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For this reason, restrictions are placed on the use and handling of mercury in proximity with aluminium. In particular, mercury is not allowed aboard aircraft under most circumstances because of the risk of it forming amalgam with exposed aluminium parts in the aircraft. In the Second World War, mercury was used to sabotage aircraft.
This amalgam is also used as a chemical reagent to reduce compounds, such as the reduction of imines to amines. Since this reaction produces waste mercury metal, it is best avoided in favor of more environmentally friendly reagents such as hydrides. The reaction tailings must be properly disposed of by a hazardous waste management company.
Normally pieces of aluminum aren't very reactive because they are covered with a thin layer of inert aluminum oxide (Al2O3). The mercury in the solution allows this protective layer to be removed, then prevents its (otherwise very rapid) re-formation by creating a thin layer of mercury over the bare aluminum. The net result is similar to the mercury electrodes often used in electrochemistry, except instead of providing electrons from an electrical supply they are provided by the aluminum (which becomes oxidized in the process.) The reaction that occurs at the surface of the amalgam may actually be a hydrogenation rather than a reduction.
The presence of water in the solution is reportedly helpful (even necessary); the electron rich amalgam will reduce water to hydroxide, creating aluminum hydroxide (Al(OH)3) and hydrogen gas (H2).
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Mercury-aluminum_amalgam". A list of authors is available in Wikipedia.|