Silver is achemical element; it hassymbolAg (from Latinargentum'silver', derived fromProto-Indo-European*h₂erǵ'shiny, white') andatomic number 47. A soft, white, lustroustransition metal, it exhibits the highestelectrical conductivity,thermal conductivity, andreflectivity of anymetal.[11] Silver is found in the Earth's crust in the pure, free elemental form ("native silver"), as analloy withgold and other metals, and in minerals such asargentite andchlorargyrite. Most silver is produced as a byproduct ofcopper, gold,lead, andzincrefining. Silver is a naturally occurring element. It is found in the environment combined with other elements such as sulfide, chloride, and nitrate. Pure silver is “silver” colored, but silver nitrate and silver chloride are powdery white and silver sulfide and silver oxide are dark-gray to black. Silver is often found as a by-product during the retrieval of copper, lead, zinc, and gold ores.[12]
Silver has long been valued as aprecious metal. Silver metal is used in manybullion coins, sometimesalongside gold:[13] while it is more abundant than gold, it is much less abundant as anative metal.[14] Its purity is typically measured on aper-mille basis; a 94%-pure alloy is described as "0.940 fine". As one of the sevenmetals of antiquity, silver has had an enduring role in most human cultures.
Silver bullion bar, 1000 ouncesSilver is extremely ductile, and can be drawn into a wire one atom wide.[15]
Silver is similar in its physical and chemical properties to its two vertical neighbours ingroup 11 of theperiodic table:copper, andgold. Its 47 electrons are arranged in theconfiguration [Kr]4d105s1, similarly to copper ([Ar]3d104s1) and gold ([Xe]4f145d106s1); group 11 is one of the few groups in thed-block which has a completely consistent set of electron configurations.[16] This distinctive electron configuration, with a single electron in the highest occupied ssubshell over a filled d subshell, accounts for many of the singular properties of metallic silver.[17]
Silver is a relatively soft and extremelyductile andmalleabletransition metal, though it is slightly less malleable than gold. Silver crystallises in aface-centred cubic lattice with bulk coordination number 12, where only the single 5s electron is delocalised, similarly to copper and gold.[18] Unlike metals with incomplete d-shells, metallic bonds in silver are lacking acovalent character and are relatively weak. This observation explains the lowhardness and high ductility ofsingle crystals of silver.[19]
Silver has a brilliant, white, metallic luster that can take a highpolish,[20] and which is so characteristic that the name of the metal itself has become acolor name.[17] Protected silver has greater opticalreflectivity thanaluminium at all wavelengths longer than ~450 nm.[21] At wavelengths shorter than 450 nm, silver's reflectivity is inferior to that of aluminium and drops to zero near 310 nm.[22]
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.[23] Thethermal conductivity of silver is among the highest of all materials, although the thermal conductivity ofcarbon (in thediamondallotrope) andsuperfluid helium-4 are higher.[16] Theelectrical conductivity of silver is the highest of all metals, greater even than copper. Silver also has the lowestcontact resistance of any metal.[16] Silver is rarely used for its electrical conductivity, due to its high cost, although an exception is inradio-frequency engineering, particularly atVHF and higher frequencies where silver plating improves electrical conductivity because thosecurrents tend to flow on the surface of conductors rather than through the interior. DuringWorld War II in the US,13540 tons of silver were used for theelectromagnets incalutrons for enrichinguranium, mainly because of the wartime shortage of copper.[24][25][26]
Silver readily formsalloys with copper, gold, andzinc. 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 tobody-centred cubic (electron concentration 1.5),complex cubic (1.615), andhexagonal close-packed phases (1.75).[18]
Naturally occurring silver is composed of two stableisotopes,107Ag and109Ag, with107Ag being slightly more abundant (51.839%natural abundance). This almost equal abundance is rare in the periodic table. Theatomic weight is 107.8682(2)u;[27][28] this value is very important because of the importance of silver compounds, particularly halides, ingravimetric analysis.[27] Both isotopes of silver are produced in stars via thes-process (slow neutron capture), as well as in supernovas via ther-process (rapid neutron capture).[29]
Twenty-eightradioisotopes have been characterised, the most stable being105Ag with ahalf-life of 41.29 days,111Ag with a half-life of 7.45 days, and112Ag with a half-life of 3.13 hours. Silver has numerousnuclear isomers, the most stable being108mAg (t1/2 = 418 years),110mAg (t1/2 = 249.79 days) and106mAg (t1/2 = 8.28 days). All of the remainingradioactive isotopes have half-lives of less than an hour, and the majority of these have half-lives of less than three minutes.[30]
