edited by
chemical of the month Mercury Vinay Kumar and Bonnie Tale Novihern Kentucky University Highland Heights. KY 41076
History Mercury and its principal ore, cinnabar, which is a compound of mercury and sulfur, have been known and used since prehistoric times. The ancient Hindus, Chinese, and Egyptians were familiar with the ore and the metal ( I , 2). Mercury is said to be the seventh metal (the others are gold, silver, copper, iron, tin, and lead) to be discovered by the ancients (3). However, according to Asimov (3), there is no mention of mercury in the Bible. Aristotle (384-322 B.C.) is credited with recording the use of mercury in connection with areligious ceremony (4). While he labeled it as "liquid silver," the Greek physician Dioscorides later referred to this as hydrargyros (Greek) or "water-silver." In Latin the name became hydrargyrum; the current chemical symbol for mercury, Hg, is based on this Latin name. The old English name for mercury was "quicksilver." The alchemists, recognizing the exceptional characteristics of this unique metal, named i t after the planet Mercury. During the ancient and medieval times, mercury and its ore were put to some very unusual uses. For example, around 950 A.D. the Moorish King, Ahd ar-Rahman 111built a palace near Cordova, Spain, in the courtyard of which a fountain of mercury used t o run continuously (3). Another king was said t o have slept on a mattress that floated in a pool of mercury. The Chinese emperors believed (erroneously, of course) that mercury could prolong their lives (4). Mercury was also used in ointments hy the Greek physicians. Paracelsus, who reacted mercury with sulfuric acid and distilled it with alcohol, claimed the product to be a cure for syphillis (4). Romans are said to have converted the cinnabar into the pigment vermillion, which was used for decorative purposes. Indians had prepared calomel (mercurous chloride) and mercuric chloride in 12th century A.D. (5). Physical Properties Mercury is the only metal that is a liquid a t room temperature. ("It is a fluid but does not moisten, and runs about, though i t has no feet" [5].)Its freezing point, -38.9"C is the lowest melting point of all the metals. The boiling point is 3566°C. Mercury is 13.6 times heavier than water. I t was this property that led Torricelli to invent the barometer in 1643. I t has a vapor pressure of 1.3 X 10-3 torr a t 20°C. I t is soluble in both polar and nonpolar liquids (6). A saturated solution glg in air-free water a t 25°C has a concentration of 6 X (6). Another important property of mercury is its ability to form alloys with all the common metals, except iron and platinum. These alloys are called amalgams. Mercury and most of its compounds are very toxic and act as cumulative poisons since they are eliminated very slowly by the human body. Chemical Properties Mercury exhibits 1+ and 2+ oxidation states. In the 1+ state, two mercury(1)ions are joined by a covalent bond to give H g P . The existence of thision has heen supported by X-ray crystallography and by the fact that mercury(1) compounds are diamagnetic. Unlike zinc and cadmium, mercury does not
DARRELL H.
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The Culver Academies CuIver. Indiana 46511
react with diluted HCI or HzSOa. It reacts with an excess of nitric acid to produce the Hg2+ ion. Mercurv comoounds are considemblv less ionic than those of zinc and cadmium. For example, ~ ~ din laqueous * solution exists ~rimarilvas molecules. Bv contrast.. He819 -- - is verv insoluble in water. At about 350°C mercury reacts slowlv with air to form HaO. Similarly, it is oxidized by-sulfur to H ~ SIn . the reactions with haloeens. mercurv(1) halides are formed when mercurv is in excess, ad merc&11) halides are formed when the haiogens are in excess. For the chemistry of organo mercury compounds, the reader should consult reference (6). Occurrence Over 90% of the world's mercury comes from the following seven countries (names in order of % production): Spain, Italy, Soviet Union, USA, China, Yugoslavia, and Mexico (4). The retail price of mercury as of June 22,1982, was about $370 per flask (76 lb or 34.5 kg). There are three common mercury-containing minerals: cinnabar, mercury(1) chloride, and metacinnabarite. Cinnabar, HgS, is generally mined from the underground deposits. The ore is crushed and roasted a t 500-600°C in the presence of air, whereupon it is converted to elemental mercury and sulfur dioxide. Uses Bailey and Smith (7) have reported nearly 3000 applications of mercury. Some of the major uses of mercury include the following: 1) It is used as a liquid contact material for electrical switches; it is the working fluid in vacuum technology. 2) It is used in the manufacture of meicury-vapor rectifiers,
