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An amalgam is an alloy of mercury with another metal. Most metals are soluble in mercury, but some (such as iron) are not. Amalgam also may be a solution of metal-like ion complexes, such as ammonium. Amalgams are commonly used in dental fillings.
Additional recommended knowledge
For some centuries, dentists have been cleaning out decay and creating dental fillings using filling material such as stone chips, resin, cork, turpentine, gum, lead and gold leaf. The renowned physician Ambroise Paré (1510 – 1590) used lead or cork to fill teeth. Amalgams have been in wide use since the mid-1800s, when they became the first true standard filling material.
Amalgams are used in dentistry because they are cheap, easy to use, and more durable than many alternatives. They are generally regarded as safe, though this is not without controversy (see below). Modern low-copper amalgams have a powder component composed of 69.4% silver, 3.6% copper, 26.2% tin, and 0.8% zinc; and they have a liquid component of 42% to 45% mercury by weight. The amalgam remains soft for a short time so it can be packed to fill any irregular volume, and then forms a hard compound.
The first people to use amalgam to fill cavities were the Chinese in the 7th century. In 1816, Auguste Taveau developed a dental amalgam from silver coins and mercury. This early amalgam was low in mercury and had to be heated in order for the silver to dissolve at any appreciable rate. More modern dental amalgams are mixed cold. Current dental amalgams contain copper to eliminate the gamma-2 phase of the silver-mercury-tin alloy. The gamma-2 phase is weaker than the other phases, so a high-copper, low-gamma-2 dental amalgam has superior strength.
Dental amalgam controversy
The Dental amalgam controversy is a debate over the use of amalgams, which contain mercury, as a dental filling. Though mercury itself is a potent neurotoxin, amalgam fillings are considered safe by most dentists. Recent random clinical trials have found no evidence of neurological harm associated with their use in children, examining a period of 5-7 years following treatment. Still, some worry about the difficulties of conclusively excluding the possibility of neurological effects. They point out that such effects may be delayed beyond the period examined in the studies; may be larger in a vulnerable subset of patients, such as those who are genetically predisposed to mercury sensitivity; or may simply have been lost in the noise of measurement. These concerns are especially significant considering the tremendous number of patients with amalgam fillings. Thus, while the studies cited provide strong evidence that there is little risk of large scale neurological harm for most patients, even a risk small enough to be easily missed could affect a large number of people.
Mercury is the preferred electrode material for the analysis of metals by anodic stripping voltammetry. The formation of amalgams facilitates the reduction of most metal ions in aqueous solutions that is normally not possible because their reduction potentials are more negative than the potential for the reduction of the solution.
Use in organic chemistry
Formation of amalgams is used to increase the reactivity of metals. In the Grignard reaction, amalgamation of the magnesium makes the reaction more facile. In the Barbier reaction and other reactions using aluminum, it is absolutely necessary, as the surface oxide coating normally makes aluminum inert.
Mercury has been used in the gold and silver mining processes due to the ease with which mercury will amalgamate with them. In gold placer mining, in which small particles 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 usable metal had been extracted from the ore, mercury was poured down a long copper trough which formed a thin coating of mercury on the surface. The waste ore was then poured down the trough, and any gold in the waste amalgamated with the mercury. This coating was occasionally scraped off and distilled to remove the mercury, leaving behind fairly high-purity gold.
Mercury amalgamation was first applied to silver ores with the invention of the patio process in Mexico in 1557. Other amalgamation processes were invented for processing silver ores, including pan amalgamation and the Washoe process.
With the invention of mercury amalgamation to treat silver ore, mercury became essential to the silver mines of the New World. The Spanish Empire transported mercury from Almadén across the Atlantic to supply the silver mines of Zacatecas and Potosí. Another source for mercury in the Spanish Empire was the mine of Huancavelica in Peru, discovered in 1563. In 1648, the Viceroy of Peru declared that Potosí and Huancavelica were "the two pillars that support this kingdom and that of Spain."
Today, mercury amalgamation has been replaced by other methods to recover gold and silver from ore. Dangers of mercury pollution have played a part in the near-disappearance of mercury amalgamation processes. Mercury amalgamation is still commonly used by small-scale gold placer miners, especially in less-developed countries, most notably Brazil.
The amalgam table is used in gold productions and was the main way to collect fine particles of gold during the 1800s. The table is simply a sloped, smooth surface with a copper sheet overlaid and smeared with mercury. The pulp from the stamp battery is directed over the table. The finely crushed gold chemically bonds with the mercury, and hence sticks to the table for later collection. However, if there is any oil in the water, it will surround the gold and cause it to float away and not bond properly with the mercury.
Thallium amalgam is used as liquid for thermometers, because it freezes at -58°C, whereas pure mercury freezes at -38°C.
Categories: Alloys | Mercury (element)
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Amalgam". A list of authors is available in Wikipedia.|