Mercury divalent

REDOX RELATIONSHIPS AMONG THE OXIDATION STATES OF MERCURY 

It might be noted that the E° value for the half-reaction Hg — Hg(II) is not the sum of the f ° values i Hg Hg i) + Hgcn- Hgun but is rather the arithmetic mean of these values. More generally, if a valence change of n electrons is broken up into changes of x electrons and n-x) electrons, the E° values are not additive (as is the case when two half-cell reactions are combined to give a full reaction) but rather fit into the linear relationship (Exercise 7)  

The chemistry of dipositive mercury is largely the chemistry of covalent compounds. Although mercury forms salts of strong oxy-acids (sulfate, nitrate, and perchlorate) that are both ionic in the solid state and quite completely dissociated in aqueous solution, compounds con- 

The fluoride of dipositive mercury stands apart from the other halides, being far more like the nitrate and perchlorate. The Hg—F bonds in the solid state are predominantly ionic, and upon dissolving HgF2 in water, the very acidic Hg2 ion reacts with the basic water molecules, yielding both a basic fluoride and HgO. 

Similar to the halides is the thiocyanate, Hg(SCN)2. This compound is also very slightly dissociated, and, consequently, mercuric ions in solution may be titrated with a standard solution of sodium thiocyanate, using a bit of ferric salt as an indicator the end point is marked by appearance of the blood-red color of the FeSCN2+ ion. Students enjoy this titration, not only because the end point is sharp, but also because the solid Hg(SCN)2 so formed may be filtered off, dried, and ignited, whereupon it evolves a voluminous, serpentlike ash (Pharaoh s serpents). 

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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). 

Feng X, Sommar J, Gardfeldt K, Lindqvist O. 2000. Improved determination of gaseous divalent mercury in ambient air using KCl coated denuders. Fresenius J Anal Chem 366(5) 423 28. 

Some metals can be converted to a less toxic form through enzyme detoxification. The most well-described example of this mechanism is the mercury resistance system, which occurs in S. aureus,43 Bacillus sp.,44 E. coli,45 Streptomyces lividans,46 and Thiobacillus ferrooxidans 47 The mer operon in these bacteria includes two different metal resistance mechanisms.48 MerA employs an enzyme detoxification approach as it encodes a mercury reductase, which converts the divalent mercury cation into elemental mercury 49 Elemental mercury is more stable and less toxic than the divalent cation. Other genes in the operon encode membrane proteins that are involved in the active transport of elemental mercury out of the cell.50 52... 

FIGURE 20.7 Distribution of molecular and ionic species of divalent mercury at different pH values. (From U.S. EPA, Assessing the Geochemical Fate of Deep-Well-Injected Hazardous Waste A Reference Guide, EPA/625/6-89/025a, U.S. EPA, Cincinnati, OH, June 1990.)... 

Divalent mercury in rats has been reported to poorly penetrate the blood-brain barrier [23], However, there is an impairment of the blood-brain barrier within hours after mercury treatment [24, 25], By means of autoradiographic techniques, it was demonstrated [26] that after a single intravenous injection of labelled mercuric chloride, large portions of the radioactive mercury were detected in the cerebellar grey matter, area postrema, hypothalamus and areas near the lateral ventricle of mice. 

A small part of divalent mercury is reduced to mercury vapour. This reduction probably accounts for the ability of certain commonly occurring microorganisms to volatilize mercury for biological media [59]. Loss of volatile radioactive mercury was observed in rats injected with salts of divalent mercury labelled with the 203Hg isotope [60]. Part of the volatile mercury was exhaled via the lungs, the remainder by way of the skin and fur. The volatile loss accounted for up to 20% of the total rate of excretion of mercury from the animals. 

Selenium lessens the toxicity of divalent mercury in animals, the protection being less at continuous mercury exposure. Selenium has been found to affect the distribution of mercuric mercury in mice [134], rats [135], rabbits [136, 137] and pigs [ 138]. Mercury forms a mercury-selenium protein complex with selenium with little biological activity [139]. Mercury is thus retained longer in the blood, liver and spleen and as a consequence lessens accumulation in the kidney. In fish, selenium pretreatment probably retarded mercury uptake rather than promoting mercury excretion [140]. 

