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= Silver =
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Introduction
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Silver is a chemical element; it has symbol Ag () and atomic number
47. A soft, whitish-gray, lustrous transition metal, it exhibits the
highest electrical conductivity, thermal conductivity, and
reflectivity of any metal. Silver is found in the Earth's crust in the
pure, free elemental 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 byproduct of copper, gold, lead, and zinc
refining.
Silver has long been valued as a precious metal. Silver metal is used
in many bullion coins, sometimes alongside gold: while it is more
abundant than gold, it is much less abundant as a native metal. Its
purity is typically measured on a per-mille basis; a 94%-pure alloy is
described as "0.940 fine". As one of the seven metals of antiquity,
silver has had an enduring role in most human cultures.
Other than in currency and as an investment medium (coins and
bullion), silver is used in solar panels, water filtration, jewellery,
ornaments, high-value tableware and utensils (hence the term
"silverware"), in electrical contacts and conductors, in specialised
mirrors, window coatings, in catalysis of chemical reactions, as a
colorant in stained glass, and in specialised confectionery. Its
compounds are used in photographic and X-ray film. Dilute solutions of
silver nitrate and other silver compounds are used as disinfectants
and microbiocides (oligodynamic effect), added to bandages,
wound-dressings, catheters, and other medical instruments.
Characteristics
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Silver bullion bar, 1000 ounces
Silver is similar in its physical and chemical properties to its two
vertical neighbours in group 11 of the periodic table: copper, and
gold. Its 47 electrons are arranged in the configuration [Kr]4d105s1,
similarly to copper ([Ar]3d104s1) and gold ([Xe]4f145d106s1); group 11
is one of the few groups in the d-block which has a completely
consistent set of electron configurations. This distinctive electron
configuration, with a single electron in the highest occupied s
subshell over a filled d subshell, accounts for many of the singular
properties of metallic silver.
Silver is a relatively soft and extremely ductile and malleable
transition metal, though it is slightly less malleable than gold.
Silver crystallises in a face-centred cubic lattice with bulk
coordination number 12, where only the single 5s electron is
delocalised, similarly to copper and gold. Unlike metals with
incomplete d-shells, metallic bonds in silver are lacking a covalent
character and are relatively weak. This observation explains the low
hardness and high ductility of single crystals of silver.
Silver has a brilliant, white, metallic luster that can take a high
polish, and which is so characteristic that the name of the metal
itself has become a color name. Protected silver has greater optical
reflectivity than aluminium at all wavelengths longer than ~450 nm. At
wavelengths shorter than 450 nm, silver's reflectivity is inferior to
that of aluminium and drops to zero near 310 nm.
Very high electrical and thermal conductivity are common to the
elements in group 11, because their single s electron is free and does
not interact with the filled d subshell, as such interactions (which
occur in the preceding transition metals) lower electron mobility. The
thermal conductivity of silver is among the highest of all materials,
although the thermal conductivity of carbon (in the diamond allotrope)
and superfluid helium-4 are higher. The electrical conductivity of
silver is the highest of all metals, greater even than copper. Silver
also has the lowest contact resistance of any metal. Silver is rarely
used for its electrical conductivity, due to its high cost, although
an exception is in radio-frequency engineering, particularly at VHF
and higher frequencies where silver plating improves electrical
conductivity because those currents tend to flow on the surface of
conductors rather than through the interior. During World War II in
the US, tons of silver were used for the electromagnets in calutrons
for enriching uranium, mainly because of the wartime shortage of
copper.
Silver readily forms alloys with copper, gold, and zinc. Zinc-silver
alloys with low zinc concentration may be considered as face-centred
cubic solid solutions of zinc in silver, as the structure of the
silver is largely unchanged while the electron concentration rises as
more zinc is added. Increasing the electron concentration further
leads to body-centred cubic (electron concentration 1.5), complex
cubic (1.615), and hexagonal close-packed phases (1.75).
Isotopes
==========
Naturally occurring silver is composed of two stable isotopes, 107Ag
and 109Ag, with 107Ag being slightly more abundant (51.839% natural
abundance). This almost equal abundance is rare in the periodic table.
The atomic weight is ; this value is very important because of the
importance of silver compounds, particularly halides, in gravimetric
analysis. Both isotopes of silver are produced in stars via the
s-process (slow neutron capture), as well as in supernovas via the
r-process (rapid neutron capture).
Twenty-eight radioisotopes have been characterised, 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. Silver has
numerous nuclear isomers, the most stable being 108mAg ('t'1/2 = 418
years), 110mAg ('t'1/2 = 249.79 days) and 106mAg ('t'1/2 = 8.28 days).
All of the remaining radioactive isotopes have half-lives of less than
an hour, and the majority of these have half-lives of less than three
minutes.
Isotopes of silver range in atomic mass from 92.950 Da (93Ag) to
129.950 Da (130Ag); 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 Clara meteorite in 1978. 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.
Chemistry
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Oxidation states and stereochemistries of silver
Oxidation state !! Coordination number !! Stereochemistry !!
Representative compound
0 (d10s1) 3 Planar Ag(CO)3
rowspan="4" | 1 (d10) 2 Linear [Ag(CN)2]−
3 Trigonal planar AgI(PEt2Ar)2
4 Tetrahedral [Ag(diars)2]+
6 Octahedral AgF, AgCl, AgBr
2 (d9) 4 Square planar [Ag(py)4]2+
rowspan="2" | 3 (d8) 4 Square planar [AgF4]−
6 Octahedral [AgF6]3−
Silver is a rather unreactive metal. This is because its filled 4d
shell is not very effective in shielding the electrostatic forces of
attraction from the nucleus to the outermost 5s electron, and hence
silver is near the bottom of the electrochemical series ('E'0(Ag+/Ag)
= +0.799 V). In group 11, silver has the lowest first ionisation
energy (showing the instability of the 5s orbital), but has higher
second and third ionisation energies than copper and gold (showing the
stability of the 4d orbitals), so that the chemistry of silver is
predominantly that of the +1 oxidation state, reflecting the
increasingly limited range of oxidation states along the transition
series as the d-orbitals fill and stabilise. Unlike copper, for which
the larger hydration energy of Cu2+ as compared to Cu+ is the reason
why the former is the more stable in aqueous solution and solids
despite lacking the stable filled d-subshell of the latter, with
silver this effect is swamped by its larger second ionisation energy.
