The Toxicity of Mercury

Almaden, some 225 km southwest of Madrid, is the richest mercury mine in the world. The mineraHzation originally occurred in connection with volcanic activity. 

Both Romans and Moors mined cinnabar there and distilled mercury. In the ore the mercury content is typically 1% but special meter-thick sections may be found with as much as 80% Hg. 

Ore from Almaden and other mercury mines is crushed and heated in the presence 

Mercury oxide is formed, which is spontaneously decomposed to mercury metal at the reaction temperature  

In this treatment sulfur is oxidized to sulfur dioxide and mercury is vaporized. For direct heating a multiple-hearth furnace may be used or alternatively a rotary kiln. These latter furnaces are operated at about 350°C at the charging head and 750°C at the discharge end. In the indirect heating system, two concentric tubes are utilized. 

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Improved sensitivities can be attained by the use of longer collection times, more efficient mass transport or pulsed wavefomis to eliminate charging currents from the small faradic currents. Major problems with these methods are the toxicity of mercury, which makes the analysis less attractive from an eiivironmental point of view, and surface fouling, which coimnonly occurs during the analysis of a complex solution matrix. Several methods have been reported for the improvement of the pre-concentration step [17,18]. The latter is, in fact. 

The biochemical basis for the toxicity of mercury and mercury compounds results from its ability to form covalent bonds readily with sulfur. Prior to reaction with sulfur, however, the mercury must be metabolized to the divalent cation. When the sulfur is in the form of a sulfhydryl (— SH) group, divalent mercury replaces the hydrogen atom to form mercaptides, X—Hg— SR and Hg(SR)2, where X is an electronegative radical and R is protein (36). Sulfhydryl compounds are called mercaptans because of their ability to capture mercury. Even in low concentrations divalent mercury is capable of inactivating sulfhydryl enzymes and thus causes interference with cellular metaboHsm and function (31—34). Mercury also combines with other ligands of physiological importance such as phosphoryl, carboxyl, amide, and amine groups. It is unclear whether these latter interactions contribute to its toxicity (31,36). 

Mercury thermometers are being phased out because of the toxicity of mercury vapor. A common replacement for mercury is the organic liquid iso amyl benzoate, which boils at 262°C. What is its boiling point in °F K ... 

C19-0014. The mercury battery, use of which is being discontinued because of the toxicity of mercury, contains HgO and Zn in contact with basic aqueous solution. The redox products are Hg and ZnO. Determine the oxidation and reduction half-reactions and the overall reaction for these batteries. 

Standards imposed to the industrial waste streams charged in heavy metals are more and more drastic in accordance with the updated knowledges of the toxicity of mercury, cadmium, lead, chromium... when they enter the human food chain after accumulating in plants and animals (Forster Wittmann, 1983). Nowadays, the use of biosorbents (Volesky, 1990) is more and more considered to complete conventional (physical and chemical) methods of removal that have shown their limits and/or are prohibitively expensive for metal concentrations typically below 100 mg.l-i. 

Owing to the toxicity of mercury and its disposal problem, solid electrodes are now very popular. In particular, electrodes made of carbon such as glassy carbon, graphite, carbon paste, and carbon fibers have gained popularity. Mercury, gold, bismuth, and other metals can be deposited as thin metal films on carbon and serves as thin metal film electrodes (TMFE) with excellent analytical advantages in trace metal analysis. The choice of working electrode is determined by the redox... 

Caution. Due to the toxicity of mercury compounds, both starting materials and products must be handled with care to avoid any skin contact. Due to the toxicity of carbon monoxide gas, this reaction must also be carried out in a well-ventilated hood. 

The biochemical basis for the toxicity of mercury and mercury compounds resulls from its ability to form covalent bonds with sulfur. Even In low coiiccninilinns divalent mercury is capable of inaelivaiing enzymes containing suirhydrvl I —Nil) groups, causing iiileil crcncc with cellular metabolism and function. 

The toxicity of mercury compound has caused a drastic reduction in their use. They were once of considerable importance in agriculture and medicine but their use is now restricted in most parts of the world. Hence their use as fungicides in crop and seed protection is prohibited in many countries and the pharmaceutical applications are very limited. 

