Mercury is used primarily for the manufacture of industrial chemicals or for electrical and electronic applications. It is used in some
liquid-in-glass thermometers, especially those used to measure high temperatures. A still increasing amount is used as gaseous mercury in
fluorescent lamps, while most of the other applications are slowly being phased out due to health and safety regulations. In some applications, mercury is replaced with less toxic but considerably more expensive
Galinstan alloy.
Medicine Historical and folk Mercury and its compounds have been used in medicine, although they are much less common today than they once were, now that the toxic effects of mercury and its compounds are more widely understood. An example of the early therapeutic application of mercury was published in 1787 by
James Lind. The first edition of
The Merck Manuals (1899) featured many then-medically relevant mercuric compounds, such as
mercury-ammonium chloride,
yellow mercury proto-iodide,
calomel, and
mercuric chloride, among others. Mercury in the form of one of its common ores, cinnabar, is used in various traditional medicines, especially in
traditional Chinese medicine. Review of its safety has found that cinnabar can lead to significant mercury intoxication when heated, consumed in
overdose, or taken long term, and can have adverse effects at therapeutic doses, though effects from therapeutic doses are typically reversible. Although this form of mercury appears to be less toxic than other forms, its use in traditional Chinese medicine has not yet been justified, as the therapeutic basis for the use of cinnabar is not clear.
Mercury(I) chloride (also known as calomel or mercurous chloride) has been used in
traditional medicine as a
diuretic, topical
disinfectant, and
laxative.
Mercury(II) chloride (also known as mercuric chloride or corrosive sublimate) was once used to treat
syphilis (along with other mercury compounds), although it is so toxic that sometimes the symptoms of its toxicity were confused with those of the syphilis it was believed to treat. It is also used as a disinfectant.
Blue mass, a pill or syrup in which mercury is the main ingredient, was prescribed throughout the 19th century for numerous conditions including constipation, depression, child-bearing and toothaches. In the early 20th century, mercury was administered to children yearly as a laxative and dewormer, and it was used in teething powders for infants. The mercury-containing organohalide
merbromin (sometimes sold as Mercurochrome) is still widely used but has been banned in some countries, such as the U.S.
Contemporary Mercury is an ingredient in
dental amalgams.
Thiomersal (called
Thimerosal in the United States) is an
organic compound used as a
preservative in
vaccines, although this use is in decline. Although it was
widely speculated that this mercury-based preservative could cause or trigger
autism in children, no evidence supports any such link. Nevertheless, thiomersal has been removed from, or reduced to trace amounts in, all U.S. vaccines recommended for children 6 years of age and under, with the exception of the inactivated influenza vaccine. Mercury is still used in some
diuretics, although substitutes such as
thiazides now exist for most therapeutic uses. In 2003, mercury compounds were found in some
over-the-counter drugs, including topical
antiseptics, stimulant laxatives,
diaper-rash ointment,
eye drops, and
nasal sprays. The
FDA has "inadequate data to establish general recognition of the safety and effectiveness" of the mercury ingredients in these products.
Production of chlorine and caustic soda Chlorine is produced from
sodium chloride (common salt, NaCl) using
electrolysis to separate metallic
sodium from chlorine gas. Usually salt is dissolved in water to produce a brine. By-products of any such
chloralkali process are hydrogen (H2) and
sodium hydroxide (NaOH), which is commonly called caustic soda or
lye. By far the largest use of mercury in the late 20th century was in the mercury cell process (also called the
Castner-Kellner process) where metallic sodium is formed as an
amalgam at a
cathode made from mercury; this sodium is then reacted with water to produce sodium hydroxide. Many of the industrial mercury releases of the 20th century came from this process, although modern plants claim to be safe in this regard.
Laboratory uses Thermometers Thermometers containing mercury were invented in the early 18th century by
Daniel Gabriel Fahrenheit, though earlier attempts at making temperature-measuring instruments filled with quicksilver had been described in the 1650s. Fahrenheit's mercury thermometer was based on an earlier design that used alcohol rather than mercury; the mercury thermometer was significantly more accurate than those using alcohol. From the early 21st century onwards, the use of mercury thermometers has been declining, and mercury-containing instruments have been banned in many jurisdictions following the 1998
Protocol on Heavy Metals. Modern alternatives to mercury thermometers include
resistance thermometers,
thermocouples, and
thermistor sensors that output to a digital display.
