Mercury complexes, equilibrium constants

Mercury-chloride complexes in dilute solutions. This slightly more difficult example will be useful in showing how to handle poorly conditioned systems of equations. It is assumed that mercury chloride HgCl2 is dissolved in pure water with a molality m = 10 5 mol kg-1. Given the equilibrium constants for chloride complex formation... 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

The value of the equilibrium constant for the reaction indicates that the favored direction of the reaction is actually from right to left. It is driven to the right by the strong complexation of Hg + with any of a large number of hgands. The third common form of mercury is as organic mercurials such as methyl mercury, CH3Hg+. 

The equilibrium constant of reaction (1), K = [Cu ][Cu ]/[Cu ], is of the order of 10 thus, only vanishingly small concentrations of aquo-copper(I) species can exist at equilibrium. However, in the absence of catalysts for the disproportionation—such as glass surfaces, mercury, red copper(I) oxide (7), or alkali (311)—equilibrium is only slowly attained. Metastable solutions of aquocopper(I) complexes may be generated by reducing copper(II) salts with europium(II) (113), chromium(II), vanadium(II) (113, 274), or tin(II) chloride in acid solution (264). The employment of chromium(II) as reducing agent is best (113), since in most other cases further reduction to copper metal is competitive with the initial reduction (274). 

The three equilibrium constants are obtained directly from the experimental data and may be combined with other constants to furnish the stability constants of the mercury(ll)-selenide complexes. 

The formation of Hg(SeCN)4 is well established by the potentiometric work of Toropova [56TOR], while her experimental data pertaining to the formation and the formation constant of Hg(SeCN)3 only comprise a few points. In their polarographic work Murayama and Takayanagi [72MUR/TAK] studied the anodic mercury wave in the presence of 0.001 to 0.003 M SeCN . The electrode process was assumed to comprise the charge transfer Hg(l) Hg + 2e combined with the formation of Hg(SeCN)2(aq) and Hg(SeCN)3. No primary data are provided and the evaluation procedure is rather involved, which makes the assessment difficult. The results are mixed equilibrium constants, since an activity coefficient correction was applied to the Hg ion. The following complexes are thus proposed to prevail in the Hg -SeCN system ... 

A non-dependence of the thermodynamic equilibrium constant on the solvent for two different types of diols was found 34>, which indicated that Ag+ as well as undissociated AgN03 formed complexes with olefins, comparable with mercury salt-olefin complexes 35>. Further formation constant investigations 36> by gas chromatography of silver complexes of cyclo-olefins had shown that methyl substitution at the double bond markedly reduced the stability and... 

TABLE 6.13 Equilibrium Constants for Exchange Reactions of Mercury Complexes ... 

Aromatic compounds undergo electrophilic mercuration. The old method involved heating the arene with mercury(II) acetate under reflux in acetic acid or in ethanol. Recently it has been found that mercury(II) trifluoroacetate in trifluoroacetic acid reacts at room temperature. The reactions are reversible the isomer ratios depend on time, tending towards the statistical. The mechanism shown above has been proposed. The equilibrium constant K for Tc-complex formation has been estimated from changes in the UV spectra of arenes which occur on addition of HgfOCOCFj). For benzene in CF3CO2H at 25°C, K = 8.2mo Mm ... 

Environmental phase distributions of elements or inorganic chemicals usually involve different chemical species and therefore speciation reactions. For example, different species of an element such as mercury have different vapor pressure and solubility. Elemental mercury, Hg(0), is fairly volatile and only sparingly soluble in water, whereas oxidized Hg(II) complexes are much less volatile but more water soluble. The distribution of mercury among the phases of air, water, and solid will thus depend on its speciation, which in turn is influenced by variable conditions of the environment, including pH, redox conditions, and the presence of other chemical species. This is approached quantitatively using equilibrium reaction constants for the various speciation reactions and illustrated using distribution diagrams that delineate the major prevalent species as a function of pH or pE, or both. 

---