The palladiumisotope107Pd decays by beta emission to107Ag 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 in107Ag abundance.Radiogenic107Ag was first discovered in theSanta Clara meteorite in 1978.[32]107Pd–107Ag correlations observed in bodies that have clearly been melted since theaccretion of theSolar System must reflect the presence of unstable nuclides in the early Solar System.[33]
Chemistry
Oxidation states and stereochemistries of silver[34]
Oxidation state
Coordination number
Stereochemistry
Representative compound
0 (d10s1)
3
Planar
Ag(CO)3
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+
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 theelectrochemical series (E0(Ag+/Ag) = +0.799 V).[17] 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.[35] Unlikecopper, for which the largerhydration 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.[35]
Most silver compounds have significantcovalent character due to the small size and high first ionisation energy (730.8 kJ/mol) of silver.[17] Furthermore, silver's Paulingelectronegativity of 1.93 is higher than that oflead (1.87), and itselectron affinity of 125.6 kJ/mol is much higher than that ofhydrogen (72.8 kJ/mol) and not much less than that ofoxygen (141.0 kJ/mol).[36] 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 unstableorganometallic compounds, forming linear complexes showing very lowcoordination numbers like 2, and forming an amphoteric oxide[37] as well asZintl phases like thepost-transition metals.[38] Unlike the preceding transition metals, the +1 oxidation state of silver is stable even in the absence ofπ-acceptor ligands.[35]
Silver does not react with air, even at red heat, and thus was considered byalchemists as anoble metal, along with gold. Its reactivity is intermediate between that of copper (which formscopper(I) oxide when heated in air to red heat) and gold. Like copper, silver reacts withsulfur and its compounds; in their presence, silver tarnishes in air to form the blacksilver sulfide (copper forms the greensulfate instead, while gold does not react). While silver is not attacked by non-oxidising acids, the metal dissolves readily in hot concentratedsulfuric acid, as well as dilute or concentratednitric acid. In the presence of air, and especially in the presence ofhydrogen peroxide, silver dissolves readily in aqueous solutions ofcyanide.[34]
The three main forms of deterioration in historical silver artifacts are tarnishing, formation ofsilver chloride due to long-term immersion in salt water, as well as reaction withnitrate 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.[39]
Silver metal is attacked by strongoxidant such aspotassium permanganate (KMnO 4) andpotassium dichromate (K 2Cr 2O 7), and in the presence ofpotassium bromide (KBr). These compounds are used in photography tobleach silver images, converting them to silver bromide that can either be fixed withthiosulfate or redeveloped tointensify the original image. Silver formscyanide complexes (silver cyanide) that are soluble in water in the presence of an excess of cyanide ions. Silver cyanide solutions are used inelectroplating of silver.[40]
The commonoxidation 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).[41] The +3 state requires very strong oxidising agents to attain, such asfluorine orperoxodisulfate, and some silver(III) compounds react with atmospheric moisture and attack glass.[42] Indeed, silver(III) fluoride is usually obtained by reacting silver or silver monofluoride with the strongest known oxidising agent,krypton difluoride.[43]
Compounds
Oxides and chalcogenides
Silver(I) sulfide
Silver and gold have rather lowchemical affinities for oxygen, lower than copper, and it is therefore expected that silver oxides are thermally quite unstable. Soluble silver(I) salts precipitate dark-brownsilver(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.[44] This and other silver(I) compounds may be oxidised by the strong oxidising agentperoxodisulfate to black AgO, a mixedsilver(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.[44]
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 ofhydrogen sulfide with silver metal or aqueous Ag+ ions. Many non-stoichiometricselenides andtellurides are known; in particular, AgTe~3 is a low-temperaturesuperconductor.[44]
The only known dihalide of silver isthe 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 synthesisehydrofluorocarbons.[45]
In stark contrast to this, all four silver(I) halides are known. Thefluoride,chloride, andbromide have the sodium chloride structure, but theiodide has three known stable forms at different temperatures; that at room temperature is the cubiczinc blende structure. They can all be obtained by the direct reaction of their respective elements.[45] 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 forligand-metal charge transfer (X−Ag+ → XAg) decreases.[45] The fluoride is anomalous, as the fluoride ion is so small that it has a considerablesolvation energy and hence is highly water-soluble and forms di- and tetrahydrates.[45] The other three silver halides are highly insoluble in aqueous solutions and are very commonly used in gravimetricanalytical methods.[27] All four arephotosensitive (though the monofluoride is so only toultraviolet light), especially the bromide and iodide which photodecompose to silver metal, and thus were used intraditional photography.[45] The reaction involved is:[46]
