Arsenic chains

The uranyl phosphate chain in the structure of Cu2[(U02)(P04)2] is topologically identical to the uranyl arsenate chain found in walpurgite [104], Copper occurs in sixfold (4+2) coordination as elongated tetragonal bipyramids to form face-sharing chains that extend along the [010] direction. The chains of Cu polyhedra share vertices with adjacent phosphate tetrahedra of the parallel uranyl phosphate chains (Fig. 25). 

When sulphur is melted viscosity changes occur as the temperature is raised. These changes are due to the formation of long-chain polymers (in very pure sulphur, chains containing about 100 (X)0 atoms may be formed). The polymeric nature of molten sulphur can be recognised if molten sulphur is poured in a thin stream into cold water, when a plastic rubbery mass known as plastic sulphur is obtained. This is only slightly soluble in carbon disulphide, but on standing it loses its plasticity and reverts to the soluble rhombic form. If certain substances, for example iodine or oxides of arsenic, are incorporated into the plastic sulphur, the rubbery character can be preserved. 

In recent years, biochemists have developed an arsenal of reactions that are relatively specific to the side chains of particular amino acids. These reactions can be used to identify functional amino acids at the active sites of enzymes or to label proteins with appropriate reagents for further study. Cysteine residues in proteins, for example, react with one another to form disulfide species and also react with a number of reagents, including maleimides (typically A ethylmaleimide), as shown in Figure 4.11. Cysteines also react effectively... 

NaAs03 has an infinite polymeric chain anion similar to that in diopside (pp. 349, 529) but with a trimeric repeat unit LiAs03 is similar but with a dimeric repeat unit whereas /6-KASO3 appears to have a cyclic trimeric anion As309 which resembles the cyc/o-trimetaphosphates (p. 530). There is thus a certain structural similarity between arsenates and phosphates, though arsenic acid and the arsenates show less tendency to catenation (p. 526). The tetrahedral As 04) group also resembles PO4) in forming the central unit in several heteropoly acid anions (p. 1014). 

One form of biological poisoning mirrors the effect of lead on a catalytic converter. The activity of an enzyme is destroyed if an alien substrate attaches too strongly to the enzyme s active site, because then the site is blocked and made unavailable to the true substrate (Fig. 13.42). As a result, the chain of biochemical reactions in the cell stops, and the cell dies. The action of nerve gases is believed to stem from their ability to block the enzyme-controlled reactions that allow impulses to travel through nerves. Arsenic, that favorite of fictional poisoners, acts in a similar way. After ingestion as As(V) in the form of arsenate ions (As043 ), it is reduced to As(III), which binds to enzymes and inhibits their action. 

Mercury, tin, lead, arsenic, and antimony form toxic lipophilic organometallic compounds, which have a potential for bioaccumulation/bioconcentration in food chains. Apart from anthropogenic organometallic compounds, methyl derivatives of mercury and arsenic are biosynthesized from inorganic precursors in the natural environment. 

Both iron(III) and copper(II) inhibit the induced chain oxidation and, when present in sufficient quantity, iXp -t-X s becomes 1.0, the effect of oxygen is completely eliminated and only the induced oxidation of arsenic(III) by peroxydi-... 

The initiation step in the chain oxidation, reaction (43), is not affected by the presence of oxygen. SO4 radicals formed in (43) give arsenic(fV), reaction (45), initiating the following propagation cycle which leads to the reformation of As(IV)... 

In the presence of oxygen the chain-oxidation of arsenic(lll) consists of... 

In the absence of oxygen the addition of arsenic(Iir) has only a slight effect on the rate of reduction of peroxydisulphate. In the presence of air the rate of reduction of persulphate increases nearly fortyfold (Table 12). The oxidation of arsenic(III) by SO4 from reaction (62) is not a chain process, thus it need not be considered in the iron(III)- and copper(II)-catalyzed reaction between peroxydisulphate and arsenic(III). 

The presence of arsenous acid causes a considerable change in the induced reaction the error in the H2O2 determination decreases to a minimum and an As(ril) error appears, while the S2OI error remains practically unchanged. Though reaction between arsenic(III) and peroxydisulphate is about ten times as rapid as that between hydrogen peroxide and peroxydisulphate, the extent of the induced reduction of peroxydisulphate remains practically unchanged. This indicates that, in the induced chain oxidation, reaction (85), is replaced by the more rapid reaction... 

Sulfide ores usually contain small amounts of mercury, arsenic, selenium, and tellurium, and these impurities volatilize during the ore treatment. All the volatilized impurities, with the exception of mercury, are collected in the dust recovery systems. On account of its being present in low concentrations, mercury is not removed by such a system and passes out with the exit gases. The problem of mercury contamination is particularly pertinent to zinc plants since the sulfidic ores of zinc contain traces of mercury (20-300 ppm). The mercury traces in zinc sulfide concentrates volatilize during roasting and contaminate the sulfuric acid that is made from the sulfur dioxide produced. If the acid is then used to produce phosphatic fertilizers, this may lead to mercury entering the food chain as a contaminant. Several processes have been developed for the removal of mercury, but these are not yet widely adopted. 

The structural chemistry of some metal dithiocarbamates, i.e. systematics, coordination modes, crystal packing, and supramolecular self-assembly patterns of nickel, zinc, cadmium, mercury,363 organotin,364 and tellurium,365 366 complexes has been thoroughly analyzed and discussed in detail. Supramolecular self-assembly frequently occurs in non-transition heavier soft metal dithiocarbamates. Thus, lead(II),367 bismuth(III)368 zinc,369 cadmium,370 and (organo)mercury371 dithiocarbamates are associated through M- S secondary bonds, to form either dimeric supermolecules or chain-like supramolecular arrays. The arsenic(III)372 and antimony(III)373 dithiocarbamates are... 

Arsenic is bioconcentrated by organisms, but not biomagnified in the food chain. 

Identification and quantification of mineral and chemical forms of arsenic in rocks, soils, and sediments that constitute the natural forms of arsenic entering water and the food chain... 

Lindsay, D.M. and J.G. Sanders. 1990. Arsenic uptake and transfer in a simplified estuarine food chain. 

Wrench, J., S.W. Fowler, and M.Y. Unlu. 1979. Arsenic metabolism in a marine food chain. Mar. Pollut. Bull. 10 18-20. 

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