Hence, Ag+ is the stable species in aqueous solution and solids, with
Ag2+ being much less stable as it oxidises water.
Most silver compounds have significant covalent character due to the
small size and high first ionisation energy (730.8 kJ/mol) of silver.
Furthermore, silver's Pauling electronegativity of 1.93 is higher than
that of lead (1.87), and its electron affinity of 125.6 kJ/mol is much
higher than that of hydrogen (72.8 kJ/mol) and not much less than that
of oxygen (141.0 kJ/mol). Due to its full d-subshell, silver in its
main +1 oxidation state exhibits relatively few properties of the
transition metals proper from groups 4 to 10, forming rather unstable
organometallic compounds, forming linear complexes showing very low
coordination numbers like 2, and forming an amphoteric oxide as well
as Zintl phases like the post-transition metals. Unlike the preceding
transition metals, the +1 oxidation state of silver is stable even in
the absence of π-acceptor ligands.
Silver does not react with air, even at red heat, and thus was
considered by alchemists as a noble metal, along with gold. Its
reactivity is intermediate between that of copper (which forms
copper(I) oxide when heated in air to red heat) and gold. Like copper,
silver reacts with sulfur and its compounds; in their presence, silver
tarnishes in air to form the black silver sulfide (copper forms the
green sulfate instead, while gold does not react). While silver is not
attacked by non-oxidising acids, the metal dissolves readily in hot
concentrated sulfuric acid, as well as dilute or concentrated nitric
acid. In the presence of air, and especially in the presence of
hydrogen peroxide, silver dissolves readily in aqueous solutions of
cyanide.
The three main forms of deterioration in historical silver artifacts
are tarnishing, formation of silver chloride due to long-term
immersion in salt water, as well as reaction with nitrate ions or
oxygen. Fresh silver chloride is pale yellow, becoming purplish on
exposure to light; it projects slightly from the surface of the
artifact or coin. The precipitation of copper in ancient silver can be
used to date artifacts, as copper is nearly always a constituent of
silver alloys.
Silver metal is attacked by strong oxidant 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 bromide that can
either be fixed with thiosulfate or redeveloped 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.
The common oxidation states of silver are (in order of commonness): +1
(the most stable state; for example, silver nitrate, AgNO3); +2
(highly oxidising; for example, silver(II) fluoride, AgF2); and even
very rarely +3 (extreme oxidising; for example, potassium
tetrafluoroargentate(III), KAgF4). The +3 state requires very strong
oxidising agents to attain, such as fluorine or peroxodisulfate, and
some silver(III) compounds react with atmospheric moisture and attack
glass. Indeed, silver(III) fluoride is usually obtained by reacting
silver or silver monofluoride with the strongest known oxidising
agent, krypton difluoride.
Oxides and chalcogenides
==========================
Silver and gold have rather low chemical affinities for oxygen, lower
than copper, and it is therefore expected that silver oxides are
thermally quite unstable. Soluble silver(I) salts precipitate
dark-brown silver(I) oxide, Ag2O, upon the addition of alkali. (The
hydroxide AgOH exists only in solution; otherwise it spontaneously
decomposes to the oxide.) Silver(I) oxide is very easily reduced to
metallic silver, and decomposes to silver and oxygen above 160 °C.
This and other silver(I) compounds may be oxidised by the strong
oxidising agent peroxodisulfate to black AgO, a mixed silver(I,III)
oxide of formula AgIAgIIIO2. Some other mixed oxides with silver in
non-integral oxidation states, namely Ag2O3 and Ag3O4, are also known,
as is Ag3O which behaves as a metallic conductor.
Silver(I) sulfide, Ag2S, is very readily formed from its constituent
elements and is the cause of the black tarnish on some old silver
objects. It may also be formed from the reaction of hydrogen sulfide
with silver metal or aqueous Ag+ ions. Many non-stoichiometric
selenides and tellurides are known; in particular, AgTe~3 is a
low-temperature superconductor.
Halides
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The only known dihalide of silver is the difluoride, AgF2, which can
be obtained from the elements under heat. A strong yet thermally
stable and therefore safe fluorinating agent, silver(II) fluoride is
often used to synthesise hydrofluorocarbons.
In stark contrast to this, all four silver(I) halides are known. The
fluoride, chloride, and bromide have the sodium chloride structure,
but the iodide has three known stable forms at different temperatures;
that at room temperature is the cubic zinc blende structure. They can
all be obtained by the direct reaction of their respective elements.
As the halogen group is descended, the silver halide gains more and
more covalent character, solubility decreases, and the colour changes
from the white chloride to the yellow iodide as the energy required
for ligand-metal charge transfer (X−Ag+ → XAg) decreases. The fluoride
is anomalous, as the fluoride ion is so small that it has a
considerable solvation energy and hence is highly water-soluble and
forms di- and tetrahydrates. The other three silver halides are highly
insoluble in aqueous solutions and are very commonly used in
gravimetric analytical methods. All four are photosensitive (though
the monofluoride is so only to ultraviolet light), especially the
bromide and iodide which photodecompose to silver metal, and thus were
used in traditional photography. The reaction involved is:
:X− + 'hν' → X + e− (excitation of the halide ion, which gives up its
extra electron into the conduction band)
:Ag+ + e− → Ag (liberation of a silver ion, which gains an electron to
become a silver atom)
The process is not reversible because the silver atom liberated is
typically found at a crystal defect or an impurity site, so that the
electron's energy is lowered enough that it is "trapped".