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 acrystal defect or an impurity site, so that the electron's energy is lowered enough that it is "trapped".[46]
Other inorganic compounds
Silver crystals forming on a copper surface in a silver nitrate solution. Video byMaxim Bilovitskiy.Crystals of silver nitrate
Whitesilver nitrate, AgNO3, is a versatile precursor to many other silver compounds, especially the halides, and is much less sensitive to light. It was once calledlunar caustic because silver was calledluna by the ancient alchemists, who believed that silver was associated with the Moon.[47][48] It is often used for gravimetric analysis, exploiting the insolubility of the heavier silver halides which it is a common precursor to.[27] Silver nitrate is used in many ways inorganic synthesis, e.g. fordeprotection and oxidations. Ag+ bindsalkenes reversibly, and silver nitrate has been used to separate mixtures of alkenes by selective absorption. The resultingadduct can be decomposed withammonia to release the free alkene.[49]
Yellowsilver carbonate, Ag2CO3 can be easily prepared by reacting aqueous solutions ofsodium carbonate with a deficiency of silver nitrate.[50] Its principal use is for the production of silver powder for use in microelectronics. It is reduced withformaldehyde, producing silver free of alkali metals:[51]
Silver fulminate, AgCNO, a powerful, touch-sensitiveexplosive used inpercussion caps, is made by reaction of silver metal with nitric acid in the presence ofethanol. Other dangerously explosive silver compounds aresilver azide, AgN3, formed by reaction of silver nitrate withsodium azide,[52] andsilver acetylide, Ag2C2, formed when silver reacts withacetylene gas in ammonia solution.[35] 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.[35]
2AgN 3 (s) → 3N 2 (g) + 2 Ag (s)
Coordination compounds
Structure of the diamminesilver(I) complex, [Ag(NH3)2]+
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 alkalineperoxodisulfate. The yellow diamagnetic [AgF4]− is much less stable, fuming in moist air and reacting with glass.[42]
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 inphosphoric acid due to complex formation. Peroxodisulfate oxidation is generally necessary to give the more stable complexes with heterocyclicamines, 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 withN- orO-donor ligands such as pyridine carboxylates.[53]
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−; andcyanide 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→}; silverthiocyanate 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-tetrahedraldiphosphine anddiarsine complexes [Ag(L–L)2]+.[54]
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 withalkenes andalkynes are known, but their bonds are thermodynamically weaker than even those of theplatinum complexes (though they are formed more readily than those of the analogous gold complexes): they are also quite unsymmetrical, showing the weakπ bonding in group 11. Ag–Cσ 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).[55]
The C–Ag bond is stabilised byperfluoroalkyl ligands, for example in AgCF(CF3)2.[56] Alkenylsilver compounds are also more stable than their alkylsilver counterparts.[57] 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 withbis(acetonitrile)palladium dichloride orchlorido(dimethyl sulfide)gold(I):[58]
Intermetallic
Different colors of silver–copper–gold alloys
Silver formsalloys with most other elements on the periodic table. The elements from groups 1–3, except forhydrogen,lithium, andberyllium, 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 (exceptboron andcarbon) 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 theeutectic mixture is used in vacuumbrazing. 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).[59]
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: dentalamalgams 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 innuclear reactors because of its high thermal neutron capturecross-section, good conduction of heat, mechanical stability, and resistance to corrosion in hot water.[59]
Etymology
The wordsilver appears inOld English in various spellings, such asseolfor andsiolfor. It iscognate withOld High Germansilabar;Gothicsilubr; orOld Norsesilfr, all ultimately deriving fromProto-Germanic*silubra. TheBalto-Slavic words for silver are rather similar to the Germanic ones (e.g.Russianсеребро [serebró],Polishsrebro,Lithuaniansidãbras), as is theCeltiberian formsilabur. They may have a common Indo-European origin, although their morphology rather suggest a non-Indo-EuropeanWanderwort.[60][61] Some scholars have thus proposed aPaleo-Hispanic origin, pointing to theBasque formzilharr as an evidence.[62]
