Amalgam Definition and Uses (Chemistry)


Definition of an amalgam and examples.
An amalgam is an alloy formed between mercury and one or more other metals.

In chemistry and materials science, an amalgam is defined as an alloy of mercury and one or more other metals. Both natural and man-made amalgams exist. Amalgams find use in dentistry, mining, mirrors, and analytical chemistry. Here is a closer look at the types of amalgams, their properties, uses, and safety.

Amalgam and Amalgamation Definitions

Arquerite Amalgam
Arquerite is a mineral that is a natural amalgam of mercury and silver. (Rob Lavinsky, iRocks.com, CC-BY-SA-3.0)

In chemistry, an amalgam is a mercury alloy and amalgamation is the process of forming a mercury alloy. Amalgams readily form between mercury and most metals. Exceptions include iron, cobalt, nickel, platinum, tungsten and tantalum. The reason these elements don’t form amalgams is that the metallic bonds between their atoms are very strong and don’t allow mercury to diffuse into their lattice. Special techniques are used to incorporate these metals into amalgams. Amalgamation is usually exothermic.

Outside of chemistry, an amalgam refers to any mixture, while amalgamation refers to a combination of diverse elements.

Properties

Mercury is a liquid at room temperature, so many amalgams are relatively soft and have a higher vapor pressure than the non-mercury metals in the alloy. Most amalgams are solid at room temperature and pressure. Health effects include allergic reactions and toxicity, both from contact and released mercury vapor. Amalgam disposal presents some problems, as most waste control systems aren’t set up to extract or recycle the mercury. So, disposal often leads to water and soil contamination. The U.S. Environmental Protection Agency prohibits amalgam disposal down the drain. In July of 2018, the European Union prohibited the use of dental amalgam for children under that age of 15 and pregnant or breastfeeding women.

List of Amalgams and Their Uses

Most amalgams are named for the other principal metal in the alloy.

  • Dental amalgam – Dental amalgam usually is a silver amalgam, although other metals may include indium, copper, zinc, palladium, and tin. Silver increases strength and corrosion resistance. Tin causes contraction, off-setting expansion due to silver. Copper improves strength, corrosion resistance, margin leakage, and creep. Zinc reduces oxidation and increases amalgam life. Indium reduces creep. Palladium reduces tarnish and corrosion. Dental amalgam remains soft long enough for a dentist to fill in cavities and then hardens.
  • Silver amalgam – Silver amalgam occurs naturally. Because silver readily alloys with mercury, it’s used in silver mining. The Patio process is used for ores, while the Washoe process separates captures silver during panning.
  • Gold amalgam – Gold amalgam is used in gold mining. A slurry of crushed ore mixed with mercury or passed over mercury-coated copper plates forms gold amalgam. Heating gold amalgam in a distillation retort vaporizes the mercury, leaving the gold. Due to environmental concerns, amalgam extraction has largely been replaced by other methods.
  • Copper amalgam – Copper amalgam is an amalgam probe, which is a device used to detect mercury in the environment. An amalgam probe is a piece of copper foil treated with a nitric acid salt solution. Dipping the probe into water that contains mercury ions forms copper amalgam and discolors the foil. Silver also reacts with copper and forms spots, but discoloration from silver rinses away, while color from copper amalgam remains.
  • Tin amalgam – In the mid-19th century, tin amalgam was a reflective mirror coating.
  • Zinc amalgam – Zinc amalgam is used in the Jones reductor in analytical chemistry and in the Clemmensen reduction in organic synthesis.
  • Sodium amalgam – Sodium amalgam is a reducing agent used in chemistry. It’s also used during high-pressure sodium lamp design to fine-tune the color and electrical properties of the lamp.
  • Thallium amalgam – Thallium amalgam has a lower freezing point (−58 °C) than pure mercury (−38.8 °C). It is used in low temperature thermometers.
  • Lead amalgam – Lead amalgam forms naturally.
  • Ammonium amalgam – The ammonium cation forms ionic bonds and acts much like a metal. Humphy Davy and Jons Jakob Berzelius discovered ammonium amalgam (H3N-Hg-H). This substance decomposes upon contact with air, water, or alcohol at room temperature to form ammonia, hydrogen gas, and mercury metal.
  • Aluminum amalgam
NileRed demonstrates formation of aluminum amalgam and its oxidation.

References

  • Callister, W. D. (2007). Materials Science and Engineering: An Introduction (7th ed.). New York:John Wiley and Sons, Inc.
  • Duwell, E. J.; Baenziger, N. C. (1955). “The Crystal Structures of KHg and KHg2“. Acta Crystallogr. 8 (11): 705–710. doi:10.1107/S0365110X55002168
  • Ham, Peter (2001). “Zinc amalgam” e-EROS Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rz003
  • Mutter, Joachim (2011). “Is dental amalgam safe for humans? The opinion of the scientific committee of the European Commission”. Journal of Occupational Medicine and Toxicology. 6: 2. doi:10.1186/1745-6673-6-2

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