Characteristics And Harmful Effects Of The Only Liquid Metal
The oldest sample of mercury 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. Mercury and cinnabar are both mentioned in ancient manuscripts. The Chinese, Hindus, Egyptians, Greeks, and Romans all recorded information about the element and its ore. Greek philosopher Theophrastus (372-287 B.C.), for example, described a method for preparing mercury. Cinnabar was rubbed together with vinegar in a clay dish. Theophrastus wrote that the cinnabar had been found in silver mines. When the metal was first made, he said, people thought it might contain gold. They were misled by the metal's shiny appearance. They soon realized, however, that it was quite different from gold.
Some mercury compounds are known to be poisonous. For example, mercuric chloride (corrosive sublimate) was often used to kill pests and, sometimes, people. On the other hand, some mercury compounds have been used as medicines. For instance, mercurous chloride (calomel) was long used as a cure for skin rashes. In the recent past the dangers of mercury have become better known. As a result, use of mercury and its compounds is now being either phased out or made cautiously.
Many reports on mercury told of its poisonous effects. Slaves who worked in Roman mercury mines, for example, often died of exposure to mercury. Strangely enough, trees and plants around these mines were not affected. Mercury was sometimes very dangerous and sometimes quite safe. People even drank from streams that ran through mercury mines. Scientists now know that mercury's effects depend on the form in which it occurs.
Mercury amalgams have also been around for a long time. An amalgam is a combination of mercury with at least one other metal. Amalgams are formed when a metal, such as silver, dissolves in mercury. The process is similar to dissolving salt in water. Amalgamation is used in mining to remove silver from ore. The silver dissolves in the mercury and a silver amalgam is formed. Heating the amalgam releases the silver. This method was used by miners as early as the sixteenth century.
Mercury is the only liquid metal. In fact, there is only one other liquid element, bromine that is a non-metal. Mercury can be frozen at a temperature of –38.85°C (–37.93°F) and boiled at 365.6°C (690.1°F). Its density is 13.59 grams per cubic centimeter. Mercury has two physical properties of special interest. First, it has very high surface tension, a property of liquids that make them act as if they are covered with a skin. For example, some water bugs are able to walk on the surface of water. With care, one can float a needle on the surface of water. These incidents are possible because of water's surface tension.
Mercury is also a very good conductor of electricity. This property is used in a number of practical devices. One such device is a mercury switch, such as the kind that turns lights on and off. A small amount of mercury can be placed into a tiny glass capsule. The capsule can be made to tip back and forth. As it tips, the mercury flows from one end to the other. At one end of the capsule, the mercury may allow an electric current to flow through a circuit. At the other end, no mercury is present, so no current can flow. Mercury switches are easy to make and very efficient. Mercury is moderately active. It does not react with oxygen in the air very readily. It reacts with some acids when they are hot, but not with most cold acids.
The abundance of mercury in the Earth's crust is estimated to be about 0.5 parts per million. That makes it one of the 20 least common elements. It very rarely occurs as an element. Instead, it is usually found as a compound. Its most common ore is cinnabar, or mercuric sulfide (HgS). Cinnabar usually occurs as a dark red powder. It is often called by the common name of vermillion or Chinese vermillion. 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 after Spain are Kyrgyzstan, Algeria, China, and Finland. In the United States, mercury is produced as a by-product of gold mining. It comes from eight gold mines in California, Nevada, and Utah.
Seven naturally occurring isotopes of mercury: mercury-196, mercury-198, mercury-199, mercury-200, mercury-201, mercury-202, and mercury-204 are known. About a dozen radioactive isotopes of mercury are also known. Two radioactive isotopes of mercury: mercury-197 and mercury-203 are used in medicine. Both isotopes are used to study the brain and the kidneys. The isotopes are injected into the body where they travel to the brain and the kidneys. Inside these two organs, the isotopes give off radiation that is detected by instruments held above the body. The pattern of radiation provides information about how well the brain and kidneys are functioning. Mercury is still prepared as it was hundreds of years ago. Cinnabar is heated in air. The compound breaks down to give mercury metal. The mercury metal is then purified by distillation.
The most important use of mercury is in the preparation of chlorine. Chlorine is produced by passing an electric current through sodium chloride. There is a problem with using this method, however. Sodium (Na) is a very reactive metal. If any water is present, the sodium will react violently with the water. This reaction makes the production of chlorine much more difficult. In 1892, two English chemists developed a method for solving this problem. They made a container with a layer of mercury on the bottom. As sodium is produced by the electric current, it dissolves in the mercury, forming an amalgam. The sodium is unable to react with water. For many years, the "mercury cell" invented in 1892 was a very popular method for producing chlorine.
But today, companies are looking for other ways to make chlorine. They are worried about the harmful effects of mercury. They are also concerned that mercury can get into the environment and harm humans, animals, and plants. The second most important use of mercury in many parts of the world is in switches and other electrical applications. Again, there are increasing concerns about the health effects of mercury. Many companies are switching to electronic switches. One application in which concerns about mercury have had little effect is fluorescent lamps. A fluorescent lamp contains mercury vapor (gas). When the lamp is turned on, an electric current passes through the mercury vapor and causes it to give off invisible radiation. The radiation strikes the inside of the glass tube, whose walls are coated with a phosphor. A phosphor is a material that gives off visible light when struck by electrons. The tube glows as the radiation strikes the phosphor.
