Aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
can form an
amalgam
Amalgam most commonly refers to:
* Amalgam (chemistry), mercury alloy
* Amalgam (dentistry), material of silver tooth fillings
** Bonded amalgam, used in dentistry
Amalgam may also refer to:
* Amalgam Comics, a publisher
* Amalgam Digital
...
in
solution
Solution may refer to:
* Solution (chemistry), a mixture where one substance is dissolved in another
* Solution (equation), in mathematics
** Numerical solution, in numerical analysis, approximate solutions within specified error bounds
* Soluti ...
with
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
Mercury(II) chloride (or mercury bichloride, mercury dichloride), historically also known as sulema or corrosive sublimate, is the inorganic chemical compound of mercury and chlorine with the formula HgCl2. It is white crystalline solid and is ...
in water.
This amalgam is used as a chemical
reagent to
reduce
Reduction, reduced, or reduce may refer to:
Science and technology Chemistry
* Reduction (chemistry), part of a reduction-oxidation (redox) reaction in which atoms have their oxidation state changed.
** Organic redox reaction, a redox react ...
compounds, such as the reduction of
imine
In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bon ...
s to
amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen
Hydrogen is the chemical element wi ...
s. The aluminium is the ultimate electron donor, and the mercury serves to mediate the electron transfer.
The reaction and the waste from it contains mercury, so special safety precautions and disposal methods are needed. As an
environmentally friendlier
Environment friendly processes, or environmental-friendly processes (also referred to as eco-friendly, nature-friendly, and green), are sustainability and marketing terms referring to goods and services, laws, guidelines and policies that clai ...
alternative,
hydride
In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride ...
s or other reducing agents can often be used to accomplish the same synthetic result. An alloy of aluminium and
gallium was proposed as a method of hydrogen generation, as the gallium renders the aluminium more reactive by preventing it from forming an oxide layer.
Mercury has this same effect on aluminium, but also serves additional functions related to electron transfer that make aluminium amalgams useful for some reactions that would not be possible with gallium.
Reactivity
Aluminium exposed to air is ordinarily protected by a molecule-thin layer of its own oxide. This
aluminium oxide layer serves as a protective barrier to the underlying unoxidized aluminium and prevents amalgamation from occurring. No reaction takes place when oxidized aluminium is exposed to mercury. However, if any elemental aluminium is exposed (even by a recent scratch), the mercury may combine with it to form the amalgam. This amalgamation can continue well beyond the vulnerable aluminium was exposed, potentially reacting with a large amount of the raw aluminium before it finally ends.
The net result is similar to the mercury electrodes often used in
electrochemistry
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...
, however 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 necessary; the
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
rich amalgam will oxidize aluminium and reduce H
+ from water, creating
aluminium hydroxide
Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic an ...
(Al(OH)
3) and
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
gas (H
2). The electrons from the aluminium reduce mercuric Hg
2+ ion to metallic mercury. The metallic mercury can then form an amalgam with the exposed aluminium metal. The amalgamated aluminium then is oxidized by water, converting the aluminium to aluminium hydroxide and releasing free metallic mercury. The generated mercury then cycles through these last two steps until the aluminium metal supply is exhausted.
:
2Al + 3Hg^2+ + 6H2O -> 2Al(OH)3 + 6H+ +3Hg
:
Hg + Al -> Hg*Al
:
2 Hg*Al + 6 H2O -> 2 Al(OH)3 + 2 Hg + 3 H2
Due to the reactivity of aluminium amalgam, restrictions are placed on the use and handling of mercury in proximity with aluminium. In particular, large amounts of mercury are not allowed aboard aircraft under most circumstances because of the risk of it forming amalgam with exposed aluminium parts in the aircraft.
Even the transportation and packaging of mercury-containing thermometers and barometers is severely restricted.
[See for example ]United States Department of Transportation
The United States Department of Transportation (USDOT or DOT) is one of the executive departments of the U.S. federal government. It is headed by the secretary of transportation, who reports directly to the President of the United States a ...
regulation 49 CFR 175.10(a)(13) Accidental mercury spills in aircraft do sometimes result in insurance
write-off
A write-off is a reduction of the recognized value of something. In accounting, this is a recognition of the reduced or zero value of an asset. In income tax statements, this is a reduction of taxable income, as a recognition of certain expenses ...
s.
See also
*
Amalgam
Amalgam most commonly refers to:
* Amalgam (chemistry), mercury alloy
* Amalgam (dentistry), material of silver tooth fillings
** Bonded amalgam, used in dentistry
Amalgam may also refer to:
* Amalgam Comics, a publisher
* Amalgam Digital
...
*
Aluminium-gallium
References
External links
Video of Mercury-Aluminum-Gallium amalgam attacking aluminum barVideo ( closeup ) of Mercury-Aluminum-Gallium amalgam attacking aluminum bar
{{Mercury compounds
Aluminium alloys
Reducing agents
Amalgams