The chemical symbol Ag is from theLatin word forsilver,argentum (compareAncient Greekἄργυρος,árgyros), from theProto-Indo-European root *h₂erǵ- (formerly reconstructed as*arǵ-), meaning'white' or'shining'. This was the usual Proto-Indo-European word for the metal, whose reflexes are missing in Germanic and Balto-Slavic.[61]
Silver was known in prehistoric times:[63] the three metals of group 11, copper, silver, and gold, occur in theelemental form in nature and were probably used as the first primitive forms ofmoney as opposed to simple bartering.[64] Unlike copper, silver did not lead to the growth ofmetallurgy, on account of its low structural strength; it was more often used ornamentally or as money.[65] Since silver is more reactive than gold, supplies of native silver were much more limited than those of gold.[64] For example, silver was more expensive than gold in Egypt until around the fifteenth century BC:[66] the Egyptians are thought to have separated gold from silver by heating the metals with salt, and then reducing thesilver chloride produced to the metal.[67]
The situation changed with the discovery ofcupellation, a technique that allowed silver metal to be extracted from its ores. Whileslag heaps found inAsia Minor and on the islands of theAegean Sea indicate that silver was being separated fromlead as early as the4th millennium BC,[16] and one of the earliest silver extraction centres in Europe wasSardinia in the earlyChalcolithic period,[68] these techniques did not spread widely until later,when it spread throughout the region and beyond.[66] The origins of silver production inIndia,China, andJapan were almost certainly equally ancient, but are not well-documented due to their great age.[67]
Silver mining and processing inKutná Hora, Bohemia, 1490s
When thePhoenicians first came to what is nowSpain, 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.[66] By the time of the Greek and Roman civilisations, silver coins were a staple of the economy:[64] the Greeks were already extracting silver fromgalena by the 7th century BC,[66] and the rise ofAthens was partly made possible by the nearby silver mines atLaurium, from which they extracted about 30 tonnes a year from 600 to 300 BC.[69] The stability of theRoman currency relied to a high degree on the supply of silver bullion, mostly from Spain, whichRoman miners produced on a scale unparalleled before thediscovery of the New World. Reaching a peak production of 200 tonnes per year, an estimated silver stock of 10,000 tonnes circulated in theRoman economy in the middle of the second century AD, five to ten times larger than the combined amount of silver available tomedieval Europe and theAbbasid Caliphate around AD 800.[70][71] 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 ofCharlemagne: by then, tens of thousands of tonnes of silver had already been extracted.[67]
Central Europe became the centre of silver production during theMiddle Ages, as the Mediterranean deposits exploited by the ancient civilisations had been exhausted. Silver mines were opened inBohemia,Saxony,Alsace, theLahn region,Siegerland,Silesia,Hungary,Norway,Steiermark,Schwaz, and the southernBlack 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 theIndustrial Revolution, before which the world production of silver was around a meagre 50 tonnes per year.[67] In the Americas, high temperature silver-leadcupellation 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.[67][72]
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, particularlyPeru,Bolivia,Chile, andArgentina:[67] the last of these countries later took its name from that of the metal that composed so much of its mineral wealth.[69] The silver trade gave way to aglobal network of exchange. As one historian put it, silver "went round the world and made the world go round."[73] 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".[74] 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."[75]
In the 19th century, primary production of silver moved to North America, particularlyCanada,Mexico, andNevada in theUnited States: some secondary production from lead and zinc ores also took place in Europe, and deposits inSiberia and theRussian Far East as well as inAustralia were mined.[67]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.[67]
16th-century fresco painting of Judas being paid thirty pieces of silver for his betrayal of Jesus
Silver plays a certain role in mythology and has found various usage as a metaphor and in folklore. The Greek poetHesiod'sWorks and Days (lines 109–201) lists differentages of man named after metals like gold, silver, bronze and iron to account for successive ages of humanity.[76]Ovid'sMetamorphoses 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:
But when goodSaturn, banish'd from above, Was driv'n to Hell, the world was underJove. Succeeding times a silver age behold, Excelling brass, but more excell'd by gold.
