An amalgam is a substance formed by the reaction of some other substance with mercury. Almost all metals can form amalgams with mercury, notable exceptions being iron and platinum. Silver-mercury amalgams are important in dentistry, and gold-mercury amalgam is used in the extraction of gold from ore.
For the alkali metals, amalgamation is distinctly exothermic, and distinct chemical forms can be identified, such as KHg and KHg2.[1] KHg is a gold-coloured compound with a melting point of 178 °C, and KHg2 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 picometres, Hg-K around 358 pm.[1]
Phases K5Hg7 and KHg11 are also known; rubidium, strontium and barium undecamercurides are known and isostructural. Sodium amalgam (NaHg2) 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 KHg2.
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Known amalgams and their uses
Dental amalgam
Dentistry has used alloys of mercury with metals such as silver, copper, indium, tin and zinc.
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Sodium amalgam
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, 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.
Ammonium amalgam
Discovered in 1808 by both Humphry Davy and Jöns Jakob Berzelius.
It is grey, soft, spongy mass which decomposes readily at room temperature or in contact with water or alcohol:
It is highly toxic and harmful to the environment. Its CAS number is 26497-91-6.
Gold amalgam
Produced when mercury is used for gold extraction.
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Aluminium amalgam
Aluminium amalgam is used as a reducing agent.
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Thallium amalgam
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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Other
Tin amalgam was used in the middle of the 19th century as a reflective mirror coating.[2]
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Amalgam probe
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.
The redox reaction involved where mercury oxidizes the copper is:
See also
References
- ^ a b E J Duwell; N C Baenziger (1955). "The Crystal Structures of KHg and KHg2". Acta Cryst. 8: 705–710. doi:.
- ^ (in German) Die Sendung mit der Maus, Sachgeschichte vom Spiegel, http://leifi.physik.uni-muenchen.de/web_ph09/umwelt_technik/10spiegelbau_maus/spegelbau.htm, retrieved on 2009-04-24
Further reading
- Prandtl, W.: Humphry Davy, Jöns Jacob Berzelius, zwei führende Chemiker aus der ersten Hälfte des 19. Jahrhunderts. Wissenschaftliche Verlagsgesellschaft, Stuttgart, 1948
- Hofmann, H., Jander, G.: Qualitative Analyse, 1972, Walter de Gruyter, ISBN 3110036533
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