Speciation ligands, cation

The OH/Al mole ratio or ligand number ris an important factor in the speciation of Al cations in the PAG product. Product composition also... 

The most widely studied interactions between biologically active ligands and organotin(lV) cations relate to the amino acids and their derivatives (N- or S-protected amino acids and peptides), though new data on several of the most commonly occurring amino acids are still being published. This is specially true for aqueous speciation studies. Nice and very detailed reviews were published in this area by Molloy and Nath. ... 

In the presence of Fe + it is possible to deprotonate polyphenols at physiological pFIs, to give phenolates which are good ligands for hard 3+ cations such as Fe " ". Speciation in iron(III) — polyphenolate systems has been discussed in relation to possible use of these ligands as iron chelating agents. 

Simple ligands can adsorb on iron oxides to form a variety of surface species including mononuclear monodentate, mononuclear bidentate and binuclear mono or bi-dentate complexes (Fig. 11.2) these complexes may also be protonated. How adsorbed ligands (and cations) are coordinated to the oxide surface can be deduced from adsorption data, particularly from the area/adsorbed species and from coadsorption of protons. Spectroscopic techniques such as FTIR and EXAFS can provide further (often direct) information about the nature of the surfaces species and their mode of coordination. In another approach, the surface species which permit satisfactory modelling of the adsorption data are often assumed to predominate. As, however, the species chosen can depend upon the model being used, this method cannot provide an unequivocal indication of surface speciation confirmation by an experimental (preferably spectroscopic) technique is necessary. 

Recent reviews on chemical speciation are published by e.g. Stumm and Brauner (1975), Florence and Batley (1980) and Leppard (1983) sometimes, with special reference to metal-organic interactions (Mantoura, 1982) or complexation in natural waters (Kramer and Duinker, 1984b). Bruland (1983) summarized the distribution and behaviour of trace elements in ocean waters. The occurrence of certain species is largely dependent on the environmental conditions. There exists a strong competition of trace metals with H+ or major cations like Ca2+ and Mg2+ in seawater, but also with other trace metals which might form more stable complexes with the ligand in question on the other side, many potential ligands or chelators compete for one trace element. 

The extent of speciation in solution depends on the stoichiometric coefficients of the components of a species the polyvalent nature and protonation behaviour of anionic complexing ligands the type and relative ability of different cations and anions to form complexes pH ionic strength, and the ratio of the total concentrations of the reactants in solution (the total cation anion ratio). 

The pK of formation of a species can have a significant effect on the variation of the extent of speciation with the total cation anion ratio in solution (Fig. 9.3). For weak complexes (p K 2), there is no effect of total cation anion ratio on speciation in solution at constant pH (Fig. 9.3(a)). If the anion complexes the cation moderately (pK 4), however, the fraction of total cation complexed by the ligand increases almost linearly as the cation anion ratio varies from 1 1 to 1 1000 (Fig. 9.3(b)). In the case of strong complexation (pK 6), species... 

Speciation in solution is considered a major factor in the mobilisation and leaching of metal cations (DeKoninck, 1980 Bloomfield, 1981 Stevenson and Fitch, 1986). Complexation increases the total soluble concentration of a metal and hence increases its potential to be leached. Organic ligands (e.g. humate, ful-vate, citrate, polyphenols) are the major complexers involved in this mechanism, but they are effective only if the soluble organic complex does not become saturated and precipitate (DeKoninck, 1980). 

With a DGT device, Cr(III) can be bound to the chelex resin because of its cationic nature, whereas Cr(VI) is not bound to the resin (it has an anionic nature) but is present in the diffusive gel layer (as in a DET probe), reaching equilibrium with Cr(VI) in the aquatic system. Hence, Cr(VI) can be measured in the diffusive layer and Cr(III) in the resin layer.44 For Mn the same procedure can be adopted. The oxidized Mn(IV) species form colloids or even larger particles and will not be sampled by the DGT probe, whereas Mn(II) species are free or labile complexes. For Fe speciation, DGTs with open pores and with restricted pores are often used. Since in aquatic systems, Fe(III) is present mostly as a ligand complex or in colloidal form, the restrictive pore size excludes these forms and makes only Fe(II) species available to the restrictive DGT,45 whereas the open-pore DGT allows the passage of Fe(II) and small and labile Fe(III) complexes. In the case of arsenic speciation, As(III) and As(V) diffuse through the diffusive gel layer of the DGT, but only As(III) is immobilized on the chelating resin layer As(V) remains in the diffusive layer as an anionic compound. 

The active sites of these enzymes can have a nitrogen ligand, usually as histidine (acid phosphatases and some protein phosphatases), a nucleophilic serine residue (alkaline phosphatases), a cysteine residue in which the thiol group can form a covalent species with the phosphate ester (protein phosphatases), or an aspartate-linked phosphate (plasma membrane ion pumps). The inhibitory form of vanadium is usually anionic vanadate V(V), but cationic vanadyl V(IV) has also shown strong inhibition of some types of phosphorylase reactions. Above neutral pH, speciation of vanadyl ions produces anionic V(IV) species capable of inhibition of enzymes in the traditional transition-state analogue manner [5],... 

Thus, there is negative feedback in the enviromnent/ metaUome system, exactly tantamount to detoxification. For so-called semi-metals some of which also form colloquial complexes, like Sb, Bi, Te, the speciation pathway of biomethylation (Thayer 1995) will remove their electrophilic properties altogether, turning the cations into ligands (donors) of their own, whereas with other elements (Ge, Sn, Pb, Pt, Au, Cd or Hg) acceptor properties are substantially altered (see the data (c and x values, Table 2.3) for R Stf R Sn and RjPb species) but do not vanish. Of course, redox processes also influence acceptor properties (cp. the data for different oxidation states of V, Fe, Ce or Tl). 

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