..... Click the link for more information. . It is not a chemically active metal, being considerably below hydrogen in the electromotive series (see metal metal, chemical element displaying certain properties by which it is normally distinguished from a nonmetal, notably its metallic luster, the capacity to lose electrons and form a positive ion, and the ability to conduct heat and electricity.
..... Click the link for more information. ). It is, however, attacked by nitric acid (forming the nitrate) and by hot concentrated sulfuric acid. Silver is almost always monovalent in its compounds, but an oxide, a fluoride, and a sulfide of divalent silver are known. It does not oxidize in air but reacts with the hydrogen sulfide present in the air, forming silver sulfide (tarnish). Silver nitrate silver nitrate , chemical compound, AgNO3, a colorless crystalline material that is very soluble in water. The most important compound of silver, it is used in the preparation of silver salts for photography, in chemical analysis, in silver plating, in
..... Click the link for more information. is the most important compound. Silver chloride, bromide, and iodide are used in still photography photography, still, science and art of making permanent images on light-sensitive materials.
See also photographic processing; motion picture photography; motion pictures.
..... Click the link for more information. because of their sensitivity to light. Solutions of certain protein complexes containing silver are used as antiseptics. A mirror can be made by coating glass with metallic silver derived from the reaction of a solution of a silver ammonia complex with an organic reducing agent such as formaldehyde. Although silver can be found uncombined in nature, most silver used today is obtained from its ores. Among these the most important are argentite or silver glance (silver sulfide), which is found associated with other metal sulfides, e.g., galena; horn silver or cerargyrite (silver chloride); two ores composed of silver and antimony (in different proportions) called pyrargyrite (or ruby silver ore) and stephanite; and another ore composed of silver and arsenic sulfides called proustite. Mexico, the United States (Idaho, Montana, Arizona, Colorado, Utah, Nevada, California, New Mexico, and Texas), the former USSR, Peru, Australia, and Canada are the leading producers. The metal is prepared in various ways depending upon the nature of its occurrence; the greatest quantity is obtained in connection with the refining of lead and copper. It is separated from lead by the Parkes process, which is based upon the fact that silver is soluble in molten zinc whereas lead is not. The cyanide process cyanide process or cyanidation, method for extracting gold from its ore. The ore is first finely ground and may be concentrated by flotation; if it contains certain impurities, it may be roasted.
..... Click the link for more information. has largely replaced an amalgam amalgam , alloy containing mercury. The alloy may be liquid or solid, depending on the proportion of mercury, although all naturally occurring amalgams, i.e., those of gold and silver, are solid. Amalgams are widely used.
..... Click the link for more information. process in which silver is dissolved in mercury. Some of the silver produced today is used, as in the past, in making coins (see coin coin, piece of metal, usually a disk of gold, silver, nickel, bronze, copper, aluminum, or a combination of such metals, stamped by authority of a government as a guarantee of its real or exchange value and used as money.
..... Click the link for more information. ; money money, term that actually refers to two concepts: the abstract unit of account in terms of which the value of goods, services, and obligations can be compared; and anything that is widely established as a means of payment.
..... Click the link for more information. ; bimetallism bimetallism , in economic history, monetary system in which two commodities, usually gold and silver, were used as a standard and coined without limit at a ratio fixed by legislation that also designated both of them as legally acceptable for all payments.
..... Click the link for more information. ). Large quantities are used for silver utensils and jewelry, and in plating tableware electrolytically from a solution of sodium silver cyanide. Alloys of silver with copper, in which the copper adds hardness, are important. Coin silver is an alloy consisting of 90% silver and 10% copper. Sterling silver contains 92.5% silver and 7.5% copper. Silver alloys are used in dental amalgams and for electrical contacts. Silver was one of the first metals to be used by humans (see silverwork silverwork, utilitarian objects and works of art created from silver. Silverwork includes ecclesiastical and domestic plate, flatware, jewelry, buttons, buckles, boxes, toilet articles, weapons, furniture, and horse trappings.
..... Click the link for more information. ).
silver
)
silver
silver [′sil·vər]
Silver
Ag, a chemical element in group I of Mendeleev’s periodic system. Atomic number, 47; atomic weight, 107.868; a white, ductile metal that can be polished to a brilliant luster. Silver occurs naturally as a mixture of the two stable isotopes 107Ag and 109Ag. Of the radioisotopes, 110Ag has the most practical value (T½ = 253 days). Silver was known as early as the fourth millennium B.C. in Egypt, Persia, and China.
