Distribution constants, separations

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. 

Dissociative interactions, mass spectrometry, 574 Distribution constants, separations, 766-769,777-779,789, 803,810,844,846,882 DLS (dynamic light scattering), 955-958... 

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... 

Chromatography is essentially a physical method of separation in trtiich the components to be separated are distributed between two phases one of which is stationary (stationeury phase) while the other (the mobile phase) percolates through it in a definite direction. The chroaatographic process occurs as a result of repeated sorption/desorption acts during the movement of the sample components along the stationary bed, and the separation is due to differences in the distribution constants of the Individual sample components. 

The addition of metal ions to the mobile phase frequently yields Improved separations of solutes capable of forming complexes (conversely the addition of ligands to the mobile phase may allow the separation of metal ions based on differences in the distribution constants of the complexes between the mobile phase and, stationary phase) [353-355]. A number of important... 

Equations 9.109 and 9.110 are valid for a boiling process in a closed system the gaseous phase develops from a liquid with initial concentration Cq, is the mass fraction of developed gas, and K is the mass distribution constant of the component of interest between gas and liquid. Equations 9.111 and 9.112 describe a multistage separation process in which n is the number of separation stages, AFg is the mass fraction of gas separated in each stage, saiAK is the mean mass distribution constant of the process. Equations 9.113 and 9.114 refer to a boiling process in an open system the gas is continuously removed from the system as the process advances. 

We study ensembles of systems with a given graph and independent and well-separated kinetic constants fcy. This means that we study asymptotic behavior of ensembles with independent identically distributed constants, log-uniform distributed in sufficiently big interval log ke[a., ft], for a — 00, ft- CO, or just a log-uniform distribution on infinite axis, logfc e M. 

In Section 2.1.3 we discussed extractions from the viewpoint of one substance being transferred from one phase to another or the separation of two solutes by selective extraction. When we have a system in which the distribution constants (K) or distribution coefficients (ftc) differ by 103, we can only recover the extracted solute in about 97% purity. Continued extractions will increase yield but not purity. Good separations, with high purity, of two or more solutes can be achieved when there is a difference in the thermodynamic behavior of the various solutes, that is, a difference in the distribution constants (K) or coefficients (Kc). A measure of this degree of separation is the separation factor, a, for pairs of solutes which is defined as... 

The sequence, position, and distribution of separated components contain a good deal of information on the mixture. If properly measured and interpreted, this can serve many analytical goals without further tests. The quality of this information naturally improves as the system is better understood, characterized, and controlled. Informational content is greatest when, through theory and/or calibration, one can identify zones or peaks located at defined positions in the sequence with specific molecular species At that point, using a suitable sensor (detector), both qualitative and quantitative analyses follow. One can, at the same time, often measure certain physicochemical constants for the components, such as partition coefficients and diffusion constants. 

Selenophene /3-diketones can be used as extractants for the separation and isolation of metals. The advantages of selenophene /3-dike-tones were revealed by comparison of their dissociation and distribution constants with those of acetylacetone, benzoylacetone, thenoyl-trifluoromethylacetone, etc. Selenophene /3-diketones containing a trifluoromethyl group and a pyridyl radical were of particular interest. 2-Acetoacetylselenophene127 is better for the extraction of thorium from water than acetylacetone, previously extensively used. 

In Fig. 5 the values of the packing parameters d and d> are plotted for constant separations R between the reacting atoms Cl and C4. The relevance of the model considerations can be tested using crystal structure data, which have become available recently for a number of reactive and unreactive diacetylene monomers. Reactivity is only observed in a small area of the map. The distribution of the points for highly reactive structures suggest the criterion for which the separation R should be less than 4 A to be a more critical condition than the requirement of a least motion pathway as calculated by Baughman Figure 5 shows that all but one reactive diacetylene... 

The peak separation will depend on the nature of the two components to be separated. The more different are the distribution constants K and K, for the components, the more different are their retention times tR, as seen in rel. (6). The separation factor a is commonly used to characterize the separation, where... 

The gas chromatographic separation in temperature gradient is affected by more than one factor [32]. Simultaneously there is variation in the gas flow, variation in the distribution constants, and variation in peak broadening. 

Distribution constants are useful because they permit us to calculate the concentration of an analyte remaining in a solution after a certain number of extractions. They also provide guidance as to the most efficient way to perform an extractive separation. Thus, we can show (see Feature 30-1) that for the simple system described by Equation 30-2, the concentration of A remaining in an aqueous solution after i extractions with an organic solvent ([A],) is given by the equation... 

From distribution studies, species M and N are known to have water/hexane distribution constants of 5.93 and 6.11 respectively K = [M]aq/[M]i,ex). The two species are to be separated by elution with hexane in a column packed with silica gel containing adsorbed water. The ratio Vs/Vm for the packing is 0.398. 

The retention time for an analyte on a column depends on its distribution constant, which in turn is related to the chemical nature of the liquid stationary phase. To separate various sample components, their distribution constants must be sufficiently different to accomplish a clean separation. At the same time, these constants must not be extremely large or extremely small because the former leads to prohibitively long retention times and the latter results in such short retention times that separations are incomplete. 

The phenomena just described are quite similar to what occurs in a liquid partition chromatographic column except that the stationary phase is moving along the length of the column at a much slower rate than the mobile phase. The mechanism of separations is identical in the two cases and depends on differences in distribution constants for analytes between the mobile aqueous phase the hydrocarbon pseudostationary phase. The process is thus true chromatography hence, the name micellar electrokinetic capillary chromatography. Figure 33-15 illustrates two typical separations by MECC. 

The result demonstrates that the distribution constants of 1 and 2 must have different values for separation to occur. If they were exactly the same, then resolution would have to be zero. 

Fig. 5. Typical equilibrium distribution of heterovalent ions between solution and resin, showing its approximation by a second-order equilibrium constant (Kn) equivalent to a constant separation factor. Courtesy of Chemical Engineering Progress (H6).

Organic chemicals are often much more soluble in organic solvents and fats than in water and are said to be lipophilic. The BCF and also to a great extent the binding to soil are dependent on the lipophilic nature of the compound. In principle, this is simple to measure experimentally by shaking a small amount of the substance in a separating funnel with n-octanol and water. The two solvents separate into two phases, and the substance distributes between them. The distribution constant at equilibrium (KOW) is defined as... 

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