In folklore, silver was commonly thought to have mystic powers: for example, abullet cast from silver is often supposed in such folklore the only weapon that is effective against awerewolf,witch, or othermonsters.[77][78][79] From this the idiom of asilver bullet developed into figuratively referring to any simple solution with very high effectiveness or almost miraculous results, as in the widely discussedsoftware engineering paper "No Silver Bullet."[80] Other powers attributed to silver include detection of poison and facilitation of passage into themythical realm of fairies.[79]
Silver production has also inspired figurative language. Clear references to cupellation occur throughout theOld Testament of theBible, such as inJeremiah'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)[66]
Silver also has more negative cultural meanings: the idiomthirty pieces of silver, referring to a reward for betrayal, references the bribeJudas Iscariot is said in theNew Testament to have taken from Jewish leaders inJerusalem to turnJesus of Nazareth over to soldiers of the high priest Caiaphas.[81] Ethically, silver also symbolizes greed and degradation of consciousness; this is the negative aspect, the perverting of its value.[82]
The abundance of silver in the Earth's crust is 0.08 parts per million, almost exactly the same as that ofmercury. It mostly occurs insulfide ores, especiallyacanthite andargentite, Ag2S. Argentite deposits sometimes also containnative silver when they occur in reducing environments, and when in contact with salt water they are converted tochlorargyrite (includinghorn silver), AgCl, which is prevalent inChile andNew South Wales.[83] Most other silver minerals are silverpnictides orchalcogenides; they are generally lustrous semiconductors. Most true silver deposits, as opposed to argentiferous deposits of other metals, came fromTertiary period vulcanism.[84]
The principal sources of silver are the ores of copper, copper-nickel, lead, and lead-zinc obtained fromPeru,Bolivia,Mexico,China,Australia,Chile,Poland andSerbia.[16] Peru, Bolivia and Mexico have been mining silver since 1546, and are still major world producers. Top silver-producing mines areCannington (Australia),Fresnillo (Mexico),San Cristóbal (Bolivia),Antamina (Peru),Rudna (Poland), andPenasquito (Mexico).[85] Top near-term mine development projects through 2015 are Pascua Lama (Chile), Navidad (Argentina), Jaunicipio (Mexico), Malku Khota (Bolivia),[86] and Hackett River (Canada).[85] InCentral Asia,Tajikistan is known to have some of the largest silver deposits in the world.[87]
Silver is usually found in nature combined with other metals, or in minerals that contain silver compounds, generally in the form ofsulfides such asgalena (lead sulfide) orcerussite (lead carbonate). So the primary production of silver requires the smelting and thencupellation of argentiferous lead ores, a historically important process.[88] 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 tolead monoxide, then known aslitharge, which captures the oxygen from the other metals present. The liquid lead oxide is removed or absorbed bycapillary action into the hearth linings.[89][90][91]
Ag(s) + 2Pb(s) +O 2(g) → 2PbO(absorbed) + Ag(l)
Today, silver metal is primarily produced instead as a secondary byproduct ofelectrolytic refining of copper, lead, and zinc, and by application of theParkes process on lead bullion from ore that also contains silver.[92] 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 iselectrolytically 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 dilutesulfuric acid and heating with lime or silica flux, before the silver is purified to over 99.9% purity via electrolysis innitrate solution.[83]
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 (6,300tonnes or 24.2% of the world's total of 26,000 t), followed by China (3,600 t) and Peru (3,100 t).[92]
In marine environments
Silver concentration is low inseawater (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.[93] Analysing the presence and dynamics of silver in marine environments is difficult due to these particularly low concentrations and complex interactions in the environment.[94] 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.[95][96] 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.[96]
In the Atlantic and Pacific, silver concentrations are minimal at the surface but rise in deeper waters.[97] 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.[95] 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.[98]