Other inorganic compounds
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White silver nitrate, AgNO3, is a versatile precursor to many other
silver compounds, especially the halides, and is much less sensitive
to light. It was once called 'lunar caustic' because silver was called
'luna' by the ancient alchemists, who believed that silver was
associated with the Moon. It is often used for gravimetric analysis,
exploiting the insolubility of the heavier silver halides which it is
a common precursor to. Silver nitrate is used in many ways in organic
synthesis, e.g. for deprotection and oxidations. Ag+ binds alkenes
reversibly, and silver nitrate has been used to separate mixtures of
alkenes by selective absorption. The resulting adduct can be
decomposed with ammonia to release the free alkene.
Yellow silver carbonate, Ag2CO3 can be easily prepared by reacting
aqueous solutions of sodium carbonate with a deficiency of silver
nitrate. Its principal use is for the production of silver powder for
use in microelectronics. It is reduced with formaldehyde, producing
silver free of alkali metals:
:Ag2CO3 + CH2O → 2 Ag + 2 CO2 + H2
Silver carbonate is also used as a reagent in organic synthesis such
as the Koenigs-Knorr reaction. In the Fétizon oxidation, silver
carbonate on celite acts as an oxidising agent to form lactones from
diols. It is also employed to convert alkyl bromides into alcohols.
Silver fulminate, AgCNO, a powerful, touch-sensitive explosive used in
percussion caps, is made by reaction of silver metal with nitric acid
in the presence of ethanol. Other dangerously explosive silver
compounds are silver azide, AgN3, formed by reaction of silver nitrate
with sodium azide, and silver acetylide, Ag2C2, formed when silver
reacts with acetylene gas in ammonia solution. In its most
characteristic reaction, silver azide decomposes explosively,
releasing nitrogen gas: given the photosensitivity of silver salts,
this behaviour may be induced by shining a light on its crystals.
: 2 (s) → 3 (g) + 2 Ag (s)
Coordination compounds
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Silver complexes tend to be similar to those of its lighter homologue
copper. Silver(III) complexes tend to be rare and very easily reduced
to the more stable lower oxidation states, though they are slightly
more stable than those of copper(III). For instance, the square planar
periodate [Ag(IO5OH)2]5− and tellurate [Ag{TeO4(OH)2}2]5− complexes
may be prepared by oxidising silver(I) with alkaline peroxodisulfate.
The yellow diamagnetic [AgF4]− is much less stable, fuming in moist
air and reacting with glass.
Silver(II) complexes are more common. Like the valence isoelectronic
copper(II) complexes, they are usually square planar and paramagnetic,
which is increased by the greater field splitting for 4d electrons
than for 3d electrons. Aqueous Ag2+, produced by oxidation of Ag+ by
ozone, is a very strong oxidising agent, even in acidic solutions: it
is stabilised in phosphoric acid due to complex formation.
Peroxodisulfate oxidation is generally necessary to give the more
stable complexes with heterocyclic amines, such as [Ag(py)4]2+ and
[Ag(bipy)2]2+: these are stable provided the counterion cannot reduce
the silver back to the +1 oxidation state. [AgF4]2− is also known in
its violet barium salt, as are some silver(II) complexes with 'N'- or
'O'-donor ligands such as pyridine carboxylates.
By far the most important oxidation state for silver in complexes is
+1. The Ag+ cation is diamagnetic, like its homologues Cu+ and Au+, as
all three have closed-shell electron configurations with no unpaired
electrons: its complexes are colourless provided the ligands are not
too easily polarised such as I−. Ag+ forms salts with most anions, but
it is reluctant to coordinate to oxygen and thus most of these salts
are insoluble in water: the exceptions are the nitrate, perchlorate,
and fluoride. The tetracoordinate tetrahedral aqueous ion [Ag(H2O)4]+
is known, but the characteristic geometry for the Ag+ cation is
2-coordinate linear. For example, silver chloride dissolves readily in
excess aqueous ammonia to form [Ag(NH3)2]+; silver salts are dissolved
in photography due to the formation of the thiosulfate complex
[Ag(S2O3)2]3−; and cyanide extraction for silver (and gold) works by
the formation of the complex [Ag(CN)2]−. Silver cyanide forms the
linear polymer {Ag-C≡N→Ag-C≡N→}; silver thiocyanate has a similar
structure, but forms a zigzag instead because of the sp3-hybridized
sulfur atom. Chelating ligands are unable to form linear complexes and
thus silver(I) complexes with them tend to form polymers; a few
exceptions exist, such as the near-tetrahedral diphosphine and
diarsine complexes [Ag(L-L)2]+.
Organometallic
================
Under standard conditions, silver does not form simple carbonyls, due
to the weakness of the Ag-C bond. A few are known at very low
temperatures around 6-15 K, such as the green, planar paramagnetic
Ag(CO)3, which dimerises at 25-30 K, probably by forming Ag-Ag bonds.
Additionally, the silver carbonyl [Ag(CO)] [B(OTeF5)4] is known.
Polymeric AgLX complexes with alkenes and alkynes are known, but their
bonds are thermodynamically weaker than even those of the platinum
complexes (though they are formed more readily than those of the
analogous gold complexes): they are also quite unsymmetrical, showing
the weak 'π' bonding in group 11. Ag-C 'σ' bonds may also be formed by
silver(I), like copper(I) and gold(I), but the simple alkyls and aryls
of silver(I) are even less stable than those of copper(I) (which tend
to explode under ambient conditions). For example, poor thermal
stability is reflected in the relative decomposition temperatures of
AgMe (−50 °C) and CuMe (−15 °C) as well as those of PhAg (74 °C) and
PhCu (100 °C).