Distribution in nature. The average silver content in the earth’s crust (clarke) is 7 × 10-6 percent by weight. It occurs primarily in medium- and low-temperature hydrothermal deposits, in the enrichment zone of sulfide deposits, and, occasionally, in sedimentary rocks (in sandstones containing carbonaceous matter) and placer deposits. More than 50 silver minerals are known. In the biosphere silver is for the most part dispersed; in seawater the silver content is 3 × l0-8 percent. Silver is one of the scarcest elements.
Physical and chemical properties. Silver has a face-centered cubic lattice (a = 4.0772 angstroms [Å] at 20°C). The atomic radius is 1.44 Å, and the ionic radius of Ag+ is 1.13 Å. Silver has a density at 20°C of 10.5 g/cm3, a melting point of 960.8°C, a boiling point of 2212°C, and a heat of fusion of 105 kilo-joules/kg (25.1 calories/g). Of all metals, silver displays the highest electrical conductivity (6,297 siemens/m [62.97 ohm-1 • cm-1] at 25°C), highest thermal conductivity (407.79 watts/m-°K [0.974 calorie/cm-°C-sec] at 18°C), and highest reflectivity (90-99 percent for wavelengths of 5,000-100,000 Å). It has a specific heat of 234.46 joules/kg-°K (0.056 calorie/g°C) and an electrical resistivity of 15.9 nanoohm-m (1.59 μΩ-cm) at 20°C. Silver is diamagnetic, with an atomic magnetic susceptibility of - 21.56 × 106 at room temperature. Its elastic modulus is 76,480 meganewtons/m2 (7,648 kilograms-force/mm2), ultimate strength is 100 meganewtons/m2 (10 kilograms-force/mm2), and Brinell hardness is 250 meganewtons/m2 (25 kilograms-force/mm2). The electronic configuration of the outer shells of the Ag atom is 4d105s ‘.
Silver exhibits chemical properties characteristic of elements in subgroup I-B of Mendeleev’s periodic system. In compounds, Ag is usually monovalent.
Silver is located at the end of the electromotive force series. Its normal electrode potential Ag ⇄ Ag+ + e- is 0.7978 volt.
Silver does not react with O2, N2, or H2 at normal temperatures. A protective film of sparingly soluble halides and the sulfide Ag2S (gray-black crystals) is formed on the surface of silver when acted upon by free halogens and sulfur. Under the effect of the hydrogen sulfide (H2S) in the atmosphere, a thin film of Ag2S is formed on the surface of silver items, thereby causing them to tarnish. The sulfide can be obtained by the action of hydrogen sulfide on soluble silver salts or on aqueous suspensions of silver salts. The solubility of Ag2S in water is 2.48 × 10-5 mole/liter (25°C). Examples of analogous compounds are the selenide Ag2Se and telluride Ag2Te.
The most stable silver oxides are Ag20 and AgO. Ag20 is formed as a thin film on the surface of silver as a result of oxygen absorption, an absorption that increases with temperature and pressure.
Ag2O is obtained by the action of KOH on a solution of AgNO3. The solubility of Ag2O in water is 0.0174 g/liter. An Ag2O suspension possesses antiseptic properties. Ag2O decomposes at 200°C. Hydrogen, carbon monoxide, and many metals reduce Ag2O to metallic Ag. Ozone oxidizes Ag2O to form AgO. At 100°C, AgO decomposes into its separate elements with an explosion. Silver dissolves in nitric acid at room temperature to yield AgNO3. Hot concentrated sulfuric acid dissolves silver to give the sulfate Ag2SO4 (the solubility of the sulfate in water is 0.79 percent by weight at 20°C). Silver does not dissolve in aqua regia because of the formation of a protective AgCl film. In the absence of oxidizing agents, HCl, HBr, and HI do not react with silver at ordinary temperatures because of the formation on the metal surface of a protective film of sparingly soluble halides. Most silver salts, with the exception of AgNO3, AgF, and AgClO4, have poor solubility. Silver forms complex compounds, most of which are soluble in water and many of which have practical uses in chemical technology and analytical chemistry. Examples are the complex ions [Ag(CN)2]-, [Ag(NH3)2]+, and [Ag(SCN)2]-.
