichols Publishing, 1979.
Smith, Jerome F. and Barbara Kelly Smith. What's Behind the New Boom In Silver and How to Maximize Your Profits. Griffin Publishing Company, 1983.
Periodicals
"A Listing of the End Users of Silver By Property." The Silver Institute, January 13, 1994, pp. 1-14.
"A Nevada Leader." News from Las Vegas (Las Vegas News Bureau), March 1995, p. 1.
"New Silver Compound To Fight Spread of Viruses." The Silver Institute Letter, December 1994-January 1995, pp. 1, 2,4.
Thorstad, Linda E. "How Heap Leaching Changed The West." World Investment News, February 1987, pp. 31, 33.
—SusanBardHall
Silver
Silver
melting point: 961°C
boiling point: 2,212°C
density: 10.53 g/cm3
most common ions: Ag+, Ag2+
Silver is a precious metal and (like gold and copper) is classified as a coinage metal. The date of its discovery is not known, but it has been identified in jewelry, coins, and religious ornaments dating to more than 2,000 years ago from ancient civilizations in South America, Egypt, Mesopotamia, and China.
Silver exists as two isotopes , 107Ag and 109Ag, occurring in similar proportions. It exhibits three valance/oxidation states: Ag(I), Ag(II), and Ag(III). The chemistry of silver was not well-known before 1980, although silver nitrate was used medicinally in the 1800s. Recent research has recognized the highly reactive nature of the silver ion and its ability to form numerous inorganic and organic complexes (halide, sulfide, nitrate, oxide, and acetylide compounds, cyano-derivatives, olefin complexes, etc.). Ag(II) complexes are less stable than those of Ag(I) and Ag(III), but unlike many other silver compounds are brightly colored red or blue. Silver ion binds readily to proteins in the human body (including albumins and metallothioneins ) and interacts with trace metals in metabolic pathways.
Silver is the sixty-third most abundant metal in Earth's crust; the average concentration of silver in water is 0.5 ppb, in soil it is 10 ppb. It is found naturally as native metal or in ores in which it is complexed with lead, copper, tellurium, mercury, arsenic , or antimony. Silver is found mainly throughout the Americas, Japan, Australia, and central Europe. Extraction is by amalgamation and displacement (using mercury), solution, or smelting methods.
Silver is used in the manufacture of photographic film chemicals such as silver nitrate, as an analytic reagent in organic chemistry, as a catalyst in photo-oxidation reactions, in electrochemical reactions, in nuclear magnetic resonance and analyses, and in silver plating.
Medically, silver is used in the manufacture of bone prostheses, cardiac implants and replacement valves, needles used in ocular surgery, peritoneal catheters, and wound sutures. It is an antiseptic ingredient used in wound management (silver nitrate, siler sulfadiazine, and cerium nitrate). A new generation of sustained silver release products is showing promise in the treatment of skin wounds, skin ulcers, and burns; silver ion is released from the dressings (Acticoat™, Actisorb™, etc.) in the presence of wound fluids, exudates, and is "activated." Activated silver ion is toxic to bacteria and yeasts. Silver is used in the making of dental amalgam fillings. Silver exhibits widespectrum antibacterial activity; it is toxic to bacteria at low concentrations (10−5 to 10−7 Ag ions per cell), the so-called oligodynamic effect.
Silver is toxic. Silver nitrate used in antiseptic mouthwashes leads to deposits of silver sulfide in the skin and a slate-gray skin discoloration (known as argyria); argyria is also caused by the colloidal silver used in antiseptics and mouthwashes. Silver absorbed through the skin is deposited in the liver and kidney and complexes with albumin and cellular proteins. Another silver hazard is allergy to silver, occurring as a result of the silver that is used in jewelry, medicinal products, coins, and antiseptics.
Alan B. G. Lansdown
Bibliography
Buckley, W. R.; Oster, C. F.; and Fassett, D. W. (1965). "Localised Argyria: Chemical Nature of the Silver Containing Granules." Archives of Dermatology 92: 697–705.
