Fig 1. Mercury is a liquid at room temperature.
Fig 2. Cinnabar. Credit: Mineral Information Institute.
Fig 3. In a mercury switch a pool of mercury completes an electrical circuit. Terminals are fused into a glass bulb, which can be tilted. In the "on" position the mercury flows around the terminals: in the "off" position it flows away and breaks the connection.
Mercury (Hg) is a silvery-white, poisonous, metallic element in group 12 (old group IIB) of the periodic table, which is liquid at room temperatures (see Figure 1). (Only three other elements, cesium, gallium, and bromine, are liquid at temperatures at or close to ordinary room temperatures.) The most common isotope of mercury is 202Hg (29.8%).
|relative atomic mass||200.59|
|relative density||13.546 (at 20°C)|
|melting point||-38.87°C (-37.97°F)|
|boiling point||356.6°C (673.86F)|
Mercury has been known since ancient times and is named after the planet Mercury and after Mercurius, the Roman messenger of the Gods. Its chemical symbol (Hg) comes from the Latin hydragyrum, "liquid silver," which has also given rise to the name "quicksilver."
Even at ordinary room temperature, mercury evaporates and the fumes are poisonous. It dissolves many metals, including sodium, potassium, tin, copper, gold, and silver, forming alloys called amalgams. As it does not amalgamate with iron, mercury is often stored in iron containers.
Most of the world's mercury is obtained from the mineral cinnabar and is chiefly found in rocks that are associated with volcanic action. It also occurs as calomel and rarely as the metal. In recent years, world production has fallen as reserves have become depleted. Italy, Slovenia, the United States and Mexico, which were formerly leading producers have been largely mined out. Most mercury production today takes place in Spain, Kyrgyzstan, China, and Tajikistan.
Cinnabar is a
heavy, reddish mineral form of mercury (II) sulfide, HgS, that is
the principal ore of mercury (Fig 2).
Cinnabar occurs in both crystallized and massive granular forms. It
occurs in the rhombohedral system as hexagonal (six-sided) crystals,
often twinned, and varies from perfectly opaque to almost transparent.
It is found in hydrothermal veins and volcanic deposits, notably in
Spain, Italy, Peru, and California. It is reduced to mercury by roasting.
Hardness 2–2.5, relative density 8.0–8.2. Hepatic
cinnabar, so called from its liver-brown color, is a variety
containing a little carbon. A black cubic form of mercury (II) sulfide, metacinnabar, also occurs.
In the form of red mercuric sulfide, cinnabar is used as a pigment under the name vermillion.
In ancient times, mercury was extracted by treating cinnabar (its sulfide, HgS), with vinegar, or simply by roasting it. Today it is obtained from cinnabar by heating the mineral to a high temperature in a furnace and letting a current of air in. The oxygen of the air combines with the cinnabar, forming sulfur dioxide and releasing mercury as a vapor. Expressed in chemical symbols the reaction is:
HgS + O2 → Hg + SO2.
The vapor is cooled and metallic mercury condenses out.
Uses of mercury
Mercury is used in thermometers, barometers, and manometers (instruments for measuring the pressure of gases and liquids); in diffusion pumps and automatic electric switches (Fig 3); for electrodes; in mercury-vapor lamps; in batteries; and for extracting gold, silver, and platinum from their ores. Various compounds of mercury are used in medicine and also in agriculture to make fungicides.
Chemistry of mercury
Compounds of mercury
It forms two series of salts, termed mercury (I), or mercurous, and mercury (II), or mercuric, compounds, and many important organometallic compounds. Various mercury compounds are used as pharmaceuticals.
Mercury (II) fulminate (Hg[ONC]2) is a white crystalline solid, sensitive to percussion, and used as a detonator. It is prepared by dissolving mercury in concentrated nitric acid (HNO3) and pouring the solution in ethanol (C2H5OH). Mercury (II) fulminate and mercury (II) cyanate (Hg[OCN]2) are isomers.
Mercury (II) chloride (HgCl2), or corrosive sublimate, is a colorless crystalline solid prepared by direct synthesis. Although highly toxic, it is used in dilute solution as an antiseptic, and also as a fungicide and a polymerization agent. Melting point 276°C, boiling point 302°C.
Mercury (I) chloride (Hg2Cl2) is a white rhombic crystalline solid, found in nature. It is used in ointments and formerly found use as a laxative. A calomel/mercury cell with potassium chloride electrolyte (the Weston cell) is used to provide a standard electromotive force. Melting point 303°C, boiling point 384°C.
If liquid mercury is swallowed, absorption via the intestines is only slight. Swallowing a small amount (e.g., from a broken thermometer) is therefore unlikely to lead to poisoning. However, liquid mercury is highly volatile and gives off a vapor that is readily absorbed into the body via the lungs. Inhalation of mercury vapor – usually as a result of industrial exposure – is the most common cause of poisoning. Mercury poisoning may be treated by chelating agents (such as penicillamine) to help the body excrete it at a faster rate. In some cases, purification of the blood by hemodialysis may also be performed, especially if the kidneys have been damaged.