Production. Most silver (approximately 80 percent) is extracted as a by-product from complex ores, as well as from gold and copper ores. The extraction of silver from silver and gold ores requires cyanidation—the dissolution of silver in an alkaline solution of sodium cyanide with an inlet for air:
2Ag + 4NaCN + ½O2 + H2O = 2Na[Ag(CN)2] + 2NaOH
Silver is removed from the resultant solutions of complex cyanides by reduction with zinc or aluminum:
2[Ag(CN)2]- + Zn = [Zn(CN)4]2- + 2Ag
Silver is smelted from copper ores together with blister copper and is then extracted from the anode mud formed during the electrolytic refining of copper. In processing lead-zinc ores, silver is concentrated in the lead-crude lead, from which it is extracted by adding metallic zinc. The zinc combines with silver to form the refractory compound Ag2Zn3, which, being insoluble in lead, rises to the surface of the lead melt as a foam that can be easily skimmed off. The zinc is then distilled from the silver at 1250°C. The silver extracted from copper or lead-zinc ores is alloyed (doré) and subjected to electrolytic refining.
Use. Silver is used primarily in alloy form for minting coins and manufacturing household items, laboratory ware, and din-nerware. Silver is used to coat radio components to improve electrical conductivity and corrosion resistance; silver contacts are used in the electrical-engineering industry. Silver solders are used in soldering titanium and titanium alloys. In vacuum technology, silver serves as a structural material. Metallic silver is used to manufacture electrodes for silver-zinc and silver-cadmium batteries. It serves as a catalyst in inorganic and organic synthesis, for example, in the oxidation of alcohols to aldehydes and acids and of ethylene to ethylene oxide. In the food-processing industry, silver apparatus are used for the preparation of fruit juices. Small concentrations of silver ions sterilize water. Large quantities of silver compounds (AgBr, AgCl, AgI) are used in the manufacture of motion-picture and photographic materials.
S. I. GINZBURG
Silver in art. Because of its attractive white color and workability, silver has been widely used in decorative art since ancient times. However, since pure silver is very soft, nonferrous metals—usually copper—are added when minting coins and producing various silver items. The processes involved in working silver and decorating silver articles include stamping, casting, filigree techniques, and embossing. Enamels, niello, engraving, and gilding are also used with silver articles.
A high standard of artistic silver work characterizes the decorative art of the Hellenistic world and of ancient Rome, Persia (vessels from the Sassanid era, third through seventh centuries A.D), and Medieval Europe. Silver articles produced by master craftsmen during the Renaissance and baroque periods (B. Cellini in Italy, jewelers of the Jamnitzer, Lenker, and Lambrecht families in Germany) are noted for their variety of form and striking silhouettes, as well as for the mastery displayed in ornamentation and the representation of figures through stamping and casting. In the 18th and early 19th centuries, France assumed the leading role in the crafting of silver articles (C. Ballin, F. T. Germain, R. J. Auguste). In the decorative art of the 19th and 20th centuries, ungilded silver has been favored, and casting and mechanical methods of working the silver have been the dominant processes. In Russian decorative art of the 19th and early 20th centuries, outstanding silver articles were produced by the firms of the Grachevs, P. A. Ovchinnikov, P. F. Sazikov, P. C. Fabergé, and I. P. Khlebnikov. A continuing, creative development of traditions in decorative arts and a desire to reveal more fully the ornamental qualities of silver are characteristic of Soviet silver articles, among which the works of native craftsmen deserve a special place.
G. A. MARKOVA
In the organism. Silver is always present in plants and animals. Its average content in marine plants is 0.025 mg per 100 g of dry matter; in terrestrial plants the average is 0.006 mg. Marine animals have between 0.3 and 1.1 mg, while terrestrial animals have only trace amounts (10-2-10-4 mg). Animals accumulate silver in certain endocrine glands, in the iris, and in erythrocytes; silver is excreted mainly with feces. Silver forms complexes with proteins (globulins in blood, hemoglobin) in the organism. It inhibits enzymes by blocking mercapto groups, which participate in the formation of the active sites in enzymes. In particular, it deprives the adenosine triphosphatase activity of myosine. The biological role of silver has not been sufficiently studied. Silver becomes fixed in inflammation zones upon parenteral injection; in the blood, it combines chiefly with serum globulins.
IU. I. RAETSKAIA
Preparations. Silver preparations exhibit an antibacterial, astringent, and caustic effect, which is associated with their ability to destroy enzyme systems in microorganisms and precipitate proteins. In medicine, the most commonly used preparations are silver nitrate, collargolum, and Protargol (used in the same cases as collargolum). Also included is bactericidal paper, a porous paper impregnated with silver nitrate and silver chloride, which is used for treating minor wounds, abrasions, and burns.
