Elemental mercury (Hg) is a heavy, silvery-white metal, unique as the only metal that exists as a liquid at standard room temperature. This liquid state, combined with its high density, uniform thermal expansion, and excellent electrical conductivity, made it historically valuable for scientific and industrial applications requiring precision and electrical function.
Historical Role in Scientific Measurement
The physical characteristics of liquid mercury made it a standard material for precise scientific and meteorological measurements for centuries. Its uniform thermal expansion meant that a change in temperature caused a consistent, proportional change in the mercury column’s height. This linearity was the principle behind the mercury-in-glass thermometer, which became the standard for clinical and laboratory temperature measurement following its invention in the 18th century.
Mercury’s high density, approximately 13.5 times that of water, was utilized in classical instruments. Evangelista Torricelli used mercury in 1643 to invent the barometer, a device for measuring atmospheric pressure. Because the metal is so dense, the required column of liquid was only about 760 millimeters high, a much more manageable size than a water-filled column, which would need to be over 10 meters tall.
This density advantage also led to its use in manometers and sphygmomanometers, which measure gas pressure differences and blood pressure. Mercury does not wet or cling to glass, ensuring accurate readings as the column rises and falls without leaving residue. Although these applications have been largely phased out due to toxicity concerns, these instruments relied entirely on mercury’s physical properties to function accurately.
Current Industrial and Electrical Applications
Despite efforts to phase it out, mercury is still used in specialized industrial and electrical processes where its unique properties are hard to replace. One significant industrial use has been in the Chlor-Alkali process for the large-scale production of chlorine gas and caustic soda. This process uses a mercury cell as a flowing cathode, where the elemental mercury forms an amalgam with sodium to separate the products.
In the electrical and lighting sectors, mercury’s high electrical conductivity and liquid state are still employed in specialized switches and relays. Liquid mercury provides a reliable, low-resistance electrical contact toggled by tilting a sealed glass tube, a design found in some older thermostats and industrial equipment. These switches are valued for their reliability in environments where arcing and contact wear would degrade mechanical switches.
Low-pressure fluorescent lighting depends on mercury vapor to function efficiently. A small amount of mercury inside the glass tube is vaporized by an electrical current, generating short-wave ultraviolet (UV) light. This UV light strikes the phosphor coating on the inside of the tube, causing it to fluoresce and emit visible light. The quantity of mercury used in modern lamps is typically only a few milligrams, but it remains necessary for their operation.
The Use of Mercury in Dental Amalgams
A common application of mercury is its use in dental amalgam, a material utilized for filling cavities in teeth. Dental amalgam is an alloy created by mixing liquid elemental mercury with a powdered alloy that primarily contains silver, tin, and copper. The resulting mixture is a pliable putty that dentists can easily place and shape within the prepared tooth cavity.
Mercury acts as a binder, chemically reacting with the alloy powder to create a strong, durable, and solid restorative material. The resulting filling is known for its longevity, affordability, and ability to withstand the forces of chewing. Though composite resins and other alternatives have become popular, dental amalgam remains a safe and effective restorative option.
Toxicity, Exposure Risks, and Safe Disposal
The use of elemental mercury is heavily regulated because of its inherent toxicity and the risk of exposure, particularly through inhalation. Elemental mercury readily evaporates at room temperature, creating an invisible, odorless, toxic vapor. This vapor is the most common route of exposure, readily absorbed by the lungs, allowing it to enter the bloodstream and cross the blood-brain barrier.
Once in the body, it can cause severe neurological damage, leading to symptoms like tremors, memory loss, and cognitive dysfunction. Chronic exposure can also affect the kidneys and the developing fetus, making it a potent neurotoxin. Improper handling or disposal of mercury-containing items is a serious environmental and health concern due to the vaporization risk.
Mercury and all mercury-containing waste (e.g., broken thermometers, fluorescent bulbs, and old electrical switches) must be treated as hazardous waste. Liquid mercury or these devices should never be thrown in the trash or poured down a drain. Improper disposal leads to environmental contamination and the formation of the more toxic methylmercury in aquatic environments. Cleanup of spilled mercury requires specialized kits and ventilation to prevent vapor inhalation; collected material must be sent to a dedicated hazardous waste collection facility for proper recycling or disposal.