Chemical separation distribution coefficients

The effect of molecular interactions on the distribution coefficient of a solute has already been mentioned in Chapter 1. Molecular interactions are the direct effect of intermolecular forces between the solute and solvent molecules and the nature of these molecular forces will now be discussed in some detail. There are basically four types of molecular forces that can control the distribution coefficient of a solute between two phases. They are chemical forces, ionic forces, polar forces and dispersive forces. Hydrogen bonding is another type of molecular force that has been proposed, but for simplicity in this discussion, hydrogen bonding will be considered as the result of very strong polar forces. These four types of molecular forces that can occur between the solute and the two phases are those that the analyst must modify by choice of the phase system to achieve the necessary separation. Consequently, each type of molecular force enjoins some discussion. 

Chemical forces are normally irreversible in nature (at least in chromatography) and thus, the distribution coefficient of the solute with respect to the stationary phase is infinite or close to infinite. Affinity chromatography is an example of the use of chemical forces in a separation process. The stationary phase is formed in such a manner that it will chemically interact with one unique solute present in the sample and thus, exclusively extract it from the other materials... 

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... 

A comprehensive survey is available in literature on the sorption behavior of elements over a range of hydrochloric acid concentrations. The compiled data show the following (i) a number of elements exhibit no sorption tendency at all (ii) many exhibit a change of behavior with hydrochloric acid concentration and (iii) several cases exhibit high distribution coefficients over at least some part of the concentration range. It can be found out that there is a possibility of separating certain metal combinations not easily obtainable by conventional chemical means. 

DOSY is a technique that may prove successful in the determination of additives in mixtures [279]. Using different field gradients it is possible to distinguish components in a mixture on the basis of their diffusion coefficients. Morris and Johnson [271] have developed diffusion-ordered 2D NMR experiments for the analysis of mixtures. PFG-NMR can thus be used to identify those components in a mixture that have similar (or overlapping) chemical shifts but different diffusional properties. Multivariate curve resolution (MCR) analysis of DOSY data allows generation of pure spectra of the individual components for identification. The pure spin-echo diffusion decays that are obtained for the individual components may be used to determine the diffusion coefficient/distribution [281]. Mixtures of molecules of very similar sizes can readily be analysed by DOSY. Diffusion-ordered spectroscopy [273,282], which does not require prior separation, is a viable competitor for techniques such as HPLC-NMR that are based on chemical separation. 

Figure 19.17 Spherical macroparticle with radius ra consisting of an aggregate of microparticles separated by micropores filled with water. A chemical with constant concentration C° diffuses into the pore volume of the macroparticle. The local dissolved pore concentration Cw is at instantaneous equilibrium with the local sorbed phase C ( K d is microscopic equilibrium coefficient). Note that the macroscopic distribution coefficient Kd is time dependent (see Eq. 19-78.)...

We have calculated the energy 0 in this way for some polymers and separation conditions (Table 2) and, using the lattice-like model and a slit-like pore, we have found the distribution coefficients, K 1, for these macromolecules as a function of N, D, 0 and 0f 65). It turned out that for such a crude model not only the calculated KJj 1 values were close to the experimental ones, but also, which is especially important, that the chemical nature of the macromolecule, the functional groups and the separation conditions (the mobile phase composition) were correctly accounted for. Two examples of such calculations are given in Figs. 8 and 9. 

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. 

Sorption coefficients quantitatively describe the extent to which an organic chemical is distributed at equilibrium between an environmental solid (i.e., soil, sediment, suspended sediment, wastewater solids) and the aqueous phase it is in contact with. Sorption coefficients depend on (1) the variety of interactions occurring between the solute and the solid and aqueous phases and (2) the effects of environmental and/or experimental variables such as organic matter quantity and type, clay mineral content and type, clay to organic matter ratio, particle size distribution and surface area of the sorbent, pH, ionic strength, suspended particulates or colloidal material, temperature, dissolved organic matter (DOM) concentration, solute and solid concentrations, and phase separation technique. 

The term partitioning constant or coefficient refers to one chemical species in each phase (for example, ionisable chemicals present in water may exist in both neutral and dissociated forms and therefore each would have a separate partitioning coefficient). For well-defined phases, such as pure water or the pure liquid state of the chemical, then partitioning with another equally well-defined phase such as air results in the use of the term partitioning constant. The Henry s Law constant describing chemical partitioning between pure air and water is one example. Distribution ratios on the other hand, such as the soil-water... 

Oil-water partition coefficient refers to the tendency of a chemical to distribute itself between lipid and aqueous phases when both are present. This can be measured for comparative purposes by adding the compound to a two-phase system such as olive oil-water or octanol-water, mixing the three components thoroughly, allowing the two phases to separate, and then determining the amount of the compound in each of the phases. Using the olive-water system, DDT has a partition coefficient of 316, indicating that the concentration of DDT in the olive oil is 316 times that in the water phase. 

Extraction or separation of dissolved chemical component [X]A from liquid phase A is accomplished by bringing liquid solution of [X]B into contact with a second phase B that is totally immiscible. A distribution of the component between the immiscible phases occurs. After the analyte is distributed between the two phases, the extracting analyte is released and/or recovered from phase A for analysis. The theory of chemical equilibrium leads us to a reversible distribution coefficient as follows ... 

The neutral species are partitioned between the liquid stationary and mobile phases (KD is the relevant distribution constant). Separation is based upon the relative values of the distribution coefficient of the different neutral species. This model most closely explains the experimental results obtained with non-bonded reversed-phase columns (e.g. n-pentanol coated onto silica gel), in which the stationary phase behaves as a bulk liquid. The ion-pair model is, however, unable to explain ion-pair interactions with chemically bonded reversed-phase columns, and the working of these clumns is more appropriately explained by a dynamic ion-exchange model. 

The next step in purification is separation of uranyl nitrate from the other metallic impurities in the dissolver solution by solvent extraction. Practically aU uranium refineries now use as solvent tributyl phosphate (TBP) dissolved in an inert hydrocarbon diluent. The first U.S. refinery used diethyl ether as solvent and later refineries have used methyl isobutyl ketone or organic amines, but practically all have now adopted TBP. It is nonvolatile, chemically stable, selective for uranium, and has a uranium distribution coefficient greater than unity when the aqueous phase contains nitric acid or inorganic nitrates. 

A compound that has a higher affinity for the stationary phase will take longer to travel through the column than one that has a higher affinity for the mobile phase. In order for separation to occur, the distribution coefficient for each of the compounds must be different for a given mobile and stationary phase. Some of the chemical properties of solutes that affect K will be discussed later in this chapter. 

The tendency of a chemical to distribute between any two separable phases is measured by the equilibrium constant of the chemical in the two phases. In this review, we shall use the term partition coefficient simply to indicate the relative tendency of a chemical to solubilize in two separable phases. Fur-... 

If Kp 1, the nonexclusion interactions take place in the separation mechanism. The absence of such interactions requires the SEC to be performed under conditions of only entropic, size exclusion interactions. In such a case, Kp = 1 and Ka = K ec- E follows from this thermodynamic approach that the distribution coefficient depends on the chemical character of the solute molecules and that of the solvent, as well as on the matrix constituting the porous particles. 

The separation factor, a, is the most important and effective term for maximizing the resolution. The separation factor reveals the relative difference of the distribution coefficient between the two analytes and can be manipulated by chemical means. Since the distribution coefficient is a characteristic of a given separation system, the selectivity can be manipulated by changing either the type of micelle or by modifying the aqueous phase. 

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