Amalgam

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An amalgam is an alloy of mercury with another metal, which, depending on the proportion of mercury, may be a liquid, a soft paste or a solid.

General properties

With the exception of iron, platinum, tantalum, tungsten, elemental mercury dissolves most metals, forming amalgam. Mercury will even form amalgam with the ammonium species, though the compound is unstable, decomposing at room temperature or in contact with water or alcohol. Amalgamation of alkali metals is exothermic.

Most important amalgams are:

  • Aluminium amalgam: formed from the contact of aluminium metal with mercury, this alloy will slowly react with oxygen to generate a very brittle mass of aluminium oxide fibers.[1] This amalgam is used as a reducing agent, to reduce many compounds, such as imines to amines.
  • Dental amalgam: Consists of amalgam with metals such as silver, copper, indium, tin and zinc, it was widely used in the past as an inexpensive and relatively easy to use and manipulate dental filling, as it remains soft for a short time so it can be packed to fill any irregular volume, and then forms a hard compound. Its use has been discarded in many countries, due to the toxicity of mercury.
  • Gold amalgam: rapidly formed by adding metallic gold to liquid mercury, best visible using gold foil.[2]
  • Potassium amalgam: At least two distinct K amalgams have been produced, KHg and KHg2. KHg is a gold-colored solid with a melting point of 178 °C, while KHg2 is silver-colored, and has a melting point of 278 °C. These amalgams are very sensitive to air and water, and work must be done under inert atmosphere.
  • Silver amalgam: A silvery solid, it is sometimes encountered in nature as the mineral/natural alloy arquerite, which consists of 87% silver and 13% mercury, or as the even rarer natural alloy moschellandsbergite (Ag2Hg3).
  • Sodium amalgam: Amalgams with 2% Na are solids at room temperature, whereas some more dilute amalgams remain liquid. Like the potassium amalgam, it is sensitive to air and water and is a very powerful reducing agent.
  • Thallium amalgam: this eutectic of this amalgam has a freezing point of −58 °C, which is lower than that of pure mercury (−38.8 °C), which allows it to be used in some low temperature thermometers.
  • Tin amalgam: A silvery solid, it didn't have much uses besides reflective mirror coating in the 19th century on some mirrors.
  • Zinc amalgam: Commonly used as reducing agent in organic synthesis for the Clemmensen reduction and in the Jones reductor. In past, the zinc plates of dry batteries were amalgamated with a little mercury to prevent deterioration in storage, process discontinued now due to the toxicity of mercury.

Availability

Some amalgams, like sodium amalgam can be purchased from lab suppliers, like Sigma-Aldrich, while others have to be prepared in situ.

Amalgams may sometimes be encountered in dentistry cabinets, though most will not give it away, usually due to the waste disposal legislation and their stock is usually registered, in most countries, though this may not be true in some third-world countries.

Mercury and mercury compounds are restricted in many countries, like EU countries.

Preparation

Amalgams can be prepared by adding a metal in powdered, foil or bead form to liquid mercury and stirred until it dissolves completely. Heating the mixture may sometimes be required. Amalgams or very reactive metals, like alkali metals, alkaline earth metals, aluminium, etc. must be done under inert atmosphere, to prevent oxidation.

Projects

Safety

Amalgams, like all mercury compounds are very toxic. They will give off mercury vapors, which are much more toxic than metallic mercury alone. Amalgams of reactive metals, like alkali metals will violently react with water, alcohols and may ignite in air.

When purifying gold from the gold amalgam, after dissolving the mercury using nitric acid, small amounts of mercury will remain trapped in the gold particles, where the nitric acid cannot reach. If you try to melt this powder, the mercury will evaporate,y turning into hazardous vapors. It's therefor necessary to further dissolve the gold in aqua regia or some other chemical, to remove all the mercury waste from the gold.

References

  1. https://www.youtube.com/watch?v=IrdYueB9pY4
  2. https://www.youtube.com/watch?v=yAGYGGmUmUw

Relevant Sciencemadness threads