Mercury cations formed

Hence mercury is a poor reducing agent it is unlikely to be attacked by acids unless these have oxidising properties (for example nitric acid), or unless the acid anion has the power to form complexes with one or both mercury cations or Hg]", so altering the... 

Cations forming insoluble chromates, such as those of silver, barium, mercury (I), mercury(II), and bismuth, do not interfere because the acidity is sufficiently high to prevent their precipitation. Bromide ion from the generation may be expected to form insoluble silver bromide, and so it is preferable to separate silver prior to the precipitation. Ammonium salts interfere, owing to competitive oxidation by bromate, and should be removed by treatment with sodium hydroxide. 

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... 

Hence mercury is a poor reducing agent it is unlikely to be attacked by acids unless these have oxidising properties (for example nitric acid), or unless the acid anion has the power to form complexes with one or both mercury cations Hg2 + or Hg2+, so altering the E values. Nitric acid attacks mercury, oxidising it to Hg2+(aq) when the acid is concentrated and in excess, and to Hgf+(aq) when mercury is in excess and the acid dilute. Hydriodic acid HI(aq) attacks mercury, because mercury(II) readily forms iodo-complexes (see below, p. 438). 

Using supporting electrolytes such as tetraalkylammonium salts, one may apply potentials as negative as -2.6 V vs. SCE in aqueous solutions, while in some nonaqueous systems even -3.0 V vs. SCE (aqueous) is accessible. Unfortunately, mercury electrodes have serious limitations in applications at positive potentials (with the exception of passivated mercury electrodes, which are described in Section VI), and this has led to extensive research in the development of solid metal and carbon electrodes. Oxidation of mercury occurs at approximately +0.4 V vs. SCE in solutions of perchlorates or nitrates, since these anions do not form insoluble salts or stable complexes with mercury cations. In all solutions containing anions that form such compounds, oxidation of the mercury proceeds at potentials less than +0.4 V vs. SCE. For example, in 0.1 M KC1 this occurs at +0.1 V, in 1.0 M KI at -0.3 V, and so on. 

Salts of the three metals form ammines, principally tetrahedral complex ions of the form [M(NH3)4]2+, but in addition mercury salts form linear, diammine complexes, and zinc and cadmium octahedral hexa-ammines, [M(NH3)g]2+. Ethylenediamine produces 6-co-ordinate complexes, [M(en)3]2+, with all three cations. 

Mercury(II) forms with 1,10-phenanthroline a cationic complex which gives ion-associates with the acid dyes Rose Bengal B [57] and eosin [58] the associates have also been used for determining mercury in aqueous medium. 

This element is a chalcophile, and in unweathered rocks is most commonly found as the mineral cinnabar (HgS). In soil environments, the cationic form, is most common, as the reduced oxidation state (+1) has a limited stability range. Reduction to the metallic elemental form, H, is easily achieved in soils by both biological and chemical reactions. Elemental mercury is somewhat volatile, and the vapor is extremely toxic to organisms. Under anaerobic conditions at least, soil microbes methylate mercury, forming volatile organomercury compounds that are bioavail-able and present a health hazard. At the same time, however, anaerobic conditions can convert Hg into the exceedingly insoluble sulhde, HgS. Some of the more important transformations possible for mercury in soil are summarized in Figure 9.9. 

A very large number of cations form insoluble hydroxides and oxides in alkaline solutions, including mercury. The only step of analysis is to add dilute sodium hydroxide solution R xmtil it reacts strongly alkaline to litmus paper (above about pH 8.3), a dense yellow precipitate of mercury(ll) oxide. 

The dimerisation reaction is favoured by aluminium cathodes at whose partially insulated surfaces high cathodic potentials exist. Simple tetraalkylammonium cations reduce reversibly at mercury to form insoluble compounds of mercury with quaternary ammonium radicals. These compounds, stable only at sub-ambient temperatures, decompose on warming with the formation of hydrocarbon and amine products. ... 

According to the results of Clarkson et al. (1980) the divalent cation should be seen as the effective form of metallic mercury, especially because metallic mercury cannot form a chemical bond. Metallic mercury is non-polar and soluble in lipids. Particularly in vapour form it dissolves readily in membrane lipids so that it penetrates the alveolar membranes quickly and efficiently when inhaled, and is diffused into the blood. There it is partially absorbed by the erythrocytes and oxidised to the divalent ion which reacts with SH groups. Inspite of the efficient diffusion into the erythrocyte, sufficient quantities of the metallic mercury dissolved in the blood can be transported to the brain, where they are also reduced to the mercury ion (Magos, 1967 Magos et al., 1973). As far as the toxicity of mercury is concerned, its oxidation produces different effects in the... 