The C-Ag bond is stabilised by perfluoroalkyl ligands, for example in
AgCF(CF3)2. Alkenylsilver compounds are also more stable than their
alkylsilver counterparts. Silver-NHC complexes are easily prepared,
and are commonly used to prepare other NHC complexes by displacing
labile ligands. For example, the reaction of the bis(NHC)silver(I)
complex with bis(acetonitrile)palladium dichloride or
chlorido(dimethyl sulfide)gold(I):
:upright=2.75
Intermetallic
===============
Silver forms alloys with most other elements on the periodic table.
The elements from groups 1-3, except for hydrogen, lithium, and
beryllium, are very miscible with silver in the condensed phase and
form intermetallic compounds; those from groups 4-9 are only poorly
miscible; the elements in groups 10-14 (except boron and carbon) have
very complex Ag-M phase diagrams and form the most commercially
important alloys; and the remaining elements on the periodic table
have no consistency in their Ag-M phase diagrams. By far the most
important such alloys are those with copper: most silver used for
coinage and jewellery is in reality a silver-copper alloy, and the
eutectic mixture is used in vacuum brazing. The two metals are
completely miscible as liquids but not as solids; their importance in
industry comes from the fact that their properties tend to be suitable
over a wide range of variation in silver and copper concentration,
although most useful alloys tend to be richer in silver than the
eutectic mixture (71.9% silver and 28.1% copper by weight, and 60.1%
silver and 28.1% copper by atom).
Most other binary alloys are of little use: for example, silver-gold
alloys are too soft and silver-cadmium alloys too toxic. Ternary
alloys have much greater importance: dental amalgams are usually
silver-tin-mercury alloys, silver-copper-gold alloys are very
important in jewellery (usually on the gold-rich side) and have a vast
range of hardnesses and colours, silver-copper-zinc alloys are useful
as low-melting brazing alloys, and silver-cadmium-indium (involving
three adjacent elements on the periodic table) is useful in nuclear
reactors because of its high thermal neutron capture cross-section,
good conduction of heat, mechanical stability, and resistance to
corrosion in hot water.
Etymology
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The word 'silver' appears in Old English in various spellings, such as
and . It is cognate with Old High German ; Gothic ; or Old Norse , all
ultimately deriving from Proto-Germanic '*silubra'. The Balto-Slavic
words for silver are rather similar to the Germanic ones (e.g. Russian
[], Polish , Lithuanian ), as is the Celtiberian form 'silabur'. They
may have a common Indo-European origin, although their morphology
rather suggest a non-Indo-European 'Wanderwort'. Some scholars have
thus proposed a Paleo-Hispanic origin, pointing to the Basque form as
an evidence.
The chemical symbol Ag is from the Latin word for 'silver', ' (compare
Ancient Greek , ), from the Proto-Indo-European root *'h₂erǵ-'
(formerly reconstructed as '*arǵ-'), meaning or . This was the usual
Proto-Indo-European word for the metal, whose reflexes are missing in
Germanic and Balto-Slavic.
History
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Silver was known in prehistoric times: the three metals of group 11,
copper, silver, and gold, occur in the elemental form in nature and
were probably used as the first primitive forms of money as opposed to
simple bartering. Unlike copper, silver did not lead to the growth of
metallurgy, on account of its low structural strength; it was more
often used ornamentally or as money. Since silver is more reactive
than gold, supplies of native silver were much more limited than those
of gold. For example, silver was more expensive than gold in Egypt
until around the fifteenth century BC: the Egyptians are thought to
have separated gold from silver by heating the metals with salt, and
then reducing the silver chloride produced to the metal.
The situation changed with the discovery of cupellation, a technique
that allowed silver metal to be extracted from its ores. While 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, and one of the earliest silver extraction centres in
Europe was Sardinia in the early Chalcolithic period, these techniques
did not spread widely until later,
when it spread throughout the region and beyond. The origins of silver
production in India, China, and Japan were almost certainly equally
ancient, but are not well-documented due to their great age.
When the Phoenicians first came to what is now Spain, they obtained so
much silver that they could not fit it all on their ships, and as a
result used silver to weight their anchors instead of lead. By the
time of the Greek and Roman civilisations, silver coins were a staple
of the economy: the Greeks were already extracting silver from galena
by the 7th century BC, and the rise of Athens was partly made possible
by the nearby silver mines at Laurium, from which they extracted about
30 tonnes a year from 600 to 300 BC. The stability of the Roman
currency relied to a high degree on the supply of silver bullion,
mostly from Spain, which Roman miners produced on a scale unparalleled
before the discovery of the New World. Reaching a peak production of
200 tonnes per year, an estimated silver stock of 10,000 tonnes
circulated in the Roman economy in the middle of the second century
AD, five to ten times larger than the combined amount of silver
available to medieval Europe and the Abbasid Caliphate around AD 800.
The Romans also recorded the extraction of silver in central and
northern Europe in the same time period. This production came to a
nearly complete halt with the fall of the Roman Empire, not to resume
until the time of Charlemagne: by then, tens of thousands of tonnes of
silver had already been extracted.
Central Europe became the centre of silver production during the
Middle Ages, as the Mediterranean deposits exploited by the ancient
civilisations had been exhausted. Silver mines were opened in Bohemia,
Saxony, Alsace, the Lahn region, Siegerland, Silesia, Hungary, Norway,
Steiermark, Schwaz, and the southern Black Forest. Most of these ores
were quite rich in silver and could simply be separated by hand from
the remaining rock and then smelted; some deposits of native silver
were also encountered. Many of these mines were soon exhausted, but a
few of them remained active until the Industrial Revolution, before
which the world production of silver was around a meagre 50 tonnes per
year. In the Americas, high temperature silver-lead cupellation
technology was developed by pre-Inca civilisations as early as AD
60-120; silver deposits in India, China, Japan, and pre-Columbian
America continued to be mined during this time.