Economic importance. In commercial dealings, silver, together with gold, has provided a universal standard and, like gold, has acquired a special use value; that is, silver has been used as money. “Gold and silver are not by nature money, but money by nature is gold and silver” (K. Marx, in K. Marx and F. Engels, Soch., 2nd. ed., vol. 13, p. 137). The commercial world chose silver as a monetary metal because of silver’s uniformity, divisibility, durability, and workability and because silver is portable (high value with low weight and volume).
Silver was originally circulated in bullion form. In countries of the ancient Middle East (Assyria, Babylonia, Egypt), as well as in Greece and Rome, silver was widely used as a monetary metal, along with gold and copper. In ancient Rome, silver coins were first minted between the fourth and third centuries B.C. The first Russian coins were minted from silver in the ninth and tenth centuries A.D.
The minting of gold coins predominated in the early Middle Ages. In the 16th century, a lack of gold supplies, coupled with the development of silver mining in Europe and the influx of silver from America (Peru and Mexico), caused silver to become the basic monetary metal of Europe. Silver monometallism and bimetallism existed in nearly every country during the initial accumulation of capital.
Since gold and silver coins were circulated according to the actual value of the content of precious metal, the relative value of the metals was established spontaneously, under the influence of market factors. In the late 18th and early 19th centuries, this system of parallel standards was replaced by a bimetallic system, wherein the state employed legislative means to set the ratio of the value of gold to silver. However, this system proved extremely unstable since the workings of the law of value led inevitably to a disparity between the market value and fixed value of gold and silver. The value of silver decreased sharply at the end of the 19th century because of improvements in methods for extracting silver from complex ores (in the 1870’s and 1880’s the value ratio of gold to silver was 1:15-1:16; by the early 20th century, it had fallen to 1:38–1:39). The growth of the world gold output accelerated the displacement of depreciated silver. Gold monometallism became widespread throughout the capitalist world in the last quarter of the 19th century. In most countries the displacement of silver currency by gold was completed by the beginning of the 20th century. Silver currency was retained in many Eastern countries (China, Iran, Afghanistan) until about the mid-1930’s. With the departure of these countries from silver monometallism, silver finally lost its value as a currency metal. In the industrially developed capitalist countries, silver is used only in the minting of coins of small denomination.
The increased use of silver in technology, dentistry, medicine, and jewelry manufacture after World War II coincided with a lag in silver output and created a shortage. Before the war, approximately 75 percent of the yearly silver output was used for monetary purposes. In the years 1950-65 this figure fell to an average of 50 percent and continued to decline in subsequent years, reaching 5 percent in 1971. Many countries have resorted to using copper-nickel alloys as monetary material. Although silver coins are still in circulation, many countries have prohibited the minting of new silver coins, and some have substantially reduced the coins’ silver content. For example, in the United States—according to a law on coin minting passed in 1965—approximately 90 percent of the silver previously used for the minting of coins was allocated to other purposes. The silver content in a 50-cent piece has been reduced from 90 to 40 percent; coins worth 10 and 25 cents, which previously contained 90 percent silver, are now minted without silver admixtures. New silver coins are minted to commemorate certain events (Olympic Games, jubilees, memorials).
In the early 1970’s, the principal consumers of silver were, in addition to the industry producing jewelry, table silver, and an-odized articles, the electrical-engineering, electronics, and motion-picture industries.
The silver market in the 1960’s and early 1970’s has been characterized by an increase in silver prices and by a consistent excess in demand for silver over the production of fresh metal. The deficiency has been made up largely by used metal, particularly the silver obtained from the remelting of coins.
L. M. RAITSIN
REFERENCES
Remy, H. Kurs neorganicheskoi khimii, vol. 1. Moscow, 1963. (Translated from German.)Plaksin, I. N. Metallurgiia blagorodnykh metallov. Moscow, 1958.
Kratkaia khimicheskaia entsiklopediia, vol. 4. Moscow, 1965.
Maksimov, M. M. Ocherk o serebre. Moscow, 1974.
Postnikova-Loseva, M. M. Russkoe iuvelirnoe ikusstvo, ego tsentry i mastera. Moscow, 1974.
Link, E. M. Eine Kunst-und Kulturgeschichte des Silbers. Berlin-Frankfurt-Warsaw, 1968. [23–879–]
Want to thank TFD for its existence? Tell a friend about us, add a link to this page, add the site to iGoogle, or visit the webmaster's page for free fun content.
| ?Page tools | ||
|---|---|---|
|