In a study of the effects of organic mercurials on the cation permeability of human red blood cells, Knauf and Rothstein (1971) found that the rate of uptake of PCMBS was diminished by 50% in cells exposed to SITS (4-acetamido-4 -isothiocyano-stilbene 2,2 -disulfonic acid), so that the total uptake could be divided into roughly equal SITS-sensitive and SITS-insensitive components. If the chloride in the bathing solution was replaced with either sulfate or phosphate, the SITS-sensitive component of PCMBS uptake was virtually unchanged whereas the SITS-insensitive component was substantially reduced. One interpretation of these results is that PCMBS enters by two routes, one the anion transporter that serves as a route for anionic complexes, and the other the lipid layer that permits the permeation of a neutral Cl complex of PCMBS, the latter perhaps being less abundant in the anion substituted media. Alternatively, a cationic form of the mercurial could permeate via some other membrane protein. 

Liquid-liquid extraction is possible in non-aqueous systems In a system consisting of a molten metal in contact with molten salt, metals can be extracted from one phase to the other. This is related to a mercury electrode where a metal can be reduced, the metal will often then dissolve in the mercury to form an amalgam that modifies its electrochemistry greatly. For example, it is possible for sodium cations to be reduced at a mercury cathode to form sodium amalgam, while at an inert electrode (such as platinum) the sodium cations are not reduced. Instead, water is reduced to hydrogen. A detergent or fine solid can be used to stabilize an emulsion, or third phase. 

A natural development of the ideas outlined above is to regard the dehydrated zeolite as a polar solid solvent and reactions (1) and (2) as dissolution processes. When Barrer and Whiteman [4] studied the reaction of mercury metal with a number of different ion-exchanged zeolites, they found that mercury uptake was limited in sodium-, calcium- and lead-exchanged forms and copious in silver- and mercury-exchanged forms, where reduction of the exchangeable cations Ag+ and Hg + (to Ag° or Hgl" ) by Hg° would be possible. The implication of these observations is that the ionization process outlined in reaction (1) is necessary for the reaction of elemental metals with zeolites - one of the main synthetic routes to ionic clusters in zeolites - to occur. Reactions (1) and (2), in fact, can usefully be regarded as models for the formation of ionic clusters in zeolites, and detailed calculations of solvation energies [1] can help... 

Table 14.2 shows that all three elements have remarkably low melting points and boiling points—an indication of the weak metallic bonding, especially notable in mercury. The low heat of atomisation of the latter element compensates to some extent its higher ionisation energies, so that, in practice, all the elements of this group can form cations in aqueous solution or in hydrated salts anhydrous mercuryfll) compounds are generally covalent. 

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). 

Thiocyanates are rather stable to air, oxidation, and dilute nitric acid. Of considerable practical importance are the reactions of thiocyanate with metal cations. Silver, mercury, lead, and cuprous thiocyanates precipitate. Many metals form complexes. The deep red complex of ferric iron with thiocyanate, [Fe(SCN)g] , is an effective iadicator for either ion. Various metal thiocyanate complexes with transition metals can be extracted iato organic solvents. 

These considerations show the essentially thermodynamic nature of and it follows that only those metals that form reversible -i-ze = A/systems, and that are immersed in solutions containing their cations, take up potentials that conform to the thermodynamic Nernst equation. It is evident, therefore, that the e.m.f. series of metals has little relevance in relation to the actual potential of a metal in a practical environment, and although metals such as silver, mercury, copper, tin, cadmium, zinc, etc. when immersed in solutions of their cations do form reversible systems, they are unlikely to be in contact with environments containing unit activities of their cations. Furthermore, although silver when immersed in a solution of Ag ions will take up the reversible potential of the Ag /Ag equilibrium, similar considerations do not apply to the NaVNa equilibrium since in this case the sodium will react with the water with the evolution of hydrogen gas, i.e. two exchange processes will occur, resulting in an extreme case of a corrosion reaction. 

Insolubility in water and other common solvents. No metals dissolve in water electrons cannot go into solution, and cations cannot dissolve by themselves. The only liquid metal, mercury, dissolves many metals, forming solutions called amalgams. An Ag-Sn-Hg amalgam is used in filling teeth. 

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