Mirrors Some
transit telescopes use a basin of mercury to form a flat and absolutely horizontal mirror, useful in determining an absolute vertical or perpendicular reference. Concave horizontal parabolic mirrors may be formed by rotating liquid mercury on a disk, the parabolic form of the liquid thus formed reflecting and focusing incident light. Such
liquid-mirror telescopes are cheaper than conventional large mirror telescopes by up to a factor of 100, but the mirror cannot be tilted and always points straight up.
Electrochemistry Liquid mercury is part of a popular secondary reference
electrode (called the
calomel electrode) in
electrochemistry as an alternative to the
standard hydrogen electrode. The calomel electrode is used to work out the
electrode potential of
half cells. The
triple point of mercury, −38.8344 °C, is a fixed point used as a temperature standard for the International Temperature Scale (
ITS-90). and the
hanging mercury drop electrode use elemental mercury. This use allows a new uncontaminated electrode to be available for each measurement or each new experiment. Mercury-containing compounds are also of use in the field of
structural biology. Mercuric compounds such as
mercury(II) chloride or
potassium tetraiodomercurate(II) can be added to
protein crystals in an effort to create heavy atom derivatives that can be used to solve the
phase problem in
X-ray crystallography via
isomorphous replacement or
anomalous scattering methods.
Niche uses Gaseous mercury is used in
mercury-vapor lamps,
fluorescent lamps, and some
neon signs. Those low-pressure lamps emit very spectrally narrow lines, which are traditionally used in
optical spectroscopy for calibration of spectral position. Commercial calibration lamps are sold for this purpose; reflecting a fluorescent ceiling light into a spectrometer is a common calibration practice. Gaseous mercury is also found in some
electron tubes, including
ignitrons,
thyratrons, and
mercury arc rectifiers. It is also used in specialist medical care lamps for skin tanning and disinfection. Gaseous mercury is added to
cold cathode argon-filled lamps to increase the
ionization and
electrical conductivity. An argon-filled lamp without mercury will have dull spots and will fail to light correctly. Lighting containing mercury can be
bombarded/oven pumped only once. When added to
neon filled tubes, inconsistent red and blue spots are produced in the light emissions until the initial burning-in process is completed; eventually it will light a consistent dull off-blue color. File:Germicidal UV discharge tube glow rotate.jpg|The deep violet glow of a mercury vapor discharge in a
germicidal lamp, whose spectrum is rich in invisible ultraviolet radiation. File:Mercuryvaporlamp.jpg|Skin tanner containing a low-pressure mercury vapor lamp and two infrared lamps, which act both as light source and
electrical ballast File:Leuchtstofflampen-chtaube050409.jpg|Assorted types of fluorescent lamps. File:Deep Space Atomic Clock-DSAC.jpg|The miniaturized
Deep Space Atomic Clock is a linear ion-trap-based mercury ion clock, designed for precise and real-time radio navigation in deep space. The
Deep Space Atomic Clock (DSAC) under development by the
Jet Propulsion Laboratory utilises mercury in a linear ion-trap-based clock. The novel use of mercury permits the creation of compact atomic clocks with low energy requirements ideal for space probes and Mars missions.
Skin whitening Mercury is effective as an active ingredient in
skin whitening compounds used to depigment skin. The
Minamata Convention on Mercury limits the concentration of mercury in such whiteners to 1 part per million. However, as of 2022, many commercially sold whitener products continue to exceed that limit, and are considered toxic.
Firearms Mercury(II) fulminate is a
primary explosive, which has mainly been used as a
primer of a
cartridge in firearms throughout the 19th and 20th centuries.