Such considerations were extended to metal complexes in 1902 by Morse, who studied the distribution of divalent mercury between toluene and water at various Hg and CT concentrations. By taking complex formation in the aqueous phase into consideration Morse could determine the formation constants of HgCr and HgCb from distribution measurements, as well as the distribution constant of the neutral complex HgCl2. The overall extraction reaction can be written... 

Yang and Zhu [107] have studied, applying several electrochemical methods and mercury electrodes, electrochemical behavior of pharmaceutically important dipeptide captopril. In acidic solution, one-electron transfer led to the formation of a univalent mercury-sulfur compound, which was strongly adsorbed at the electrode surface and gradually transformed into the divalent mercury-sulfur compound. 

Toxicity. The toxic el frets of mercury and mercury compounds are well known, and several detailed discussions on mercury toxicity are available. Toxicity lo the central nervous system is more prominent aftci exposure lo mercury vapor than lo divalent mercury. 

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. 

In the gas phase, divalent mercury has been shown to be linear and therefore to be sp hybridized. However, in solution the X—R—X, R—Hg—X, or R—Hg—R bond angle in divalent mercury compounds varies from 130 to 180°. The variation in geometry is not yet entirely understood, so we shall follow Jensen s example and assume that, even in solution, divalent mercury is sp hybridized and that if a divalent mercury compound donates one empty orbital to coordinate with a Lewis base it rehybridizes to sp2 (F. R. Jensen and B. Rickborn, Electrophilic Substitution of Organomercurials, pp. 35, 36). 

Acetaldehyde is an intermediate in acetic acid and vinyl acetate production. Since 1916 it has been produced from the addition of water to acetylene, a reaction catalyzed by divalent mercury in sulphuric acid (20%)/water. Acetylene was made from coal. In Germany in particular, a lot of research was carried out on the use of acetylene as a chemical feedstock. 

The electrodeposition of mercury, Hg, has been studied in both basic and acidic EMICI-AICI3 ionic liquids [73]. Mercury dichloride, HgCl2, is soluble in the acidic ionic liquid to form a divalent mercury species, Hg " ", which can be reduced irreversibly to a cluster cation, Hg ... 

The formal potentials of Hg /Hg and Hg /Hg are reported as 1.21 and 1.093 V, respectively. In case of the basic ionic liquid, HgCl2 dissolves and forms a divalent mercury chlorocomplex anion, HgCl . The reduction of HgCl results in the deposition of metallic Hg ... 

The electronic bands of an infinite crystal can cross as a function of some parameter (pressure, concentration etc.). If one treats the e /r,2 term of the electron repulsion correctly, one sees that the crossing transition of the two bands is a first-order phase transition, between the metallic and insulating states. This transition was predicted by Mott in 1946 and has carried his name ever since. In fact, the original Mott criterion does not predict such a transition for Hg, but the criterion was derived for monovalent atoms. For divalent mercury it should not be applicable. Also the semiempirical Herzfeld criterion, which was very successful in predicting the insulator to metal transition in compressed xenon, predicts bulk Hg to be non-metallic. All this seems to imply that Hg is a rather special case. 

G dfeldt K., Munthe J., Stromberg D., and Lindqvist (2003) A kinetic study on the abiotic methylation of divalent mercury in the aqueous phase. Sci. Total Environ. 304, 127-136. 

Metallic mercury can be oxidized to inorganic divalent mercury by the hydrogen peroxidase-catalase pathway, which is present in most tissues. The inorganic divalent cation can, in turn, be reduced to... 

Yoshida et al. 1990, 1992). Following repeated exposure (5 weeks) of rats to mercury vapor (1 mg/m3), high levels were detected in the blood and brain (Warfvinge et al. 1992). The absorption of inorganic divalent mercury has not been measured, but it is estimated to be approximately 40% in dogs (Morrow et al. 1964). 

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