thermometers, barometers, tachometers, thermostats, manometers, and mercury-vaporUV lamps. 3) In microgas analysis, mercury is used as a sealingliquid for the evolved gases. 4) In the manufacture of chlorine and sodium hydroxide, mercury is used as cathode. 5) In the calomel electrode and in the Weston standard cell, mercury is used in contact with solutions which contain mercury(1)saltsas the solid phase. 6) Many dry cells contain amalgams of zinc and cadmium to prevent impurities from shortening the life of the battery. Mercury batteries contain HgO. The latter is also used in skin ointments. 7) For filling dental cavities, mercury is ground with Ag&; the resulting semisolid amalgam, on standing,sets to form a hard solid mixture of stable and nontoxic compounds Ag,Hg8 and add amalgams are also used for filling SmHp (8).Silver and . teeth.. 8) Merbaohen. is used as a diuretic. Mer. . an oreanomereurial. " curichrome and merthiolate are popular antiseptics. Ammoniated mercury ointment containa L10%HgNH2CLand is used in the treatment of various skin diseases. 9) HgS04 catalyzes the oxidation of naphthalene to phthalic acid, and acetylene to acetaldehyde. 10) Compounds like phenylmercury acetate and methoxyethylmercury acetate were once used to prevent fungi from growing in lumber, paint, paper, and seeds, and to kill plant fungus The stfuchlre, properties, and uses of a variety of chemicals are highlightedin this feature, which is aimed at inneasing the use of descriptive chemist~.
Volume 59
Number 11 November 1982
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diseases. Shipbuilders used paint containing Hg to prevent marine animals and plants from growing on the hulls. of ships. In 1972,the U.S. Government halted the use of Hg compounds for most of these pupme4 (9),and 91 countries approved a ban on ocean dumoine . .. of mercurv. Hg,CIL tml~~rnel~ la ured na a purgative and HgCI2(corrosive suldimatr) is wed a i an insecticide, in rat poison, and as a
12) Mercury fulminate, Hg(CNO)% is used as a detonator.
Mercury in the Environment
Mercury can he introduced directly into the environment during the manufacture of some of its products. The indirect contamination may he caused by inadvertent or accidental release of mercury and mercury-containing industrial, agricultural, and consumer products. For example, an undetermined amount of mercurv finds its wav into waterwavs and sewer systems as a resultif indiscriminant disposal df merw-containing paints, paper ~roducts.hroken thermometers. tooih fillings, disinfedaits, discarded cosmetics, pharma: ceuticals. and garden fungicides. Movement of Mercury and its Transmethylation
The lakes, rivers, and oceans are the main routes for the transport of mercury throughout the environment. Manv parameters including the temperature, redox potentiai, and presence of naturally occurring chelating agents affect its m o v b e n t . The normalbackground mercu~eoncentration in the soil ranges between 10 and 150 pph (10). The term "transmethylation" refers to the transfer of a methyl group from one substrate to another, usually as part of biochemical reaction. This nrocess is also called "bioloeical methylation" or simply "hiomethylation." ~icroobial transmethylation of mercuric ion has been reported by Jensen and Jernelov (11) and Wood et al. (12). Biotransformation of mercuric ion bv bacteria and molds results in the formation of monomethyl "or dimethylmercuric compounds (usually termed "methvl mercurv"). The amounts of mono- and dimethylmercur; compou~dsproduced in a given system are a function of microbial soecies. mercunr concentration.. oreanic pollution loading, temperatuke, and of that system. Based on the availahle research data, Thayer (13) has made the following general observations:
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The toxic effects of oreanomercurials first became known in 1953, when people of ~ i n a m a t aJapan, , became afflicted with a mysterious disease (later nicknamed Minamata disease) that affected their central nervous system. In all 121 cases were r e ~ o r t e dwith 46 resulting deaths. An extensive investigation ievealed that the wastewater effluent of a nearby vinylchloride plant had deposited a methvl mercuric compo"nd (produced unknowingly during the Eata~yticaction of mercuric chloride on acetylene) in the Minamata Bay. This compound was biologically concentrated by the fish and shellfish of the bay and then transferred to the victims when they ate the fish. The mercury content in the fish was found to he as high as 20 ppm (15). Additional accounts of the effect of methylmercuric poisoning in humans have also been reported (16-19). The Canadian and the American eoveruments became aware of the mercury problem in 1969khen i t was reported that Dow Chemical Comoanv's - . .~ l a natt Sarnia. Canada. was responsible for dumping mercury in the St. Cl&r River (19). In A~ril1970,the US. FDA reoorted that analvses of fish in ~ a k St. e Clair showed mercury ievels of 0.5-1.4 ppm mercury. (In unpolluted stream, river, and lake water, the normal amount of background mercury has been reported to he 0.01-0.1 pph 1211). Soon after that 0.5 porn (drv weieht) was established as the maximum limit foi safety by tge FDA. However, according to some prominent phvsicians (20). this limit may not he safe for a-pregnant woman's child since mercury tends to concentrate in the fetus. Because of the toxicity of mercury, i t is important for everyone to recognize that the mercury pollution is a serious threat not only to man but also to the entire ecosystem. Luckily, the government and the industries are doing their share in tackling the problem. For example, some industrial establishments are recirculating the waste water and have installed lazoons and settline oonds for mercurv t o collect: others are installing scruhhersh the smoke stacGs and are making sure that hazardous materials do not escaoe from their olants. In the field of medical research, the problem of mercury poisoning is being successfully overcome by binding the toxic metal with polymers (22). As a result of all these great efforts, the prospects look highly encouraging for a healthier tomorrow.
