An amalgam is an
alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductilit ...
of
mercury with another
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typi ...
. It may be a liquid, a soft paste or a solid, depending upon the proportion of mercury. These alloys are formed through
metallic bonding, with the electrostatic attractive force of the
conduction electrons
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in wh ...
working to bind all the positively charged metal ions together into a
crystal lattice structure. Almost all metals can form amalgams with mercury, the notable exceptions being
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
,
platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Pla ...
,
tungsten, and
tantalum.
Silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
-mercury
amalgams are important in
dentistry
Dentistry, also known as dental medicine and oral medicine, is the branch of medicine focused on the teeth, gums, and mouth. It consists of the study, diagnosis, prevention, management, and treatment of diseases, disorders, and conditions ...
, and
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...
-mercury amalgam is used in the
extraction of gold from
ore. Dentistry has used alloys of mercury with metals such as silver, copper, indium, tin and zinc.
Important amalgams
Zinc amalgam
Zinc amalgam finds use in organic synthesis (e.g., for the
Clemmensen reduction).
It is the reducing agent in the
Jones reductor, used in analytical chemistry. Formerly the zinc plates of dry
batteries
Battery most often refers to:
* Electric battery, a device that provides electrical power
* Battery (crime), a crime involving unlawful physical contact
Battery may also refer to:
Energy source
*Automotive battery, a device to provide power t ...
were amalgamated with a small amount of mercury to prevent deterioration in storage. It is a binary solution (liquid-solid) of mercury and zinc.
Potassium amalgam
For the
alkali metals, amalgamation is exothermic, and distinct chemical forms can be identified, such as KHg and KHg
2.
KHg is a gold-coloured compound with a melting point of 178 °C, and KHg
2 a silver-coloured compound with a melting point of 278 °C. These amalgams are very sensitive to air and water, but can be worked with under dry nitrogen. The Hg-Hg distance is around 300
picometres, Hg-K around 358 pm.
Phases K
5Hg
7 and KHg
11 are also known;
rubidium,
strontium and
barium undecamercurides are known and isostructural.
Sodium amalgam (NaHg2) has a different structure, with the mercury atoms forming hexagonal layers, and the sodium atoms a linear chain which fits into the holes in the hexagonal layers, but the potassium atom is too large for this structure to work in KHg
2.
Sodium amalgam
Sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
amalgam is produced as a byproduct of the
chloralkali process and used as an important reducing agent in organic and inorganic chemistry. With water, it decomposes into concentrated
sodium hydroxide solution, hydrogen and mercury, which can then return to the chloralkali process anew. If absolutely water-free alcohol is used instead of water, an
alkoxide of sodium is produced instead of the alkali solution.
Aluminium amalgam
Aluminium
Aluminium (aluminum in AmE, American and CanE, Canadian English) is a chemical element with the Symbol (chemistry), symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately o ...
can form an amalgam through a reaction with mercury. Aluminium amalgam may be prepared by either grinding aluminium pellets or wire in mercury, or by allowing aluminium wire or foil to react with a solution of
mercuric chloride. This amalgam is used as a 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 su ...
s. The aluminium is the ultimate electron donor, and the mercury serves to mediate the electron transfer.
The reaction itself and the waste from it contain mercury, so special safety precautions and disposal methods are needed. As an environmentally friendlier alternative, hydrides or other reducing agents can often be used to accomplish the same synthetic result. Another environmentally friendly alternative is an alloy of aluminium and gallium which similarly renders the aluminium more reactive by preventing it from forming an oxide layer.
Tin amalgam
Tin amalgam was used in the middle of the 19th century as a reflective
mirror coating.
Other amalgams
A variety of amalgams are known that are of interest mainly in the research context.
*
Ammonium amalgam is a grey, soft, spongy mass discovered in 1808 by
Humphry Davy
Sir Humphry Davy, 1st Baronet, (17 December 177829 May 1829) was a British chemist and inventor who invented the Davy lamp and a very early form of arc lamp. He is also remembered for isolating, by using electricity, several elements for the ...
and
Jöns Jakob Berzelius. It decomposes readily at room temperature or in contact with water or alcohol:
*:
*
Thallium
Thallium is a chemical element with the symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists William Crookes a ...
amalgam has a freezing point of −58 °C, which is lower than that of pure mercury (−38.8 °C) so it has found a use in low temperature thermometers.
*
Gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...
amalgam: Refined gold, when finely ground and brought into contact with mercury where the surfaces of both metals are clean, amalgamates readily and quickly to form alloys ranging from AuHg
2 to Au
8Hg.
*
Lead
Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is density, denser than most common materials. Lead is Mohs scale of mineral hardness#Intermediate ...
forms an amalgam when filings are mixed with mercury and is also listed as a naturally occurring alloy called leadamalgam in the
Nickel–Strunz classification.
