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| [[File:Amalgam-200675.jpg|thumb|[[Arquerite]], a natural amalgam of silver and mercury]]
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| An '''amalgam''' is a substance formed by the reaction of [[mercury (element)|mercury]] with another [[metal]]. Almost all metals can form amalgams with mercury, the notable exception being [[iron]]. [[Silver]]-mercury [[amalgam (dentistry)|amalgams]] are important in [[dentistry]], and [[gold]]-mercury amalgam is used in the [[Gold extraction|extraction of gold]] from [[ore]].
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| == Known amalgams and their uses ==
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| === Dental amalgam ===
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| {{main|Amalgam (dentistry)}}
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| [[File:Filling.jpg|thumb|An amalgam dental filling]]
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| Dentistry has used alloys of mercury with metals such as silver, [[copper]], [[indium]], [[tin]] and [[zinc]]. Amalgam is an "excellent and versatile restorative material" and is used in dentistry for a number of reasons.{{Citation needed|date=July 2011}} It is inexpensive and relatively easy to use and manipulate during placement; it remains soft for a short time so it can be packed to fill any irregular volume, and then forms a hard compound. Amalgam possesses greater [[longevity]] when compared to other direct restorative materials, such as composite. However, this difference has decreased with continual development of composite resins.
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| Amalgam is typically compared to resin-based composites because many applications are similar and many physical properties and costs are comparable.
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| ===Potassium amalgam===
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| For the [[alkali metals]], amalgamation is exothermic, and distinct chemical forms can be identified, such as KHg and KHg<sub>2</sub>.<ref name="duwell">{{cite journal | title=The Crystal Structures of KHg and KHg2 |author=E J Duwell |author2=N C Baenziger |journal=Acta Cryst. | volume=8 |year=1955 |pages=705–710 | doi=10.1107/S0365110X55002168 | issue=11}}</ref> KHg is a gold-coloured compound with a melting point of 178 °C, and KHg<sub>2</sub> a silver-coloured compound with a melting point of 278 °C. These amalgams are very sensitive to air and water, but can be worked with under dry nitrogen. The Hg-Hg distance is around 300 [[picometre]]s, Hg-K around 358 pm.<ref name="duwell" />
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| Phases K<sub>5</sub>Hg<sub>7</sub> and KHg<sub>11</sub> are also known; [[rubidium]], [[strontium]] and [[barium]] undecamercurides are known and isostructural. [[Sodium amalgam|Sodium amalgam (NaHg<sub>2</sub>)]] has a different structure, with the mercury atoms forming hexagonal layers, and the sodium atoms a linear chain which fits into the holes in the hexagonal layers, but the potassium atom is too large for this structure to work in KHg<sub>2</sub>.
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| === Sodium amalgam ===
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| {{main|Sodium amalgam}}
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| Sodium amalgam is produced as a by product of the [[chloralkali process]] and used as an important reducing agent in organic and inorganic chemistry. With water it decomposes into concentrated [[sodium hydroxide]] solution,<!-- is there an English word for this? --> hydrogen and mercury, which can then return to the chloralkali process anew. If absolutely water-free alcohol is used instead of water, an [[alkoxide]] of sodium is produced instead of the alkali solution.
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| === Ammonium amalgam ===
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| In 1808, an ammonium amalgam was discovered by [[Humphry Davy]] and [[Jöns Jakob Berzelius]]
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| It is a grey, soft, spongy mass which decomposes readily at room temperature or in contact with water or alcohol:
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| :<math>\mathrm{2 \ H_3N{-}Hg{-}H \ \xrightarrow{\Delta T} \ 2 \ NH_3 + H_2 + 2 \ Hg}</math>
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| It is highly toxic and harmful to the environment.
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| === Aluminium amalgam ===
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| {{main|Aluminium amalgam}}
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| Aluminium amalgam is used as a reducing agent.
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| {{Expand section|date=April 2009|reason=one paragraph and/or link to a related article?}}
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| === Thallium amalgam ===
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| [[Thallium]] amalgam has a freezing point of −58 °C, lower than mercury's, and so has found a use in low temperature thermometers.
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| {{Expand section|date=April 2009|reason=one paragraph and/or link to a related article?}}
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| === Tin amalgam ===
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| Tin amalgam was used in the middle of the 19th century as a reflective [[silvering|mirror coating]].<ref>{{Cite web |title=Die Sendung mit der Maus, Sachgeschichte vom Spiegel |url=http://leifi.physik.uni-muenchen.de/web_ph09/umwelt_technik/10spiegelbau_maus/spegelbau.htm |language=German |accessdate=2009-04-24 |postscript=<!--None-->| archiveurl= http://web.archive.org/web/20090417222633/http://leifi.physik.uni-muenchen.de/web_ph09/umwelt_technik/10spiegelbau_maus/spegelbau.htm| archivedate= 17 April 2009 <!--DASHBot-->| deadurl= no}}</ref>
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| == Use in mining ==
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| Mercury has been used in gold and silver mining because of the convenience and the ease with which mercury and the precious metals will amalgamate. In gold placer mining, in which minute specks of gold are washed from sand or gravel deposits, mercury was often used to separate the gold from other heavy minerals.
