Hg
Mercury is a chemical element that occurs naturally in the environment and exists in several forms.
| Previous Element: Gold Next Element: Thallium |
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| Physical Properties | ||
|---|---|---|
| Color | silvery-white | |
| Phase at Room Temp. | liquid | |
| Density (g/cm3) | 13.534 | |
| Hardness (Mohs) | --- | |
|
Melting Point (K) |
234.33 | |
|
Boiling Point (K) |
630 | |
| Heat of Fusion (kJ/mol) | 2.3 | |
| Heat of Vaporization (kJ/mol) | 59 | |
| Heat of Atomization (kJ/mol) | 61 | |
| Thermal Conductivity (J/m sec K) | 8.3 | |
| Electrical Conductivity (1/mohm cm) | 10.163 | |
| Source | Cinnabar (sulfide) | |
| Atomic Properties | ||
| Electron Configuration | [Xe]6s24f145d10 | |
|
Number of Isotopes |
45 (7 natural) | |
| Electron Affinity (kJ/mol) | --- | |
| First Ionization Energy (kJ/mol) | 1007 | |
| Second Ionization Energy (kJ/mol) | 1810 | |
| Third Ionization Energy (kJ/mol) | 3300 | |
| Electronegativity | 2.0 | |
| Polarizability (Å3) | 5.4 | |
| Atomic Weight | 200.59 | |
| Atomic Volume (cm3/mol) | 14.8 | |
| Ionic Radius2- (pm) | --- | |
| Ionic Radius1- (pm) | --- | |
| Atomic Radius (pm) | 151 | |
| Ionic Radius1+ (pm) | 133 | |
| Ionic Radius2+ (pm) | 116 | |
| Ionic Radius3+ (pm) | --- | |
| Common Oxidation Numbers | +1, +2 | |
| Other Oxid. Numbers | --- | |
| Abundance | ||
| In Earth's Crust (mg/kg) | 8.5x10-2 | |
| In Earth's Ocean (mg/L) | 3.0x10-5 | |
| In Human Body (%) | 0.000009% | |
| Regulatory / Health | ||
| CAS Number | 7439-97-6 | |
| OSHA Permissible Exposure Limit (PEL) | Ceiling: 0.1 mg/m3 | |
| OSHA PEL Vacated 1989 | TWA: 0.05mg/m3 Ceiling: 0.1 mg/m3 (Potential for skin absorption) | |
|
NIOSH Recommended Exposure Limit |
TWA: 0.05mg/m3 Ceiling: 0.1 mg/m3 IDLH: 10 mg/m3 (Potential for skin absorption) | |
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Sources: |
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These forms can be organized under three headings: metallic mercury (also known as elemental mercury), inorganic mercury, and organic mercury. All forms of mercury can enter the body and are potentially toxic. Metallic mercury is a shiny, silver-white metal that is a liquid at room temperature and traditionally called "quick silver". Metallic mercury is the elemental or pure form of mercury (i.e., it is not combined with other elements). Metallic mercury metal is the familiar liquid metal used in thermometers and some electrical switches. At room temperature, some of the metallic mercury will evaporate and form mercury vapors. Mercury vapors are colorless and odorless. The higher the temperature, the more vapors will be released from liquid metallic mercury. Some people who have breathed mercury vapors report a metallic taste in their mouths. Metallic mercury has been found at 714 hazardous waste sites nationwide.
It is the only liquid metal, although gallium and cesium are liquid on hot days (bromine is also a liquid, but it is non-metallic). It has been known to humans for at least 4000 years. Mercury is a stable metal, not reacting with air, water, most acids and most bases. It is a poor conductor of heat (for a metal) but a good conductor of electricity. Alloys of mercury with other metals are called amalgams. Mercury's ability to amalgamate with many metals is often useful in purifying the other metal, especially gold. Mercury is used for street lights, advertising lights, fungicides, pesticides, herbicides, dental preparations, batteries, caustic soda and chlorine production, catalysts, electrical equipment and silent electrical switches. It is also used where a heavy liquid is needed, such as barometers or thermometers. Mercury is very poisonous, it damages the central nervous system. It is absorbed easily by the body, but cannot be excreted easily. Mercury is volatile and it is possible to breath mercury vapors without ever touching the metal. Mercury must be handled with adequate protection. The symbol for mercury, Hg, comes from mercury's Latin name hydrargyrum, which means "liquid silver".