1) the methylmernrric ion, a potent neurotoxin, has the exeptional
abilitv to oermeate membranes.. esoeeiallv the "bload-brain . bar&," the protective membrane aroundihhe brain, 2) it is excreted from the M y considerably more slowly than other mercury compounds, 3) dkylated (methyl or ethyl) forms of mereuryare more toxic than aryl or inorganic mercury. Thaver further states that the toxicitv of oreanometals arises fro& their reactions with the thiol groups, wLich comprise the active sites on various enzvmes, such as dihvdroli~ovltransacetylase. Soon after its formation, the-volaiile liquid, (CHdzHg, escapes into the atmosphere. Oxidation or photolysis decomposes it, causing the mercury t o return to the ground. Methylmercuric ion, in its aqueous solution is absorbed by unicellular organisms, thereby entering the aquatic food chain. As little fish eat the mercury-containing plankton and are in turn eaten by bigger fish (tuna. swordfish, etc.), the concentration of mercury compounds increases as one goes up the predatory hierachy. This phenomenon is known as biomagnification. According to a study of biomethylation of toxic elements, including mercury, Wood et al. (14). vitamin B12 (methyl. corrinoid) derivatives (which are capable of transferring methyl groups as radicals, carbonium ions, or carhanions), are believed to he the methylating agentsfor inorganic mercury salts.
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Literature Cited PA, 1948,p. 48. (2) Farber, E., "The Evolution of Chcmiatry," Ronald Prrss.NewVark, 1952,~. 21. (3) Aaimov. Isaac, "The Solar System and Baek,"Doublda"and Co.be.. Garden Cit".
N.Y., 1970.p~.164-176. M.,"TheEnvironmmtdMermry Problem," CRC P-Ckdmd,Ohio. 1972. pp. &7.
(4) DTtri, Flank
(6) W e e k , Mary E , '"Direouery of the Elements," Mack Printing Co. Eaaton. PA,
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(11) Jmm. s. and Jcmelw.A..Nhture.223.753 (1969). (12) Wmd. J. M.. Roeen. C. G..and Kennedy. S. F,Noture, 220,173(1968). (13) Theyer,John S., J. CHEM. EDUC., 54 [Ill,662 (1977). (14) ~ i d kW. ~ P., , uizike8,~. J., wmd J. M., science. 197,329 (1977). (15) Keens", C.w., Kleinfelter,D. c.,andwood,J. H.."~enera~collepe chemiatry:6th Ed., Hame. and Row, NewYork, N.Y., 1980, p. 692. (16) ~ a k i rF.. , et or.. scipnce.181,230 (19%). (17) Moniague.K.and Monta~ue.P.,"Mereury."SienaCiub,SanPlaneiam,lwl. (IS) Putman. J. J..Nofio~lG a o p q h i c , 507 (October 1972). R ; ~D. ~ C. . and ~ ~ iG.b ~ ~ ~216., 759 (1982). (20) ~ c tu~pp.90-102. . (21) Jon-". I. R. and B w k , R. W. "Proe. Sa. Csn. S~m~.,"Ottawa, Fch. 15.1971, p. 5. (22) Jones,Mark A. and Nyssen.&rard A,. Chernteeh.. 648 (September 1918).
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