Solvent extraction equilibria anionic extractants

Equation (31) is true only when standard chemical potentials, i.e., chemical solvation energies, of cations and anions are identical in both phases. Indeed, this occurs when two solutions in the same solvent are separated by a membrane. Hence, the Donnan equilibrium expressed in the form of Eq. (32) can be considered as a particular case of the Nernst distribution equilibrium. The distribution coefficients or distribution constants of the ions, 5 (M+) and B X ), are related to the extraction constant the... 

Table 4.15 gives the equilibrium constants (for extraction, amine) for this reaction with trioctylamine in various solvents. Although the ion pairs are only slightly soluble in water, they can exchange the anion L with other anions, X, in the aqueous phase. (Note that we use L to indicate any anion, while X is used for (an alternative) inorganic anion.)... 

The extraction efficiency depends on the nature of cephalosporin molecule (i. e., its dissociation constant), solvent, and extractant through equilibrium relationship. The extractant should be able to provide stripping of anion to another aqueous phase to affect the separation. 

The interface itself is dynamics, non-planar and ill-defined, due to important local solvent mixing, and the position of the complexes with respect to the interface fluctuates much more than at classical aqueous interfaces. There is thus likely an equilibrium involving two types of positions for the strontium complex at the interface, which is consistent with cation exchange, as well as anion co-extraction mechanisms. 

One would expect the organic phase of other amine extraction systems in which more than one metal anion can be formed to exhibit similar equilibria. It is fortunate that in this system not only is the solvent not present in the coordination sphere of either complex but also the equilibrium constant between the two is of an order of magnitude which allows concentration of both to be measured readily by spectrophoto-metric methods. This allows the effect of the dielectric constant of the solvent on the ratio of the species to be studied easily without the perturbing effect of specific interactions caused by differences in the tendency of the solvents to enter the coordination sphere. 

Before evaluating Eq. [30], the parameters of kinetics, mass transfer, and thermodynamic equilibrium must be established. The aim of this work is to evaluate the equilibrium and extraction of a quaternary salt in an organic solvent/aqueous solution. The studies on distribution equilibrium of the quaternary salts enable one to clarify the true mechanism through which the reactant anion is transferred. 

Brandstrom [48] indicated that the distribution of quaternary salt between two (liquid-liquid) phases exists as complicated multiequilibrium constants, which depend on the structure of the anion, cation, and solvent, as well as on pH, ionic strength, and concentrations in the aqueous solution. Such equilibrium properties have not yet been evaluated completely. The relationship between quaternary salt and extraction constant is an important consideration for PTC work. 

In liquid/liquid extraction, a dissolved component ( , extracted component, solute) is transferred from a loaded liquid R, raffinate phase) to another liquid (I, extract phase, solvent). A prerequisite for this is immiscibility of raffinate and extract phases. If the driving force for mass transfer is not only attainment of the physical liquid/liquid equilibrium, but there is also a chemical driving force such as solvation, chelation, and anion or cation exchange, then the term reactive extraction is used ... 

The report consists of tables compiling equilibrium constants. All the solvents included in the list in Appendix A of the Introduction to this Series (Part I) have been looked up in the literature. In all cases, only systems in which equilibrium constants have been calculated are included in this compilation. The tables on pages 2-17 are for distribution reactions of carboyxlic and sulfonic acid extractants, for their dimerization and other reactions in the organic phase, and for extraction reactions of metal ions from aqueous solutions. The inorganic anions in these solutions are usually irrelevant, since they do not participate in the extraction reaction, hence are not properly called ligands. The extractants themselves are acids, which exchange their hydrogen ions (rarely also alkali metal ions]), for the extracted metal ions, which form compounds with extractants. Since the extractants are monofunctional carboxylic or sulfonic acids, no chelates are formed. 

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