With the discovery of America and the plundering of silver by the
Spanish conquistadors, Central and South America became the dominant
producers of silver until around the beginning of the 18th century,
particularly Peru, Bolivia, Chile, and Argentina: the last of these
countries later took its name from that of the metal that composed so
much of its mineral wealth. The silver trade gave way to a global
network of exchange. As one historian put it, silver "went round the
world and made the world go round." Much of this silver ended up in
the hands of the Chinese. A Portuguese merchant in 1621 noted that
silver "wanders throughout all the world... before flocking to China,
where it remains as if at its natural centre". Still, much of it went
to Spain, allowing Spanish rulers to pursue military and political
ambitions in both Europe and the Americas. "New World mines",
concluded several historians, "supported the Spanish empire."
In the 19th century, primary production of silver moved to North
America, particularly Canada, Mexico, and Nevada in the United States:
some secondary production from lead and zinc ores also took place in
Europe, and deposits in Siberia and the Russian Far East as well as in
Australia were mined. Poland emerged as an important producer during
the 1970s after the discovery of copper deposits that were rich in
silver, before the centre of production returned to the Americas the
following decade. Today, Peru and Mexico are still among the primary
silver producers, but the distribution of silver production around the
world is quite balanced and about one-fifth of the silver supply comes
from recycling instead of new production.
File:Proto-Elamite kneeling bull holding a spouted
vessel.jpg|Proto-Elamite kneeling bull holding a spouted vessel;
3100-2900 BC; 16.3×6.3×10.8 cm; Metropolitan Museum of Art (New York
City)
Horus as falcon god with Egyptian crown from the 27th dynasty
(05).jpg|Ancient Egyptian figurine of Horus as falcon god with an
Egyptian crown; ; silver and electrum; height: 26.9 cm; Staatliche
Sammlung für Ägyptische Kunst (Munich, Germany)
Silver tetradrachm MET DP139641.jpg|Ancient Greek tetradrachm; 315-308
BC; diameter: 2.7 cm; Metropolitan Museum of Art
Silver-gilt bowl MET DP105813.jpg|Ancient Greek gilded bowl; 2nd-1st
century BC; height: 7.6 cm, diameter: 14.8 cm; Metropolitan Museum of
Art
Silver plate MET DP231273.jpg|Roman plate; 1st-2nd century AD; height:
0.1 cm, diameter: 12.7 cm; Metropolitan Museum of Art
Silver bust of Serapis MET DT6658.jpg|Roman bust of Serapis; 2nd
century; 15.6×9.5 cm; Metropolitan Museum of Art
Schaal met voorstellingen uit de geschiedenis van Diana en Actaeon
door Paulus Willemsz van Vianen in 1613.jpg|Auricular basin with
scenes from the story of Diana and Actaeon; 1613; length: 50 cm,
height: 6 cm, width: 40 cm; Rijksmuseum (Amsterdam, the Netherlands)
Silver Tureen (a), lid (b) -pair with 1975.1.2560a-c- MET SLP2561a
b-1.jpg|French Rococo tureen; 1749; height: 26.3 cm, width: 39 cm,
depth: 24 cm; Metropolitan Museum of Art
Coffeepot MET DP103144 (cropped),.jpg|French Rococo coffeepot; 1757;
height: 29.5 cm; Metropolitan Museum of Art
Ewer MET DT236853.jpg|French Neoclassical ewer; 1784-1785; height:
32.9 cm; Metropolitan Museum of Art
Elkington & Co. - Neo-Rococo Coffee Pot - 2003.243 - Cleveland
Museum of Art.jpg|Neo-Rococo coffeepot; 1845; overall: 32×23.8×15.4
cm; Cleveland Museum of Art (Cleveland, Ohio, US)
Dessert Spoon (France), ca. 1890 (CH 18653899-2).jpg|French Art
Nouveau dessert spoons; circa 1890; Cooper Hewitt, Smithsonian Design
Museum (New York City)
Jardiniere And Liner (Germany), ca. 1905-10 (CH 18444035)
(cropped).jpg|Art Nouveau jardinière; circa 1905-1910; height: 22 cm,
width: 47 cm, depth: 22.5 cm; Cooper Hewitt, Smithsonian Design Museum
Handspiegel met gedreven Jugendstilornament, BK-1967-10.jpg|Hand
mirror; 1906; height: 20.7 cm, weight: 88 g; Rijksmuseum (Amsterdam,
the Netherlands)
Mystery watch.jpg|Mystery watch; ca. 1889; diameter: 5.4 cm, depth:
1.8 cm; Musée d'Horlogerie of Le Locle (Switzerland)
Symbolic role
======================================================================
Silver plays a certain role in mythology and has found various usage
as a metaphor and in folklore. The Greek poet Hesiod's 'Works and
Days' (lines 109-201) lists different ages of man named after metals
like gold, silver, bronze and iron to account for successive ages of
humanity. Ovid's 'Metamorphoses' contains another retelling of the
story, containing an illustration of silver's metaphorical use of
signifying the second-best in a series, better than bronze but worse
than gold:
In folklore, silver was commonly thought to have mystic powers: for
example, a bullet cast from silver is often supposed in such folklore
the only weapon that is effective against a werewolf, witch, or other
monsters. From this the idiom of a silver bullet developed into
figuratively referring to any simple solution with very high
effectiveness or almost miraculous results, as in the widely discussed
software engineering paper "No Silver Bullet." Other powers attributed
to silver include detection of poison and facilitation of passage into
the mythical realm of fairies.
Silver production has also inspired figurative language. Clear
references to cupellation occur throughout the Old Testament of the
Bible, such as in Jeremiah's rebuke to Judah: "The bellows are burned,
the lead is consumed of the fire; the founder melteth in vain: for the
wicked are not plucked away. Reprobate silver shall men call them,
because the Lord hath rejected them." (Jeremiah 6:19-20) Jeremiah was
also aware of sheet silver, exemplifying the malleability and
ductility of the metal: "Silver spread into plates is brought from
Tarshish, and gold from Uphaz, the work of the workman, and of the
hands of the founder: blue and purple is their clothing: they are all
the work of cunning men." (Jeremiah 10:9)
Silver also has more negative cultural meanings: the idiom thirty
pieces of silver, referring to a reward for betrayal, references the
bribe Judas Iscariot is said in the New Testament to have taken from
Jewish leaders in Jerusalem to turn Jesus of Nazareth over to soldiers
of the high priest Caiaphas. Ethically, silver also symbolizes greed
and degradation of consciousness; this is the negative aspect, the
perverting of its value.