Mining Mercury is used in illegal gold mining to help separate gold particles from a mixture of sand or gravel and water. Small gold particles may form mercury-gold amalgam and therefore increase the gold recovery rates. • In
Islamic Spain, it was used for filling decorative pools. Later, the American artist
Alexander Calder built a
mercury fountain for the Spanish Pavilion at the
1937 World Exhibition in Paris. The fountain is now on display at the
Fundació Joan Miró in
Barcelona. • The
Fresnel lenses of old
lighthouses used to float and rotate in a bath of mercury which acted like a bearing. • Mercury
sphygmomanometers,
barometers,
diffusion pumps,
coulometers, and many other laboratory instruments took advantage of mercury's properties as a very dense, opaque liquid with a nearly linear thermal expansion. • As an electrically conductive liquid, it was used in
mercury switches (including
home mercury light switches installed prior to 1970), tilt switches used in old fire detectors and in some home thermostats. • Owing to its acoustic properties, mercury was used as the propagation medium in
delay-line memory devices used in early digital computers of the mid-20th century, such as the
SEAC computer. • In 1911,
Heike Kamerlingh Onnes discovered
superconductivity through the cooling of mercury below 4 kelvin shortly after the discovery and production of
liquid helium. Its superconductive properties were later determined to be unusual compared to other later-discovered superconductors, such as the more popular
niobium alloys. • Experimental
mercury vapor turbines were installed to increase the efficiency of fossil-fuel electrical power plants. The South Meadow power plant in Hartford, CT employed mercury as its
working fluid, in a
binary configuration with a secondary water circuit, for a number of years starting in the late 1920s in a drive to improve plant efficiency. Several other plants were built, including the Schiller Station in Portsmouth, NH, which went online in 1950. The idea did not catch on industry-wide due to the weight and toxicity of mercury, as well as the advent of
supercritical steam plants in later years. • Similarly, liquid mercury was used as a
coolant for some
nuclear reactors; however,
sodium is proposed for reactors cooled with liquid metal, because the high density of mercury requires much more energy to circulate as coolant. • Mercury was a propellant for early
ion engines in
electric space propulsion systems. Advantages were mercury's high molecular weight, low ionization energy, low dual-ionization energy, high liquid density and liquid storability at
room temperature. Disadvantages were concerns regarding environmental impact associated with ground testing and concerns about eventual cooling and condensation of some of the propellant on the spacecraft in long-duration operations. The first spaceflight to use electric propulsion was a mercury-fueled ion thruster developed at
NASA Glenn Research Center and flown on the Space Electric Rocket Test "
SERT-1" spacecraft launched by
NASA at its
Wallops Flight Facility in 1964. The SERT-1 flight was followed up by the SERT-2 flight in 1970. Mercury and
caesium were preferred propellants for ion engines until
Hughes Research Laboratory performed studies finding
xenon gas to be a suitable replacement. Xenon is now the preferred propellant for ion engines, as it has a high molecular weight, little or no reactivity due to its
noble gas nature, and high liquid density under mild cryogenic storage. Other applications made use of the chemical properties of mercury: • The
mercury battery is a non-rechargeable
electrochemical battery, a
primary cell, that was common in the middle of the 20th century. It was used in a wide variety of applications and was available in various sizes, particularly button sizes. Its constant voltage output and long shelf life gave it a niche use for camera light meters and hearing aids. The mercury cell was effectively banned in most countries in the 1990s due to concerns about the mercury contaminating landfills. • Mercury was used for preserving wood, developing
daguerreotypes,
silvering mirrors,
anti-fouling paints,
herbicides, interior latex paint, handheld maze games, cleaning, and road leveling devices in cars. Mercury compounds have been used in
antiseptics, laxatives,
antidepressants, and in
antisyphilitics. • Mercury was once used as a gun barrel bore cleaner. • From the mid-18th to the mid-19th centuries, a process called "
carroting" was used in the making of
felt hats. Animal skins were rinsed in an orange solution (the term "carroting" arose from this color) of the mercury compound
mercuric nitrate, Hg(NO3)2. This process separated the fur from the pelt and matted it together. This solution and the vapors it produced were highly toxic. The
United States Public Health Service banned the use of mercury in the felt industry in December 1941. The psychological symptoms associated with mercury poisoning inspired the phrase "
mad as a hatter".
Lewis Carroll's "
Mad Hatter" in his book ''
Alice's Adventures in Wonderland'' was a play on words based on the older phrase, but the character himself does not exhibit symptoms of mercury poisoning. • Historically, mercury was used extensively in
hydraulic gold mining (see #Mining. Large-scale use of mercury stopped in the 1960s. However, mercury is still used in small scale, often clandestine, gold prospecting. It is estimated that 45,000 metric tons of mercury used in California for
placer mining have not been recovered. Mercury was also used in silver mining to extract the metal from ore through the
patio process. ==Toxicity and safety==