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 throughbioaccumulation, association with particulate matters, andsorption.[93] 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.[99]
In one study, the presence of excess ionic silver and silvernanoparticles caused bioaccumulation effects on zebrafish organs and altered the chemical pathways within their gills.[100] 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.[101] 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.[102]
The earliest known coins were minted in the kingdom ofLydia inAsia Minor around 600 BC.[103] The coins of Lydia were made ofelectrum, which is a naturally occurringalloy of gold and silver, that was available within the territory of Lydia.[103] Since that time,silver standards, in which the standard economicunit of account is a fixed weight of silver, have been widespread throughout the world until the 20th century. Notablesilver coins through the centuries include theGreek drachma,[104] the Romandenarius,[105] the Islamicdirham,[106] thekarshapana from ancient India andrupee from the time of theMughal Empire (grouped with copper and gold coins to create a trimetallic standard),[107] and theSpanish dollar.[108]
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.[109]
Today, silver bullion has theISO 4217 currency code XAG, one of only fourprecious metals to have one (the others beingpalladium,platinum, and gold).[110] Silver coins are produced from cast rods or ingots, rolled to the correct thickness, heat-treated, and then used to cutblanks from. These blanks are then milled and minted in a coining press; modern coining presses can produce 8000 silver coins per hour.[109]
Silver prices are normally quoted introy ounces. One troy ounce is equal to 31.1034768 grams. The London silver fix is published every working day at noonLondon time.[111] This price is determined by several major international banks and is used byLondon bullion market members for trading that day. Prices are most commonly shown as theUnited States dollar (USD), thePound sterling (GBP), and theEuro (EUR).
The major use of silver besides coinage throughout most of history was in the manufacture ofjewellery and other general-use items, and this continues to be a major use today. Examples includetable silver for cutlery, for which silver is highly suited due to its antibacterial properties.Western concert flutes are usually plated with or made out ofsterling silver;[113] in fact, most silverware is only silver-plated rather than made out of pure silver; the silver is normally put in place byelectroplating. Silver-plated glass (as opposed to metal) is used for mirrors,vacuum flasks, and Christmas tree decorations.[114]
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 ofhydrogen sulfide and its derivatives. Including precious metals such as palladium, platinum, and gold gives resistance to tarnishing but is quite costly;base metals likezinc,cadmium,silicon, andgermanium 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.[113]
In medicine, silver is incorporated into wound dressings and used as an antibiotic coating in medical devices. Wound dressings containingsilver sulfadiazine orsilver nanomaterials are used to treat external infections. Silver is also used in some medical applications, such asurinary catheters (where tentative evidence indicates it reduces catheter-relatedurinary tract infections) and inendotracheal breathing tubes (where evidence suggests it reduces ventilator-associatedpneumonia).[115][116] The silverion isbioactive and in sufficientconcentration readily killsbacteriain 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.[117][118] Bacteria can develop resistance to the antimicrobial action of silver.[119] Silver compounds are taken up by the body likemercury 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.[114]Silver diammine fluoride, the fluoride salt of acoordination complex with the formula [Ag(NH3)2]F, is a topicalmedicament (drug) used to treat and preventdental caries (cavities) and relieve dentinal hypersensitivity.[120]
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 forRF,VHF, and higher frequencies, particularly in tuned circuits such ascavity filters where conductors cannot be scaled by more than 6%.Printed circuits andRFID antennas are made with silver paints,[16][121] Powdered silver and its alloys are used in paste preparations for conductor layers and electrodes, ceramic capacitors, and other ceramic components.[122]
Brazing alloys
Silver-containingbrazing alloys are used for brazing metallic materials, mostlycobalt,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, andphosphorus. Silver provides increased workability and corrosion resistance during usage.[123]
