Amalgam (chemistry)
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An amalgam is an
Important amalgams
Zinc amalgam
Zinc amalgam finds use in organic synthesis (e.g., for the Clemmensen reduction).[4] It is the reducing agent in the
Potassium amalgam
For the
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.
Sodium amalgam
Sodium amalgam is produced as a byproduct 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.
Aluminium amalgam
Tin amalgam
Tin amalgam was used in the middle of the 19th century as a reflective mirror coating.[7]
Other amalgams
A variety of amalgams are known that are of interest mainly in the research context.
- Jöns Jakob Berzelius. It decomposes readily at room temperature or in contact with water or alcohol:
- Thallium amalgam has a freezing point of −58 °C, which is lower than that of pure mercury (−38.8 °C) so it has found a use in low temperature thermometers.
- Gold amalgam: Refined gold, when finely ground and brought into contact with mercury where the surfaces of both metals are clean, amalgamates readily and quickly forms alloys ranging from AuHg2 to Au8Hg.[8]
- Lead forms an amalgam when filings are mixed with mercury[citation needed] and is also listed as a naturally occurring alloy called leadamalgam in the Nickel–Strunz classification.[9]
Dental amalgam
Dentistry has used alloys of mercury with metals such as silver, copper, indium, tin and zinc. Amalgam is an "excellent and versatile restorative material"[10] and is used in dentistry for a number of reasons. 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.
Amalgam is typically compared to resin-based composites because many applications are similar and many physical properties and costs are comparable.
Dental amalgam has been studied and is generally considered to be safe for humans,[11][12] though the validity of some studies and their conclusions have been questioned.[13]
In July 2018 the EU, in consideration of the persistent pollution and environmental toxicity of amalgam's mercury, prohibited amalgam for dental treatment of children under 15 years and of pregnant or breastfeeding women.[14]
Use in mining
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.
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.
Mercury amalgamation was first used on silver ores with the development of the
Gold amalgam
Gold extraction (mining)
Gold amalgam 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
Gold extraction (ore processing)
Where stamp mills 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.
Gold extraction (retorting)
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.
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.
Amalgam probe
Mercury salts are, compared to mercury metal and amalgams, 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
- Hg2+ + Cu → Hg + Cu2+.
See also
References
- ^ Callister, W. D. "Materials Science and Engineering: An Introduction" 2007, 7th edition, John Wiley and Sons, Inc. New York, Section 4.3 and Chapter 9.
- ^ "Mercury Amalgamation".
- ^ "Which one of the following metals does not form amalgams?". BYJU'S.
- ^ .
- ^ "Die Sendung mit der Maus, Sachgeschichte vom Spiegel" (in German). Archived from the original on 17 April 2009. Retrieved 2009-04-24.
- ^ "Mercury Amalgamation". mine-engineer.com. Retrieved 8 April 2018.
- ^ webmineral.com/data/Leadamalgam.shtml
- PMID 21217947.
- ^ "The "Mercury Toxicity" Scam:: How Anti-Amalgamists Swindle People". www.quackwatch.com. Archived from the original on 2018-11-15. Retrieved 2017-09-12.
- ^ "Statement on Dental Amalgam". www.ada.org.
- PMID 21232090.
- ^ "Mercury Regulation EU". www.europa.eu. 25 September 2023.
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 3-11-003653-3
External links
- Media related to Amalgam at Wikimedia Commons
- The dictionary definition of amalgam at Wiktionary
- The Periodic Table of Videos. University of Nottingham.
- The American Cyclopædia. 1879. .