Occurrence and production
======================================================================
The abundance of silver in the Earth's crust is 0.08 parts per
million, almost exactly the same as that of mercury. It mostly occurs
in sulfide ores, especially acanthite and argentite, Ag2S. Argentite
deposits sometimes also contain native silver when they occur in
reducing environments, and when in contact with salt water they are
converted to chlorargyrite (including horn silver), AgCl, which is
prevalent in Chile and New South Wales. Most other silver minerals are
silver pnictides or chalcogenides; they are generally lustrous
semiconductors. Most true silver deposits, as opposed to argentiferous
deposits of other metals, came from Tertiary vulcanism.
The principal sources of silver are the ores of copper, copper-nickel,
lead, and lead-zinc obtained from Peru, Bolivia, Mexico, China,
Australia, Chile, Poland and Serbia. Peru, Bolivia and Mexico have
been mining silver since 1546, and are still major world producers.
Top silver-producing mines are Cannington (Australia), Fresnillo
(Mexico), San Cristóbal (Bolivia), Antamina (Peru), Rudna (Poland),
and Penasquito (Mexico). Top near-term mine development projects
through 2015 are Pascua Lama (Chile), Navidad (Argentina), Jaunicipio
(Mexico), Malku Khota (Bolivia), and Hackett River (Canada). In
Central Asia, Tajikistan is known to have some of the largest silver
deposits in the world.
Silver is usually found in nature combined with other metals, or in
minerals that contain silver compounds, generally in the form of
sulfides such as galena (lead sulfide) or cerussite (lead carbonate).
So the primary production of silver requires the smelting and then
cupellation of argentiferous lead ores, a historically important
process. Lead melts at 327 °C, lead oxide at 888 °C and silver melts
at 960 °C. To separate the silver, the alloy is melted again at the
high temperature of 960 °C to 1000 °C in an oxidising environment. The
lead oxidises to lead monoxide, then known as litharge, which captures
the oxygen from the other metals present. The liquid lead oxide is
removed or absorbed by capillary action into the hearth linings.
Bayley, J., Crossley, D. and Ponting, M. (eds). (2008).
[
https://www.researchgate.net/publication/271133104_Metals_and_Metalworking_A_Research_Framework_for_Archaeometallurgy
'Metals and Metalworking. A research framework for archaeometallurgy'.
Historical Metallurgy Society. p. 6.
: (s) + 2(s) + (g) → 2(absorbed) + Ag(l)
Today, silver metal is primarily produced instead as a secondary
byproduct of electrolytic refining of copper, lead, and zinc, and by
application of the Parkes process on lead bullion from ore that also
contains silver. In such processes, silver follows the non-ferrous
metal in question through its concentration and smelting, and is later
purified out. For example, in copper production, purified copper is
electrolytically deposited on the cathode, while the less reactive
precious metals such as silver and gold collect under the anode as the
so-called "anode slime". This is then separated and purified of base
metals by treatment with hot aerated dilute sulfuric acid and heating
with lime or silica flux, before the silver is purified to over 99.9%
purity via electrolysis in nitrate solution.
Commercial-grade fine silver is at least 99.9% pure, and purities
greater than 99.999% are available. In 2022, Mexico was the top
producer of silver (6,300 tonnes or 24.2% of the world's total of
26,000 t), followed by China (3,600 t) and Peru (3,100 t).
In marine environments
========================
Silver concentration is low in seawater (pmol/L). Levels vary by depth
and between water bodies. Dissolved silver concentrations range from
0.3 pmol/L in coastal surface waters to 22.8 pmol/L in pelagic deep
waters. Analysing the presence and dynamics of silver in marine
environments is difficult due to these particularly low concentrations
and complex interactions in the environment. Although a rare trace
metal, concentrations are greatly impacted by fluvial, aeolian,
atmospheric, and upwelling inputs, as well as anthropogenic inputs via
discharge, waste disposal, and emissions from industrial companies.
Other internal processes such as decomposition of organic matter may
be a source of dissolved silver in deeper waters, which feeds into
some surface waters through upwelling and vertical mixing.
In the Atlantic and Pacific, silver concentrations are minimal at the
surface but rise in deeper waters. Silver is taken up by plankton in
the photic zone, remobilized with depth, and enriched in deep waters.
Silver is transported from the Atlantic to the other oceanic water
masses. In North Pacific waters, silver is remobilised at a slower
rate and increasingly enriched compared to deep Atlantic waters.
Silver has increasing concentrations that follow the major oceanic
conveyor belt that cycles water and nutrients from the North Atlantic
to the South Atlantic to the North Pacific.
There is not an extensive amount of data focused on how marine life is
affected by silver despite the likely deleterious effects it could
have on organisms through bioaccumulation, association with
particulate matters, and sorption. Not until about 1984 did scientists
begin to understand the chemical characteristics of silver and the
potential toxicity. In fact, mercury is the only other trace metal
that surpasses the toxic effects of silver; the full silver toxicity
extent is not expected in oceanic conditions because of its tendency
to transfer into nonreactive biological compounds.
In one study, the presence of excess ionic silver and silver
nanoparticles caused bioaccumulation effects on zebrafish organs and
altered the chemical pathways within their gills. In addition, very
early experimental studies demonstrated how the toxic effects of
silver fluctuate with salinity and other parameters, as well as
between life stages and different species such as finfish, molluscs,
and crustaceans. Another study found raised concentrations of silver
in the muscles and liver of dolphins and whales, indicating pollution
of this metal within recent decades. Silver is not an easy metal for
an organism to eliminate and elevated concentrations can cause death.
