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. It has ...

can form an amalgam 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 * Solutio ...

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 compounds, such as the reduction of imines 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 Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent suc ...

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 cla ...

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 of ...

s or other reducing agents can often be used to accomplish the same synthetic result. An alloy of aluminium and

gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (alumini ...

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 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 and ...

(Al(OH)₃) and hydrogen gas (H₂). The electrons from the aluminium reduce mercuric Hg²⁺ 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 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 ...

References

External links

Video of Mercury-Aluminum-Gallium amalgam attacking aluminum bar

Video ( closeup ) of Mercury-Aluminum-Gallium amalgam attacking aluminum bar
{{Mercury compounds Aluminium alloys Reducing agents Amalgams

Reactivity

See also

References

External links