Occurrence
Inorganic mercury compounds occur when mercury combines with elements such as chlorine, sulfur, or oxygen. These mercury compounds are also called mercury salts. Most inorganic mercury compounds are white powders or crystals, except for mercuric sulfide (also known as cinnabar) which is red and turns black after exposure to light.
When mercury combines with carbon, the compounds formed are called "organic" mercury compounds or organomercurials. There is a potentially large number of organic mercury compounds; however, by far the most common organic mercury compound in the environment is methylmercury (also known as monomethylmercury). In the past, an organic mercury compound called phenylmercury was used in some commercial products. Another organic mercury compound called dimethylmercury is also used in small amounts as a reference standard for some chemical tests. Dimethylmercury is the only organic mercury compound that has been identified at hazardous waste sites. It was only found in extremely small amounts at two hazardous waste sites nationwide, but it is very harmful to people and animals. Like the inorganic mercury compounds, both methylmercury and phenylmercury exist as "salts" (for example, methylmercuric chloride or phenylmercuric acetate). When pure, most forms of methylmercury and phenylmercury are white crystalline solids. Dimethylmercury, however, is a colorless liquid.
Several forms of mercury occur naturally in the environment. The most common natural forms of mercury found in the environment are metallic mercury, mercuric sulfide (cinnabar ore), mercuric chloride, and methylmercury. Some microorganisms (bacteria and fungi) and natural processes can change the mercury in the environment from one form to another. The most common organic mercury compound that microorganisms and natural processes generate from other forms is methylmercury. Methylmercury is of particular concern because it can build up or biomagnify in certain edible freshwater and saltwater fish and marine mammals to levels that are many times greater than levels in the surrounding water.
Extraction and Uses
Mercury is mined as cinnabar ore, which contains mercuric sulfide. The metallic form is refined from mercuric sulfide ore by heating the ore to temperatures above 1000 degrees Fahrenheit. This vaporizes the mercury in the ore, and the vapors are then captured and cooled to form the liquid metal mercury. There are many different uses for liquid metallic mercury. It is used in producing of chlorine gas and caustic soda, and in extracting gold from ore or articles that contain gold. It is also used in thermometers, barometers, batteries, and electrical switches. Silver-colored dental fillings typically contain about 50% metallic mercury. Metallic mercury is still used in some herbal or religious remedies in Latin America and Asia, and in rituals or spiritual practices in some Latin American and Caribbean religions such as Voodoo, Santeria, and Espiritismo. These uses may pose a health risk from exposure to mercury both for the user and for others who may be exposed to mercury vapors in contaminated air.
Some inorganic mercury compounds are used as fungicides. Inorganic salts of mercury, including ammoniated mercuric chloride and mercuric iodide, have been used in skin-lightening creams. Mercuric chloride is a topical antiseptic or disinfectant agent. In the past, mercurous chloride was widely used in medicinal products including laxatives, worming medications, and teething powders. It has since been replaced by safer and more effective agents. Other chemicals containing mercury are still used as antibacterials. These products include mercurochrome (contains a small amount of mercury, 2%), and thimerosal and phenylmercuric nitrate, which are used in small amounts as preservatives in some prescription and over-the-counter medicines. Mercuric sulfide and mercuric oxide may be used to color paints, and mercuric sulfide is one of the red coloring agents used in tattoo dyes.
Methylmercury is produced primarily by microorganisms (bacteria and fungi) in the environment, rather than by human activity. Until the 1970s, methylmercury and ethylmercury compounds were used to protect seed grains from fungal infections. Once the adverse health effects of methylmercury were known, the use of methymercury and ethylmercury as fungicides was banned. Up until 1991, phenylmercuric compounds were used as antifungal agents in both interior and exterior paints, but this use was also banned because mercury vapors were released from these paints.
