Aluminium amalgam

Aluminium amalgam is a solution of aluminium in mercury. Aluminium amalgam may be prepared by either grinding aluminium pellets or wire in mercury, or by allowing aluminium wire to react with a solution of mercury(II) chloride in water.

This amalgam is 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 mercury-containing reaction waste is troublesome to dispose of as well.

This reaction was popularized by Alexander Shulgin in his book PiHKAL, but has few virtues beyond being inexpensive and using chemicals that are readily available.

Reactivity
Aluminium in air is ordinarily protected by a molecule-thin layer of its own oxide which is not porous to oxygen. Mercury coming into contact with this oxide does no harm. However, if any elemental aluminium is exposed (even by a recent scratch), the mercury may combine with it, starting the process described above, and potentially damaging a large part of the aluminium before it finally ends (Ornitz 1998).

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 aluminium 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 oxidize aluminium and reduce H+ from water, creating aluminium hydroxide (Al(OH)3) and hydrogen gas (H2).

Due to the reactivity of aluminium amalgam, 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.