Mercury
Mercury is a transition metal, one of the elements found in the middle of the periodic table in Rows 4, 5, and 6 between Groups 2 and 3. Its atomic number if 80, its atomic mass is 200.59, and its chemical symbol is Hg.
Properties
Mercury is the only liquid metal and the only liquid element except for bromine. Mercury has a freezing point of -37.93°F (-38.85°C), a boiling point of 673.9°F (356.6°C), and a density of 13.59 grams per cubic centimeter.
Two physical properties of special interest are mercury's high surface tension and its superior ability to conduct an electric current. The metal's surface tension is great enough to permit a steel needle to be floated on its surface. Its ability to conduct an electric current allows the metal to be used in a "mercury switch," a small glass capsule in which the metal can flow from one end of the capsule to the other. In this way, the capsule can open and close an electric circuit depending on the angle at which it is tipped.
Mercury is a moderately active metal that does not react with oxygen at room temperature very readily, although it does react with oxygen and most acids at elevated temperatures.
Occurrence and Extraction
Mercury is a relatively rare element in the Earth's crust with an abundance estimated to be about 0.5 parts per million. It usually does not occur as an element, and its most common compound is cinnabar (mercuric sulfide; HgS). Cinnabar usually occurs as a dark red powder more commonly known as vermilion or Chinese vermilion.
The largest producer of mercury outside the United States is Spain. U.S. production numbers are not announced in order to protect U.S. industries from revealing important company secrets. Other producers of the metal are Kyrgyzstan, Algeria, China, and Finland. In the United States, mercury is produced as a byproduct of gold mining in California, Nevada, and Utah.
Discovery and Naming
Mercury is rather easily prepared from cinnabar by a process that was known to ancient humans. The Greek philosopher Theophrastus (372-287B.C.), for example, described a method for preparing mercury by rubbing cinnabar with vinegar in a clay dish. The oldest sample of mercury metal dates to about the fifteenth or sixteen century B.C.. It was found in an Egyptian tomb at Kurna, stored in a small glass container.
The poisonous effects of mercury were also known to ancient civilizations. Slaves who worked in Roman mercury mines, for example, often died of exposure to the metal. Strangely enough, trees and plants around mercury mines were unaffected by the element.
Mercury amalgams have also been known and used for centuries. One of the most important uses in ancient societies was to remove silver from its ores. Silver dissolves in mercury, forming an amalgam. The amalgam can then be heated to drive off the mercury, leaving pure silver behind.
Mercury has long been known also by the name quicksilver, because it is a silver liquid. The Latin term for "watery silver" is hydrargyrum, a word from which the element was given its chemical symbol of Hg.
Mercury is now extracted from its ores by a method that has been used for hundreds of years. Cinnabar is heated in air until the mercuric sulfide of which it is made breaks down to yield pure mercury metal: HgS --heat Hg + S.
Uses
The most important use of mercury metal today is in the manufacture of chlorine. Chlorine is produced by the electrolysis of aqueous sodium chloride: 2NaCl --electric current%rarr; 2Na + Cl2. The problem with this reaction is that sodium reacts with water as soon as it is formed, sometimes violently. To avoid this problem a layer of mercury may be placed on the bottom of the container in which the electrolysis is carried out. In this case, the sodium dissolves in the mercury as soon as it forms, preventing an explosive reaction between sodium and water. For many years, the "mercury cell" was a very popular method for producing chlorine. Today, companies are looking for other methods to make chlorine because of the toxic hazards posed by mercury to workers who handle the material and to the environment.
The second most important use of mercury in the United States is in switches and other electrical applications. Again, increasing concerns about the environmental hazards of mercury are prompting companies to move to greater reliance on electronic switches. The toxic effects of mercury also account for the decreasing use of the element in fluorescent lamps and mercury batteries, two applications which once counted for a significant portion of the mercury consumed each year.
Some compounds of mercury that are still being used include mercuric arsenate (HgHAsO4) for waterproofing paints; mercuric chloride (mercury bichloride; corrosive sublimate; HgCl2) for tanning of leather, preservation of wood, textile printing, and engraving; mercuric cyanide [Hg(CN)2] in germicidal soaps and photography; and mercuric oxide (HgO) as a red or yellow pigment in paint, a fungicide, and a disinfectant.
Health Issues
Scientists have become fully aware of the toxic effects of mercury only in the past few decades. One of the most important discoveries they have made is that the form in which mercury occurs determines how toxic it will be. Mercury fumes, for example, tend to enter the body easily and exert toxic effects much more easily than liquid mercury metal itself. Short term, limited contact with mercury can cause nausea, vomiting, diarrhea, and stomach pain. More extensive exposure can lead to kidney damage and death in a relatively short period of time.
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