Pathways for mercury in the environment
Mercury is a naturally occurring metal found throughout the environment. Mercury enters the environment as the result of the normal breakdown of minerals in rocks and soil from exposure to wind and water, and from volcanic activity. Mercury releases from natural sources have remained relatively constant in recent history, resulting in a steady rise in environmental mercury. Human activities since the start of the industrial age (e.g., mining, burning of fossil fuels) have resulted in additional release of mercury to the environment. Estimates of the total annual mercury releases that result from human activities range from one-third to two-thirds of the total mercury releases. A major uncertainty in these estimates is the amount of mercury that is released from water and soils that were previously contaminated by human activities as opposed to new natural releases. The levels of mercury in the atmosphere (i.e., the air you breathe in the general environment) are very, very low and do not pose a health risk; however, the steady release of mercury has resulted in current levels that are three to six times higher than the estimated levels in the preindustrial era atmosphere.
Approximately 80% of the mercury released from human activities is elemental mercury released to the air, primarily from fossil fuel combustion, mining, and smelting, and from solid waste incineration. About 15% of the total is released to the soil from fertilizers, fungicides, and municipal solid waste (for example, from waste that contains discarded batteries, electrical switches, or thermometers). An additional 5% is released from industrial wastewater to water in the environment.
With the exception of mercury ore deposits, the amount of mercury that naturally exists in any one place is usually very low. In contrast, the amount of mercury that may be found in soil at a particular hazardous waste site because of human activity can be high (over 200,000 times natural levels). The mercury in air, water, and soil at hazardous waste sites may come from both natural sources and human activity.
Most of the mercury found in the environment is in the form of metallic mercury and inorganic mercury compounds. Metallic and inorganic mercury enters the air from mining deposits of ores that contain mercury, from the emissions of coal-fired power plants, from burning municipal and medical waste, from the production of cement, and from uncontrolled releases in factories that use mercury. Metallic mercury is a liquid at room temperature, but some of the metal will evaporate into the air and can be carried long distances. In air, the mercury vapor can be changed into other forms of mercury, and can be further transported to water or soil in rain or snow. Inorganic mercury may also enter water or soil from the weathering of rocks that contain mercury, from factories or water treatment facilities that release water contaminated with mercury, and from incineration of municipal garbage that contains mercury (for example, in thermometers, electrical switches, fluorescent light bulbs, or batteries that have been thrown away). Inorganic or organic compounds of mercury may be released to the water or soil if mercury-containing fungicides are used. However, Electric heaters and ranges also generate fumes of toxic mercury in their normal operation, making electric heating appliances a toxic risk in home use.
Microorganisms (bacteria, phytoplankton in the ocean, and fungi) convert inorganic mercury to methylmercury. Methylmercury released from microorganisms can enter the water or soil and remain there for a long time, particularly if the methylmercury becomes attached to small particles in the soil or water. Mercury usually stays on the surface of sediments or soil and does not move through the soil to underground water. If mercury enters the water in any form, it is likely to settle to the bottom where it can remain for a long time.
Mercury can enter and accumulate in the food chain. The form of mercury that accumulates in the food chain is methylmercury. Inorganic mercury does not accumulate up the food chain to any extent. When small fish eat the methylmercury in food, it goes into their tissues. When larger fish eat smaller fish or other organisms that contain methylmercury, most of the methylmercury originally present in the small fish will then be stored in the bodies of the larger fish. As a result, the larger and older fish living in contaminated waters build up the highest amounts of methylmercury in their bodies. Saltwater fish (especially sharks and swordfish) that live a long time and can grow to a very large size tend to have the highest levels of mercury in their bodies. Plants (such as corn, wheat, and peas) have very low levels of mercury, even if grown in soils containing mercury at significantly higher than background levels. Mushrooms, however, can accumulate high levels if grown in contaminated soils.
| Disclaimer: This article is taken wholly from, or contains information that was originally published by, the Environmental Protection Agency, Agency for Toxic Substances and Disease Registry, and USGS. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the Environmental Protection Agency, Agency for Toxic Substances and Disease Registry, and USGS should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content. |
