Ligand distribution coefficient

This expression can be solved for the ligand distribution coefficient . 

Both Table XIX and Table XX show that distribution coefficients cover a much larger range for complexes than for the parent ligands, and that variation of ligand substituent can have a very large effect on... 

Fig. 6.4 Calculated, estimated, or apparent salting out constants for various chemicals (a) selected aromatic compounds, (b) selected aliphatic compounds, (c) natural or surrogate hgands, (d) anthropogenic ligands, (e) sediment organic matter (SOM), (f) transition metal complexes, (g) trace metal complexes in the Mersey Estuary. Reprinted with permission from Turner A, Martino M, Le Roux SM (2002) Trace metal distribution coefficients in the Mersey Estuary UK Evidence for salting out of metal complexes. Environ Sci Technol 36 4578-4584. Copyright 2002 American Chemical Society...

In qualitative terms, microscopic interactions are caused by differences in crystal chemical properties of trace element and carrier, such as ionic radius, formal charge, or polarizability. This type of reasoning led Onuma et al. (1968) to construct semilogarithmic plots of conventional mass distribution coefficients K of various trace elements in mineral/melt pairs against the ionic radius of the trace element in the appropriate coordination state with the ligands. An example of such diagrams is shown in figure 10.6. 

Let s derive an equation for the distribution coefficient of a metal between two phases when essentially all the metal in the aqueous phase (ag) is in the form Mn+ and all the metal n the organic phase (org) is in the form ML (Figure 23-3). We define the partition coeffi- ients for ligand and complex as follows ... 

We see that the distribution coefficient for metal ion extraction depends on pH and ligand concentration. It is often possible to select a pH where D is large for one metal and small for another. For example. Figure 23-4 shows that Cu2+ could be separated from Pb2+ and Zn2+ by extraction with dithizone at pH 5. Demonstration 23-1 illustrates the pH dependence of an extraction with dithizone. Box 23-1 describes crown ethers that are used to extract polar reagents into nonpolar solvents for chemical reactions. 

The value of A ex is, of course, fixed for any given cation-extracting agent-solvent system. Analytically important distribution coefficients, D, can be varied by changing the concentration of ligand relative to that of the metal and more significantly by adjusting the pH for 50% extraction D = 1 and equation (21) reduces to... 

As reviewed by Fidelis and Mioduski57) the formation constant K of a complex with a given" ligand (or the distribution coefficient for extraction in another solvent, or an ion-exchange resin) shows a ratio (in the case of two consecutive lanthanides) which provides perceptible variations (from a constant) not only at the half-filled shell (q = 7) Gd(III) but also at the plateaux q = 3 and 4, as well as 10 and 11. These quarter-shell effects can be rationalized 217,218) by the refined spin-pairing energy theory. If D of Eq. (3) is decreased 1 % (65 cm 1) by the nephelauxetic effect in a... 

The distribution coefficient of the ligand is one of the most important factors governing the magnitude of Kex the smaller Z)l is, the larger the Kcx value will be. However, if the ligand itself is very soluble in water, the LnL complex will probably also be quite soluble, which in turn is unfavourable for a high extractability in the organic phase. A balance has therefore to be found. 

Various aspects of structure activity relationships in 4,5-epoxymorphinans have been reviewed,(201,46 U467) and specific consideration has been given to quantum chemical studies with particular reference to polar group variation,(468) stereoisomeric ligands as receptor probes, 469 partition and distribution coefficients, 470 471 and antagonists. 145 ... 

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