{{Anchor|XAG}}Monetary use
======================================================================
The earliest known coins were minted in the kingdom of Lydia in Asia
Minor around 600 BC. The coins of Lydia were made of electrum, which
is a naturally occurring alloy of gold and silver, that was available
within the territory of Lydia. Since that time, silver standards, in
which the standard economic unit of account is a fixed weight of
silver, have been widespread throughout the world until the 20th
century. Notable silver coins through the centuries include the Greek
drachma, the Roman denarius, the Islamic dirham, the karshapana from
ancient India and rupee from the time of the Mughal Empire (grouped
with copper and gold coins to create a trimetallic standard), and the
Spanish dollar.
The ratio between the amount of silver used for coinage and that used
for other purposes has fluctuated greatly over time; for example, in
wartime, more silver tends to have been used for coinage to finance
the war.
Today, silver bullion has the ISO 4217 currency code XAG, one of only
four precious metals to have one (the others being palladium,
platinum, and gold). Silver coins are produced from cast rods or
ingots, rolled to the correct thickness, heat-treated, and then used
to cut blanks from. These blanks are then milled and minted in a
coining press; modern coining presses can produce 8000 silver coins
per hour.
Price
=======
Silver prices are normally quoted in troy ounces. One troy ounce is
equal to 1 ozt. The London silver fix is published every working day
at noon London time. This price is determined by several major
international banks and is used by London bullion market members for
trading that day. Prices are most commonly shown as the United States
dollar (USD), the Pound sterling (GBP), and the Euro (EUR).
Jewellery and silverware
==========================
The major use of silver besides coinage throughout most of history was
in the manufacture of jewellery and other general-use items, and this
continues to be a major use today. Examples include table silver for
cutlery, for which silver is highly suited due to its antibacterial
properties. Western concert flutes are usually plated with or made out
of sterling silver; in fact, most silverware is only silver-plated
rather than made out of pure silver; the silver is normally put in
place by electroplating. Silver-plated glass (as opposed to metal) is
used for mirrors, vacuum flasks, and Christmas tree decorations.
Because pure silver is very soft, most silver used for these purposes
is alloyed with copper, with finenesses of 925/1000, 835/1000, and
800/1000 being common. One drawback is the easy tarnishing of silver
in the presence of hydrogen sulfide and its derivatives. Including
precious metals such as palladium, platinum, and gold gives resistance
to tarnishing but is quite costly; base metals like zinc, cadmium,
silicon, and germanium do not totally prevent corrosion and tend to
affect the lustre and colour of the alloy. Electrolytically refined
pure silver plating is effective at increasing resistance to
tarnishing. The usual solutions for restoring the lustre of tarnished
silver are dipping baths that reduce the silver sulfide surface to
metallic silver, and cleaning off the layer of tarnish with a paste;
the latter approach also has the welcome side effect of polishing the
silver concurrently.
Medicine
==========
In medicine, silver is incorporated into wound dressings and used as
an antibiotic coating in medical devices. Wound dressings containing
silver sulfadiazine or silver nanomaterials are used to treat external
infections. Silver is also used in some medical applications, such as
urinary catheters (where tentative evidence indicates it reduces
catheter-related urinary tract infections) and in endotracheal
breathing tubes (where evidence suggests it reduces
ventilator-associated pneumonia). The silver ion is bioactive and in
sufficient concentration readily kills bacteria 'in vitro'. Silver
ions interfere with enzymes in the bacteria that transport nutrients,
form structures, and synthesise cell walls; these ions also bond with
the bacteria's genetic material. Silver and silver nanoparticles are
used as an antimicrobial in a variety of industrial, healthcare, and
domestic application: for example, infusing clothing with nanosilver
particles thus allows them to stay odourless for longer. Bacteria can
develop resistance to the antimicrobial action of silver. Silver
compounds are taken up by the body like mercury compounds, but lack
the toxicity of the latter. Silver and its alloys are used in cranial
surgery to replace bone, and silver-tin-mercury amalgams are used in
dentistry. Silver diammine fluoride, the fluoride salt of a
coordination complex with the formula [Ag(NH3)2]F, is a topical
medicament (drug) used to treat and prevent dental caries (cavities)
and relieve dentinal hypersensitivity.
Electronics
=============
Silver is very important in electronics for conductors and electrodes
on account of its high electrical conductivity even when tarnished.
Bulk silver and silver foils were used to make vacuum tubes, and
continue to be used today in the manufacture of semiconductor devices,
circuits, and their components. For example, silver is used in high
quality connectors for RF, VHF, and higher frequencies, particularly
in tuned circuits such as cavity filters where conductors cannot be
scaled by more than 6%. Printed circuits and RFID antennas are made
with silver paints, Powdered silver and its alloys are used in paste
preparations for conductor layers and electrodes, ceramic capacitors,
and other ceramic components.
Brazing alloys
================
Silver-containing brazing alloys are used for brazing metallic
materials, mostly cobalt, nickel, and copper-based alloys, tool
steels, and precious metals. The basic components are silver and
copper, with other elements selected according to the specific
application desired: examples include zinc, tin, cadmium, palladium,
manganese, and phosphorus. Silver provides increased workability and
corrosion resistance during usage.
Chemical equipment
====================
Silver is useful in the manufacture of chemical equipment on account
of its low chemical reactivity, high thermal conductivity, and being
easily workable. Silver crucibles (alloyed with 0.15% nickel to avoid
recrystallisation of the metal at red heat) are used for carrying out
alkaline fusion. Copper and silver are also used when doing chemistry
with fluorine. Equipment made to work at high temperatures is often
silver-plated. Silver and its alloys with gold are used as wire or
ring seals for oxygen compressors and vacuum equipment.
Catalysis
===========
Silver metal is a good catalyst for oxidation reactions; in fact it is
somewhat too good for most purposes, as finely divided silver tends to
result in complete oxidation of organic substances to carbon dioxide
and water, and hence coarser-grained silver tends to be used instead.