Dental amalgam
Dentistry has used alloys of mercury with metals such as silver,
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...
,
indium
Indium is a chemical element with the symbol In and atomic number 49. Indium is the softest metal that is not an alkali metal. It is a silvery-white metal that resembles tin in appearance. It is a post-transition metal that makes up 0.21 par ...
,
tin and
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...
. Amalgam is an "excellent and versatile restorative material" and is used in dentistry for a number of reasons. It is inexpensive and relatively easy to use and manipulate during placement; it remains soft for a short time so it can be packed to fill any irregular volume, and then forms a hard compound. Amalgam possesses greater
longevity when compared to other direct restorative materials, such as composite. However, this difference has decreased with continual development of composite resins.
Amalgam is typically compared to resin-based composites because many applications are similar and many physical properties and costs are comparable.
Dental amalgam has been studied and is generally considered to be safe for humans, though the validity of some studies and their conclusions have been questioned.
In July 2018 the EU, in consideration of the persistent pollution and environmental toxicity of amalgam's mercury, prohibited amalgam for dental treatment of children under 15 years and of pregnant or breastfeeding women.
Use in mining
Mercury has been used in gold and silver mining because of the convenience and the ease with which mercury and the precious metals will amalgamate. In gold placer mining, in which minute specks of gold are washed from sand or gravel deposits, mercury was often used to separate the gold from other heavy minerals.
After all of the practical metal had been taken out from the ore, the mercury was dispensed down a long copper trough, which formed a thin coating of mercury on the exterior. The waste ore was then transferred down the trough, and gold in the waste amalgamated with the mercury. This coating would then be scraped off and refined by evaporation to get rid of the mercury, leaving behind somewhat high-purity gold.
Mercury amalgamation was first used on silver ores with the development of the
patio process in Mexico in 1557. There were also additional amalgamation processes that were created for processing silver ores, including
pan amalgamation and the
Washoe process.
Gold amalgam
Gold extraction (mining)
Gold amalgam has proved effective where gold fines ("flour gold") would not be extractable from ore using hydro-mechanical methods. Large amounts of mercury were used in ''
placer mining
Placer mining () is the mining of stream bed ( alluvial) deposits for minerals. This may be done by open-pit (also called open-cast mining) or by various surface excavating equipment or tunneling equipment.
Placer mining is frequently used for ...
'', where deposits composed largely of decomposed granite slurry were separated in long runs of "riffle boxes", with mercury dumped in at the head of the run. The amalgam formed is a heavy solid mass of dull gray color. (The use of mercury in 19th century placer mining in California, now prohibited, has caused extensive pollution problems in riverine and estuarine environments, ongoing to this day.) Sometimes substantial slugs of amalgam are found in downstream river and creek bottoms by amateur
wet-suited miners seeking gold nuggets with the aid of an engine-powered water vacuum/dredge mounted on a float.
Gold extraction (ore processing)
Where
stamp mills were used to crush gold-bearing ore to fines, a part of the extraction process involved the use of mercury-wetted copper plates, over which the crushed fines were washed. A periodic scraping and re-mercurizing of the plate resulted in amalgam for further processing.
Gold extraction (retorting)
Amalgam obtained by either process was then heated in a distillation retort, recovering the mercury for reuse and leaving behind the gold. As this released mercury vapors to the atmosphere, the process could induce adverse health effects and long term pollution.
Today, mercury amalgamation has been replaced by other methods to recuperate gold and silver from ore in developed nations. Hazards of mercurial toxic waste have played a major role in the phasing out of the mercury amalgamation processes. However, mercury amalgamation is still regularly used by small-scale gold placer miners (often illegally), particularly in developing countries.
Amalgam probe
Mercury salts are, compared to mercury metal and amalgams, highly toxic due to their solubility in water. The presence of these salts in water can be detected with a probe that uses the readiness of mercury ions to form an amalgam with copper. A
nitric acid solution of salts under investigation is applied to a piece of copper foil, and any mercury ions present will leave spots of silvery-coloured amalgam. Silver ions leave similar spots but are easily washed away, making this a means of distinguishing silver from mercury.
The
redox reaction involved where mercury oxidizes the copper is:
:Hg
2+ + Cu → Hg + Cu
2+.
See also
*
Caesium#Physical properties
*
Pan amalgamation
*
Patio process
References
Further reading
* Prandtl, W.: ''Humphry Davy, Jöns Jacob Berzelius, zwei führende Chemiker aus der ersten Hälfte des 19. Jahrhunderts''. Wissenschaftliche Verlagsgesellschaft, Stuttgart, 1948
* Hofmann, H., Jander, G.: ''Qualitative Analyse'', 1972, Walter de Gruyter,
External links
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