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. Silvercrucibles (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 withfluorine. 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.[124]
Catalysis
Silver metal is a good catalyst foroxidation reactions; in fact it is somewhat too good for most purposes, as finely divided silver tends to result in complete oxidation of organic substances tocarbon 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 ofethylene toethylene oxide at 230–270 °C. Dehydrogenation ofmethanol toformaldehyde is conducted at 600–720 °C over silver gauze or crystals as the catalyst, as is dehydrogenation ofisopropanol toacetone. In the gas phase,glycol yieldsglyoxal andethanol yieldsacetaldehyde, while organicamines are dehydrated tonitriles.[124]
Photography
Before the advent ofdigital photography, which is now dominant, the photosensitivity of silver halides was exploited for use in traditional film photography. Thephotosensitive emulsion used in black-and-white photography is a suspension of silver halide crystals ingelatin, possibly mixed in with some noble metal compounds for improved photosensitivity,developing, andtuning[clarify].
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.[125]
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 (267,000,000troy ounces or 8,304.6tonnes) the market contracted almost 70% by 2013.[126]
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.[127] They are also used medicinally in antibacterials and antifungals in much the same way as larger silver particles.[118] In addition, according to theEuropean Union Observatory for Nanomaterials (EUON), silver nanoparticles are used both in pigments, as well as cosmetics.[128][129]
Miscellanea
A tray ofSouth Asian sweets, with some pieces covered with shiny silvervark
Pure silver metal is used as a food colouring. It has theE174 designation and is approved in theEuropean Union.[130] Traditional Indian and Pakistani dishes sometimes include decorative silver foil known asvark,[131] and in various other cultures, silverdragée are used to decorate cakes, cookies, and other dessert items.[132]
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 inradiation detectors.Zeolite sieves incorporating Ag+ ions are used todesalinate 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 likehypochlorite treatments do, colloidal silver is not effective enough for more contaminated open swimming pools. Smallsilver iodide crystals are used incloud seeding to cause rain.[118]
TheTexas Legislature designated silver the official precious metal of Texas in 2007.[133]
Silver compounds have low toxicity compared to those of most otherheavy 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 bymetallothionein. Silver fluoride and silver nitrate are caustic and can cause tissue damage, resulting ingastroenteritis,diarrhoea, fallingblood pressure, cramps, paralysis, orrespiratory arrest. Animals repeatedly dosed with silver salts have been observed to experienceanaemia, slowed growth,necrosis of the liver, and fatty degeneration of the liver and kidneys; rats implanted with silver foil or injected withcolloidal silver have been observed to develop localised tumours.Parenterally admistered colloidal silver causes acute silver poisoning.[135] 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.[135]
In large doses, silver and compounds containing it can be absorbed into thecirculatory system and become deposited in various body tissues, leading toargyria, which results in a blue-grayish pigmentation of the skin, eyes, andmucous 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 forcyanosis, a blue tint on skin, caused by lack of oxygen.[135][16]
Metallic silver, like copper, is an antibacterial agent, which was known to the ancients and first scientifically investigated and named theoligodynamic effect byCarl 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 ofsulfur due to the extreme insolubility of silver sulfide.[135]
Some silver compounds are very explosive, such as the nitrogen compounds silver azide, silveramide, and silver fulminate, as well assilver 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 andacetylene should be kept away from silver equipment. Salts of silver with strongly oxidising acids such assilver chlorate and silver nitrate can explode on contact with materials that can be readily oxidised, such as organic compounds, sulfur and soot.[135]
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