Lansdown, Alan B. G. (2002). "Silver," Parts 1 and 2. Journal of Wound Care 10: 125–130; 173–177.
Lansdown, Alan B. G.; Myers, S. R.; Clarke, J. A.; et al. (2003). "A Reappraisal of the Role of Cerium in Burn Wound Management." Journal of Wound Care, 12/3: 113–118.
Lowbury, Edmund J. L. (1992). "Special Problems in Hospital Antisepsis." In Principles and Practice of Disinfection, Preservation and Sterilisation, ed. A. D. Russell, W.B. Hugo, and G. A. G. Aycliffe. London: Blackwell Scientific Publications.
Mietzner, S.; Schwille, R. C.; Farley, A.; et al. (1997). "Efficiency of Thermal Treatment and Copper Silver Ionization for Control of Legionnella pneumophila in High Volume Hot Water Plumbing Systems in Hospitals." American Journal of Infection Control 25: 452–457.
Patai, Paul, and Rappaport, Zvi, eds. (1999). The Chemistry of Organic Derivatives of Gold and Silver. Chichester, UK: Wiley.
Thompson, N. R. (1973). "Silver." In Comprehensive Inorganic Chemistry, ed. J. C. Bailar; H. J. Emelius; R. Nyholm; et al. London: Pergamon Press.
silver
sil·ver / ˈsilvər/ • n. 1. a precious shiny grayish-white metal, the chemical element of atomic number 47. (Symbol: Ag) 2. a shiny gray-white color or appearance like that of silver: the dark hair was now highlighted with silver.3. silver dishes, containers, or cutlery: thieves stole $5,000 worth of silver the family silver. ∎ household cutlery of any material: it is important to wash table silver in hot soapy water immediately after each meal.4. coins made from silver or from a metal that resembles silver. ∎ chiefly Scot. money.5. short for silver medal.• adj. made wholly or chiefly of silver: silver jewelry. ∎ colored like silver: a silver Mercedes. ∎ denoting a twenty-fifth anniversary.• v. [tr.] [often as adj.] (silvered) coat or plate with silver: large silvered candlesticks. ∎ provide (mirror glass) with a backing of a silver-colored material in order to make it reflective. ∎ poetic/lit. (esp. of the moon) give a silvery appearance to: the brilliant moon silvered the turf. ∎ turn (a person's hair) gray or white. ∎ [intr.] (of a person's hair) turn gray or white.PHRASES: be born with a silver spoon in one's mouth be born into a wealthy family of high social standing.the silver screen the movie industry; movies collectively: stars of the silver screen.
silver
silver age in classical Greek and Roman literature, the second age of the world, inferior to the simplicity and happiness of the first or golden age; in general use, a period regarded as notable but inferior to a golden age, such as that of so-called silver Latin literature.
silver bullet a simple and seemingly magical solution to a complicated problem, from the traditional belief that only a bullet made of silver could kill a werewolf.
silver cord used in biblical allusion to Ecclesiastes 12:6 to indicate the dissolution of life.
silver-fork designating a school of novelists of about 1830 distinguished by an affectation of gentility.
Silver Latin literary Latin from the death of Augustus (ad 14) to the mid second century.
silver screen the cinema; originally, a cinematographic projection screen covered with metallic paint to produce a highly reflective silver-coloured surface; the term is recorded from the 1920s.
Silver Star a decoration for gallantry awarded to members of the US Army and Navy.
Silver State an informal name for Nevada, referring to its silver mines.
Silver Stick in the UK, (the bearer of) a silver-tipped rod borne on State occasions by a particular officer of the Life Guards or their successors the Household Cavalry Regiment.
silver wedding the twenty-fifth anniversary of a wedding.
See also born with a silver spoon in one's mouth, every cloud has a silver lining, cross someone's palm with silver at cross3, selling off the family silver, thirty pieces of silver.