Mercury

Mercury, denoted by the chemical symbol Hg and atomic number 80, is a dense, silvery-white metal known for being the only metallic element that remains liquid at room temperature. Historically referred to as quicksilver, mercury has fascinated scientists and civilisations for centuries due to its unique physical properties and wide range of uses. Despite its usefulness, mercury is also highly toxic, and its industrial and environmental management has become a major global concern.

Physical and Chemical Characteristics

Mercury has a melting point of –38.83°C and a boiling point of 356.73°C, making it liquid across a broad temperature range. It is exceptionally dense, with a density of 13.53 g/cm³, and exhibits a bright metallic lustre. Chemically, mercury is relatively unreactive; it does not oxidise easily in air but can form compounds with chlorine, sulphur, and other elements.
Its ability to form amalgams—alloys with most metals except iron and platinum—has historically underpinned many of its applications. Mercury also conducts electricity and responds to temperature and pressure changes, which made it a valuable component in precision instruments before modern substitutes emerged.

Occurrence and Extraction

Mercury occurs naturally in the Earth’s crust, primarily as the mineral cinnabar (mercury(II) sulphide, HgS). The principal deposits are found in China, Kyrgyzstan, Spain, and Algeria, though many mines have been closed due to environmental restrictions.
Extraction traditionally involved roasting cinnabar ore to release mercury vapour, which was then condensed into its metallic form. Today, environmental regulations have largely curtailed mercury mining, with global production increasingly sourced from recycled products and recovery from industrial waste streams.

Everyday Applications

Mercury’s unique physical properties once made it a common material in household and everyday items, though its use has significantly declined due to toxicity concerns.

• Thermometers and Barometers: Mercury’s consistent rate of expansion with temperature made it ideal for use in clinical thermometers and barometric pressure gauges. However, these have largely been replaced by digital or alcohol-based devices.
• Electrical Switches and Relays: Mercury was once used in tilt switches, thermostats, and relays due to its conductivity and smooth motion. These are now replaced by electronic alternatives.
• Lighting: Mercury vapour lamps and fluorescent tubes rely on mercury vapour to produce ultraviolet light, which is converted into visible light by phosphor coatings. Compact fluorescent lamps (CFLs) still use small amounts of mercury.
• Batteries: Mercury oxide batteries were widely used in hearing aids, watches, and cameras for their stable voltage output, though environmental regulations have phased out their manufacture in most countries.
• Dental Amalgams: Mercury mixed with silver, tin, and copper has been used in dental fillings for over a century. While still in use in some regions, many health systems are phasing out mercury-based amalgams in favour of resin composites.

Industrial and Technological Uses

Mercury’s industrial applications stem from its role as a catalyst, electrode material, and reactant in various chemical and electrochemical processes.

• Chlor-alkali Industry: Mercury was traditionally used as a cathode in the electrolysis of brine to produce chlorine and sodium hydroxide. The mercury-cell method has now been largely replaced by membrane cell technology to eliminate contamination risks.
• Gold and Silver Extraction: In artisanal and small-scale mining, mercury is used to form amalgams with gold and silver, which are later heated to recover the metals. This process, although banned in many countries, persists in developing regions, contributing significantly to environmental pollution.
• Laboratory and Scientific Instruments: Mercury serves as a component in manometers, vacuum pumps, and diffusion devices due to its stability and liquid properties under varying pressures.
• Chemical Manufacturing: It acts as a catalyst in the production of certain organic compounds, including vinyl chloride and acetaldehyde, though safer alternatives are being implemented.

Economic Importance

Mercury has historically played a significant economic role, particularly from the late 19th to mid-20th centuries, when it was indispensable in scientific and industrial innovation.

• Trade and Value: Although demand has sharply declined, mercury remains traded in limited quantities for laboratory, lighting, and artisanal uses. Its price fluctuates based on regulatory developments, recycling capacity, and global demand for mercury-containing products.
• Artisanal Gold Mining: The most significant economic use of mercury today lies in small-scale gold mining, especially in regions of Africa, Asia, and South America. Despite bans, its availability and low cost make it appealing for miners who lack access to modern extraction technologies.
• Recycling Sector: Increasingly, the mercury economy is shifting towards recovery and recycling. Mercury is reclaimed from obsolete equipment, industrial waste, and fluorescent lighting. This secondary production reduces the need for mining and helps limit environmental contamination.

Environmental and Health Implications

Mercury’s toxicity is a major environmental and public health concern. When released into the atmosphere, mercury can settle into water bodies, where microorganisms convert it into methylmercury, a potent neurotoxin that accumulates in fish and enters the human food chain.
Exposure to mercury—through inhalation, ingestion, or occupational contact—can damage the nervous system, kidneys, and lungs, and is especially harmful to developing foetuses and young children. As a result, global initiatives such as the Minamata Convention on Mercury (2013) aim to reduce mercury emissions, ban certain uses, and encourage safe disposal and substitution.
Key sources of environmental mercury include:

• Artisanal and small-scale gold mining.
• Coal combustion in power plants.
• Industrial processes and waste incineration.
• Improper disposal of mercury-containing products.

Efforts are being made worldwide to control emissions, improve recycling, and promote mercury-free alternatives in lighting, instrumentation, and manufacturing.

Economic Transition and Global Regulations

The decline in mercury use has led to a structural transformation of industries once dependent on the element. Countries party to the Minamata Convention are required to phase out primary mercury mining, control exports, and restrict its use in products such as thermometers, batteries, and cosmetics.
This transition has generated economic shifts towards the development of alternative technologies such as:

• Digital temperature and pressure sensors in place of mercury thermometers.
• LED lighting replacing fluorescent lamps.
• Mercury-free dental materials using resin or glass ionomer cement.
• Membrane-based chlorine production in the chemical industry.

These changes have spurred innovation while reducing the environmental burden of mercury pollution.

Future Prospects and Research

The global trend is moving towards the elimination of mercury from industrial and consumer use. However, mercury will continue to hold niche importance in scientific research, analytical chemistry, and certain high-precision instruments where alternatives are still under development.