For instance, 15% silver supported on α-Al2O3 or silicates is a
catalyst for the oxidation of ethylene to ethylene oxide at 230-270
°C. Dehydrogenation of methanol to formaldehyde is conducted at
600-720 °C over silver gauze or crystals as the catalyst, as is
dehydrogenation of isopropanol to acetone. In the gas phase, glycol
yields glyoxal and ethanol yields acetaldehyde, while organic amines
are dehydrated to nitriles.
Photography
=============
Before the advent of digital photography, which is now dominant, the
photosensitivity of silver halides was exploited for use in
traditional film photography. The photosensitive emulsion used in
black-and-white photography is a suspension of silver halide crystals
in gelatin, possibly mixed in with some noble metal compounds for
improved photosensitivity, developing, and .
Colour photography requires the addition of special dye components and
sensitisers, so that the initial black-and-white silver image couples
with a different dye component. The original silver images are
bleached off and the silver is then recovered and recycled. Silver
nitrate is the starting material in all cases.
The market for silver nitrate and silver halides for photography has
rapidly declined with the rise of digital cameras. From the peak
global demand for photographic silver in 1999 (267,000,000 troy ounces
or 8,304.6 tonnes) the market contracted almost 70% by 2013.
Nanoparticles
===============
Nanosilver particles, between 10 and 100 nanometres in size, are used
in many applications. They are used in conductive inks for printed
electronics, and have a much lower melting point than larger silver
particles of micrometre size. They are also used medicinally in
antibacterials and antifungals in much the same way as larger silver
particles. In addition, according to the European Union Observatory
for Nanomaterials (EUON), silver nanoparticles are used both in
pigments, as well as cosmetics.
Miscellanea
=============
Pure silver metal is used as a food colouring. It has the E174
designation and is approved in the European Union. Traditional Indian
and Pakistani dishes sometimes include decorative silver foil known as
'vark', and in various other cultures, silver 'dragée' are used to
decorate cakes, cookies, and other dessert items.
Photochromic lenses include silver halides, so that ultraviolet light
in natural daylight liberates metallic silver, darkening the lenses.
The silver halides are reformed in lower light intensities. Colourless
silver chloride films are used in radiation detectors. Zeolite sieves
incorporating Ag+ ions are used to desalinate seawater during rescues,
using silver ions to precipitate chloride as silver chloride. Silver
is also used for its antibacterial properties for water sanitisation,
but the application of this is limited by limits on silver
consumption. Colloidal silver is similarly used to disinfect closed
swimming pools; while it has the advantage of not giving off a smell
like hypochlorite treatments do, colloidal silver is not effective
enough for more contaminated open swimming pools. Small silver iodide
crystals are used in cloud seeding to cause rain.
The Texas Legislature designated silver the official precious metal of
Texas in 2007.
Precautions
======================================================================
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Silver compounds have low toxicity compared to those of most other
heavy metals, as they are poorly absorbed by the human body when
ingested, and that which does get absorbed is rapidly converted to
insoluble silver compounds or complexed by metallothionein. Silver
fluoride and silver nitrate are caustic and can cause tissue damage,
resulting in gastroenteritis, diarrhoea, falling blood pressure,
cramps, paralysis, or respiratory arrest. Animals repeatedly dosed
with silver salts have been observed to experience anaemia, slowed
growth, necrosis of the liver, and fatty degeneration of the liver and
kidneys; rats implanted with silver foil or injected with colloidal
silver have been observed to develop localised tumours. Parenterally
admistered colloidal silver causes acute silver poisoning. Some
waterborne species are particularly sensitive to silver salts and
those of the other precious metals; in most situations, silver is not
a serious environmental hazard.
In large doses, silver and compounds containing it can be absorbed
into the circulatory system and become deposited in various body
tissues, leading to argyria, which results in a blue-grayish
pigmentation of the skin, eyes, and mucous membranes. Argyria is rare,
and so far as is known, does not otherwise harm a person's health,
though it is disfiguring and usually permanent. Mild forms of argyria
are sometimes mistaken for cyanosis, a blue tint on skin, caused by
lack of oxygen.
Metallic silver, like copper, is an antibacterial agent, which was
known to the ancients and first scientifically investigated and named
the oligodynamic effect by Carl Nägeli. Silver ions damage the
metabolism of bacteria even at such low concentrations as 0.01-0.1
milligrams per litre; metallic silver has a similar effect due to the
formation of silver oxide. This effect is lost in the presence of
sulfur due to the extreme insolubility of silver sulfide.
Some silver compounds are very explosive, such as the nitrogen
compounds silver azide, silver amide, and silver fulminate, as well as
silver acetylide, silver oxalate, and silver(II) oxide. They can
explode on heating, force, drying, illumination, or sometimes
spontaneously. To avoid the formation of such compounds, ammonia and
acetylene should be kept away from silver equipment. Salts of silver
with strongly oxidising acids such as silver chlorate and silver
nitrate can explode on contact with materials that can be readily
oxidised, such as organic compounds, sulfur and soot.
See also
======================================================================
* Silver coin
* Silver medal
* Free silver
* List of countries by silver production
* List of silver compounds
* Silver as an investment
* Silverpoint drawing
External links
======================================================================
* [
http://www.periodicvideos.com/videos/047.htm Silver] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
http://www.silverinstitute.org/ The Silver Institute], industry
association website
* [
https://www.theodoregray.com/PeriodicTable/Elements/047/index.html
Collection of silver items and samples] from Theodore Gray
* [
https://www.cdc.gov/niosh/npg/npgd0557.html Silver entry] in the
'NIOSH Pocket Guide to Chemical Hazards' published by the U.S. Centers
for Disease Control and Prevention's National Institute for
Occupational Safety and Health
* [
https://www.bloomberg.com/markets/commodities/futures/metals Silver
prices] - current spot prices on the global commodities markets, from
Bloomberg L.P.
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Silver