Lamp manufacturers have reduced the amount of mercury in fluorescent lamps by about 60 percent. They developed ways to make the Lamps work just as well with less mercury. However, mercury lamps are much more popular. Each lamp now contains much less mercury. But there are many more lamps than ever before. For a time, mercury batteries were quite popular. In the early 1980s, more than 1,000 tons of mercury a year was used to make mercury batteries. These batteries are a special environmental problem, however. People tend to just throw them away when they no longer work. The cases split open easily, releasing mercury into the environment. As a result, much less mercury is now being used to make such batteries. In 1996, less than one ton of mercury was used in these batteries. They are now restricted almost entirely to military and medical uses. Mercury is also used in dental applications, measuring instruments (such as mercury thermometers and barometers), and coatings for mirrors. Use of mercury compounds is decreasing because of health their concerns.
In a tragic irony, a scientist who was helping to improve the environment died as a result of her efforts. On June 8, 1997, Dartmouth College chemistry professor Karen Wetterhahn died of mercury poisoning. Less than a year earlier, she had been experimenting with dimethyl mercury when she spilled a tiny amount on her hands. Dimethyl mercury is one of the most toxic of mercury compounds.
Wetterhahn was studying the effects that heavy metals (mercury, chromium, lead, and arsenic) have on living things. She was concerned about how these elements pollute the environment and cause disease in people. In August 1996, as Wetterhahn was transferring some dimethyl mercury to a tube, the accident occurred. She was wearing latex gloves, but they were not adequate protection against the dangerous chemical. The mercury seeped into her skin. Wetterhahn did not begin to feel the effects of the exposure until six months later. She then started losing her balance, slurring her speech, and suffering vision and hearing loss. Tests showed her system had eighty times the lethal dose of mercury. Wetterhahn died of mercury poisoning on June 8, 1997.
Wetterhahn's death prompted some safety changes. Bright stickers on latex glove boxes should warn against using the gloves with hazardous chemicals. Workshops were held to teach proper glove selection. The dangers of dimethyl mercury were stressed. And scientists were urged to use a less dangerous chemical than dimethyl mercury. Overall, her death heightened awareness in the scientific community of potential laboratory dangers.
Though the toxicity of some mercury compounds has been known for many centuries, interestingly enough, scientists have become aware of the fact that mercury metal and most of its compounds are highly toxic only quite recently. One form of mercury chloride known as calomel, for example, was sometimes used as a poison to kill people. It was also once used extensively to kill fungi and control maggots in agricultural crops. But even as recently as fifty years ago, there was relatively little concern about mercury metal and many mercury compounds. High school chemistry students often played with tiny droplets of mercury in the laboratory. So much so they used mercury to coat pennies and other pieces of metal.
Mercury was also widely used in dentistry. It was used to make amalgams, alloys of mercury with other metals, used to fill teeth. Most people even today are likely to have dental fillings that contain a small amount of mercury metal. In the last fifty years, chemists have learned a great deal more about the toxic effects of both mercury metal and most of its compounds. They now know that mercury itself enters the body very easily. Its vapors pass through the skin into the blood stream. Its vapors can also be inhaled. And, of course, it can also be swallowed. In any of these cases, mercury gets into blood and then into cells. There it interferes with essential chemical reactions and can cause illness and death.
Sometimes, these effects occur over very long periods of time. People who work with mercury, for example, may take in small amounts of mercury over months or years. Health problems develop very slowly. These problems can include inflammation of the mouth and gums; loosening of the teeth; damage to the kidneys and muscles; shaking of the arms and legs; and depression, nervousness, and personality changes.
Back in the 1800s, most of the negative effects of mercury and its compounds were not yet known. Hatmakers of that time commonly used a mercury compound in their craft. It was used to treat the felt and beaver fur that lined the hats. Eventually, exposure to the mercury began to cause changes in the hatmakers' bodies. Their personalities and behavior became erratic. Recognizing the bizarre personalities of many hatmakers, people often used the expression "mad as a hatter." In fact, author Lewis Carroll (1832-98) created a character for Alice's Adventures in Wonderland that owes its origins to the symptoms of mercury poisoning: The Mad Hatter.
People can also be exposed to large doses of mercury over short periods of time. In such cases, even more serious health problems can arise. These include nausea, vomiting diarrhea, stomach pain, damage to the kidneys, and death in only a week or so. So is mercury still safe to use in dental fillings? That question is the source of considerable controversy. Some people say that so little mercury is lost from fillings that the metal presents no danger to people. Other people think that dentists should take no chances with this dangerous metal. They should stop using mercury fillings entirely.
About the Author
Dr. Badruddin Khan teaches Chemistry in the University Of Kashmir, Srinagar, India.
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