A major limitation of field-based stripping sensing is the toxicity of mercury (used as the working electrode). Intensive research efforts... 

CAUTION The toxicity of mercury is such that the element and its compounds should not be allowed to contaminate air or water. Wear eye protection, nitrile rubber gloves, and laboratory coat. Work in the fume hood. Dissolve the waste mercury salts (10 g) as far as possible in water (100 mL). Adjust the solution to pH 10 with 10% sodium hydroxide solution. Add 20% sodium sulfide solution while stirring until no further precipitation occurs. Withdraw a small sample of supernatant liquid and add sodium sulfide solution to check that precipitation is complete. Allow the precipitate to settle, and then decant or filter the supernatant liquid into the drain with at least 50 times its volume of water. Package the dry mercuric sulfide for disposal in a secure landfill.13-15... 

There are several types of mercury electrodes. Of these, the dropping mercury electrode (DME), the hanging mercury drop electrode (HMDE), and the mercury film electrode (MFE) are the most frequently used. Related solid amalgam electrodes have been introduced more recently to address concerns related to the toxicity of mercury. 

To reach lower pressures, a secondary pump is used, such as a diffusion pump or a sublimation pump (both must remain connected to a primary or "roughing" pump). There are two kinds of diffusion pumps. A mercury diffusion pump can reach 10 6 Torr, but the toxicity of mercury vapor has decreased its use dramatically. A silicone oil diffusion pump can reach 10-7 Torr. For even lower pressures, a Ti sublimation pump is used it can reach about 10 11 Torr. It is usually connected to a sorption primary pump. The ultimate low pressure attained in a laboratory on earth is about 10 13Torr. 

The toxicity of mercury depends on its exact form (and valence state) ... 

Perfluoroalkyl derivatives of mercury were the first fluorocarbon-organometallic compounds to be reported. Alkylmercurials are valuable in that they are able to alkylate other metals, but the toxicity of mercurials greatly inhibits the use of these systems. Perfluoroalkyl iodides react with mercury on heating or irradiation with ultraviolet light to give perfluoroalkylmercury(ll) iodides [58-60] (Figure 10.17). 

A number of adverse reactions to phenylmercuric salts have been reported and concern at the toxicity of mercury compounds may preclude the use of phenylmercuric salts under certain circumstances see Section 14. 

Phenylmercuric nitrate and other phenylmercuric salts are widely used as antimicrobial preservatives in parenteral and topical pharmaceutical formulations. However, concern over the use of phenylmercuric salts in pharmaceuticals has increased as a result of greater awareness of the toxicity of mercury and other mercury compounds. This concern must, however, be balanced by the effectiveness of these materials as antimicrobial preservatives and the low concentrations in which they are employed. 

Potassium dichromate and atrazine may increase the toxicity of mercury, although these effects have been noted only with metallic and inorganic mercury. Ethanol increases the toxicity of methylmercury in experimental animals. Vitamins D and E, thiol compounds, selenium, copper, and possibly zinc are antagonistic to the toxic effects of mercury. 

Discontinued applications. The use of phenylmercuric acetate as a fungicide in interior latex paints was banned in 1990 (Reese 1990), and its use in exterior paint was banned in 1991 (Hefflin et al. 1993). Both of these bans were prompted because of releases of mercury vapors as the paint degraded. Alkyl mercurial compounds were used until the mid-1970s as a treatment to disinfect grain seeds. Most other agricultural applications of mercury compounds in bactericides and fungicides have been banned due to the toxicity of mercury. Mercuric nitrate was used in the production of felt hats to hydrolyze rabbit fur. The use of mercury as a wood preservative has ceased due to the use of polyurethane (Drake 1981). 

ImuraN, Naganuma A. 1991. Possible mechanism of detoxifying effect of selenium on the toxicity of mercury compounds. In Suzuki T, ImuraN, Clarkson TW, eds. Advances in mercury toxicology. New York, NY Plenum Press, 275-288. 

Chronic exposure to low levels of mercury can also result in neurotoxic effects, as evidenced by the increased neurological symptoms observed in workers in a mercury thermometer manufacturing plant. 13 a good discussion of the toxicity of mercury is presented in the article by Magos and Clarkson. 14 ... 

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