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| After all of the practical metal had been taken out from the ore, the mercury was dispensed down a long copper trough, which formed a thin coating of mercury on the exterior. The waste ore was then transferred down the trough, and gold in the waste amalgamated with the mercury. This coating would then be scraped off and refined by evaporation to get rid of the mercury, leaving behind somewhat high purity gold.
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| Mercury amalgamation was first used on silver ores with the development of the patio process in Mexico in 1557. There were also additional amalgamation processes that were created for processing silver ores, including pan amalgamation and the Washoe process.
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| === Gold amalgam ===
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| Produced when mercury is used for gold extraction.
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| {{Expand section|date=July 2011|reason=more content or link to related article?}}
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| ==== Gold extraction (mining) ====
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| This has proved effective where gold fines ("flour gold") would not be extractable from ore using hydro-mechanical methods. Large amounts of mercury were used in ''[[placer mining]]'', where deposits composed largely of decomposed granite slurry were separated in long runs of "riffle boxes", with mercury dumped in at the head of the run. The amalgam formed is a heavy solid mass of dull gray color. (The use of mercury in 19th century placer mining in California, now prohibited, has caused extensive pollution problems in riverine and estuarine environments, ongoing to this day.) Sometimes substantial slugs of amalgam are found in downstream river and creek bottoms by amateur [[wet suit|wet-suited]] miners seeking gold nuggets with the aid of an engine-powered water vacuum mounted on a float.
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| ====Gold extraction (ore processing)====
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| [[File:Grabill - Clean Up day at the Deadwood Terra Gold Stamp Mill.jpg|thumb|Interior of the Deadwood Terra Gold [[Stamp mill|Stamp Mill]]. Crushed ore is washed over mercury coated copper sheets, and fine gold particles form an amalgam with the mercury. The amalgam was scraped off and the gold then separated from the amalgam by heating and evaporating the mercury which was then recovered by a [[condensation|condenser]] for reapplication to the plates]]
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| Where [[stamp mill]]s were used to crush gold-bearing ore to fines, a part of the extraction process involved the use of mercury-wetted copper plates, over which the crushed fines were washed. A periodic scraping and re-mercurizing of the plate resulted in amalgam for further processing.
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| ===Gold extraction (retorting)===
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| Amalgam obtained by either process was then heated in a distillation retort, recovering the mercury for reuse and leaving behind the gold. As this released mercury vapors to the atmosphere, the process could induce adverse health effects and long term pollution.
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| Today, mercury amalgamation has been replaced by other methods to recuperate gold and silver from ore in developed nations. Hazards of mercurial toxic waste have played a major role in the phasing out of the mercury amalgamation processes. However mercury amalgamation is still regularly used by small-scale gold placer miners (often illegally) & particularly in developing countries.
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| == Amalgam probe ==
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| [[File:Amalgamprobe.jpg|thumb|The amalgam probe]]
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| Mercury salts are, compared to mercury metal and amalgam, highly toxic due to their solubility in water. The presence of these salts in water can be detected with a probe that uses the readiness of mercury ions to form an amalgam with copper. A [[nitric acid]] solution of salts under investigation is applied to a piece of copper foil, and any mercury ions present will leave spots of silvery-coloured amalgam. Silver ions leave similar spots but are easily washed away, making this a means of distinguishing silver from mercury.
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| The [[redox reaction]] involved where mercury oxidizes the copper is:
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| :Hg<sup>2+</sup> + Cu → Hg + Cu<sup>2+</sup>.
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| Mercury does not form amalgam with iron, platinum, tungsten or tantalum.
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| == See also ==
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| * [[Patio process]]
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| * [[Pan amalgamation]]
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| == References ==
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| {{Reflist}}
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| == Further reading ==
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| {{Commons category|Amalgam}}
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| * Prandtl, W.: ''Humphry Davy, Jöns Jacob Berzelius, zwei führende Chemiker aus der ersten Hälfte des 19. Jahrhunderts''. Wissenschaftliche Verlagsgesellschaft, Stuttgart, 1948
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| * Hofmann, H., Jander, G.: ''Qualitative Analyse'', 1972, Walter de Gruyter, ISBN 3-11-003653-3
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| ==External links==
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| {{Wiktionary|amalgam}}
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| * {{cite web|last=Poliakoff|first=Martyn|title=Amalgams|url=http://www.periodicvideos.com/videos/feature_amalgams.htm|work=[[The Periodic Table of Videos]]|publisher=[[University of Nottingham]]|authorlink=Martyn Poliakoff|year=2009}}
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| * {{Cite AmCyc|Amalgam}}
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| {{DEFAULTSORT:Amalgam (Chemistry)}}
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| [[Category:Mercury (element)]]
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| [[Category:Alloys]]
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