Distribution parameters, solvent extraction

The primary parameter in solvent extraction is the measured distribution ratio, where it is up to the writer to define what is being measured, indicating this by an appropriate index. In the Nernst distribution experiment described earlier, the analytically measured concentration of benzoic acid is in the aqueous phase [Bz]aq,tot = [HBz]aq + [Bz ]aq, and in the organic phase [Bz] , tot= [HBzJorg + [HjBzjlo . Thus the measured distribution ratio, abbreviated Z)bz, becomes... 

Because of the many parameters involved in solvent extraction, chemical as well as physical, it is a difficult task to draw reliable conclusions about the reactions that are responsible for the observed distribution values. In the introductory part of this chapter, section 4.2, the extraction reaction described by was shown to be a product of a number of parameters related to the different steps involved in the formation of the extracted complex. Sections 4.4-4.11 have described these various subreactions, which are described by Eq. (4.46) ... 

The studies described above have the purpose of identifying the reacting species in a solvent extraction process and developing a quantitative model for then-interactions. The fundamental parameter measured is the distribution ratio, from which extraction curves are derived. Solvent extraction work can still be carried out with simple batchwise (or point-by-point) technique, but continuous on-line measurements give faster and more accurate results. 

The suitability of using solvent extraction for a given separation is determined by thermodynamic and kinetic considerations. The main thermodynamic parameter is the solute distribution ratio, Dyi, between the organic and the aqueous phase. This is given by [Eq. (4.3), Chapter 4] ... 

Decontamination of soils using supercritical fluids is an attractive process compared to extraction with liquid solvents because no toxic residue is left in the remediated soil and, in contrast to thermal desorption, the soils are not burned. In particular, typical industrial wastes such as PAHs, PCBs, and fuels can be removed easily [7 to 21]. The main applications are in preparation for analytical purposes, where supercritical fluid extraction acts as a concentration step which is much faster and cheaper than solvent-extraction. The main parameters for successful extraction are the water content of the soil, the type of soil, and the contaminating substances, the available particle-size distribution, and the content of plant material, which can act as adsorbent material and therefore prolong the extraction time. For industrial regeneration, further the amount of soil to be treated has to taken into account, because there exists, so far, no possibility of continuous input and output of solid material for high pressure extraction plants, so that the process has to be run discontinuously. 

In the second step, with the hydrolysis constants and the specific interaction parameter for ZrOH" and for Zr3(OH) fixed to the values optimised as detailed above, the equilibrium constants and interaction parameter for all other species in the overall hydrolysis model were obtained by a global fit of the potentiometric, solubility, solvent extraction and ion exchange data mentioned above. The fit was extended to the determination of equilibrium constants for heterogeneous reactions ion exchange constants, solubility constants and liquid/liquid distribution coefficients. The fit was based on a preselection of the stoichiometries of dominant species which included invariably the species Zr(OH)4(aq), Zr ) ), Zr (OH)Jj and Zr4(OH)i6(aq) and various other mono-, di-, tri- and tetravalent species to improve the fit. The potential formation of chloride complexes of Zr was considered for chloride containing solutions, using the stability constants determined in Section V-4. If all fitted results were found insensitive to the equilibrium constants of a given species, the respective species was removed from the list of species. 

Long-chain, aliphatic amines ate effective extractants for separation of carboxylic acids from dilute aqueous solution (Yang et al., 1991). Generally, the amine extractants are dissolved in a diluent, an organic solvent that dilutes the extractant. It controls the viscosity and density of the solvent phase. In order to improve the amine s solvation power, diluents such as oleyl alcohol, chloroform, methyl isobutyl ketone, and 1-octanol have been used. The diluents affect the basicity of the amine, the stabiUty of the acid amine complex formed and its solvation power. The pH of the aqueous phase is an important parameter for the reactive extraction of organic acids (Kahya et al., 2001). In the present study, various pure diluents are used for extraction of propionic acid from aqueous solution. On the basis of distribution coefficients, reactive extraction is also carried out with amine extractant for the recovery of propionic acid. 

LLE is also called solvent extraction. It is used for both sample cleanup and concentration of the analyte. LLE is based on the phenomenon that a compound will distribute between two nonmiscible liquid phases. The equilibrium is strongly determined by the physicochemical parameters of the two liquids and can be advantageously used to concentrate some while dilute other components of the sample. 

Many solvent properties are related to density and vary with pressure in a SCF. These include the dielectric constant (er), the Hildebrand parameter (S) and n [5], The amount a parameter varies with pressure is different for each substance. So, for example, for scC02, which is very nonpolar, there is very little variation in the dielectric constant with pressure. However, the dielectric constants of both water and fluoroform vary considerably with pressure (Figure 6.3). This variation leads to the concept of tunable solvent parameters. If a property shows a strong pressure dependence, then it is possible to tune the parameter to that required for a particular process simply by altering the pressure [6], This may be useful in selectively extracting natural products or even in varying the chemical potential of reactants and catalysts in a reaction to alter the rate or product distributions of the reaction. 

The mechanism of separation of biological molecules such as proteins and amino acids, and the parameters that affect the extraction distribution coefficient and the kinetics of extraction have been studied more extensively than the extraction of inorganic solutes. This is mainly due to the variety of size and structure of these molecules and, furthermore, to the fact that their characteristics may be adversely affected by their contact with solvents and surfactants. 

The distribution ratios obtained were compared to the corresponding distribution ratios between water and Toctanol. The authors also calculated solvent parameters of [C4Cilm][PFg] (Section 9.3). It was shown that phenolate-ion associates with [C4CiIm][PFg] more strongly than other ions. The authors also mention the possibility of extraction of amino acids into [C4CiIm][PPg] in the presence of crown ether dibenzo-18-crown-6, though at rather moderate efficiency. 

Selection of the radiolysis conditions is of primary importance. If studies carried out with a pure extractant enable the intrinsic stability of the molecules to be verified, radiolysis in solution and especially in a basic medium are indispensable to guarantee the approaches good representativity, as much from the point of view of species formation as from that of their distribution (potential elimination of the shortest degradation products, the most polar to the aqueous phase). The characteristics of the irradiation source (nature, dose rate, integrated dose) and also the temperature are essential parameters. Thus, the nature of the irradiation depends on the composition of the fuel, and the dose rate is dependent on the bum-up and cooling time of the fuel, while the exposure time of a solvent depends on the implementation conditions of the proposed process (flowsheet and nature of the contactors). 

In this context, another empirical solvent parameter called SI should be mentioned. SI stands for Solvent /nfluence (in Russian, BP for Ejmstsae FacTBopHTena). This parameter was introduced by Shmidt et al. in 1967 and was derived from the study of many different extraction equilibria, i.e. of the distribution of organic and inorganic compounds between two immiscible liquid phases [298-301]. It was found that in the extraction of metal salts using various extraction reagents, the distribution coefficients of the extractable compound depend on the specific electrophilic and/or nucleophilic properties of the solvents used as diluent. From a large number of well-studied extraction systems, Eq. (7-12d) has been derived. 

Lo used the three-term solubility parameter (Barton, 1983) and a graphical procedure to identify solvents for liquid-liquid extraction. In a 2D space constructed from the polar component of the solubility parameter 6p and the hydrogen bonding component of the solubility parameter 6, the distribution coefficient of the solute B, mg, is given by... 

PTRs have received considerable attention as effective agents to facilitate the use of hydrogen peroxide, but require many parameters to be adjusted to obtain good selectivity. Among them are the nature and concentration of the PTRs, where in general, the length of the alkyl groups determines the extraction efficiency, the solvent, the pH of the aqueous phase, and the presence of salts like NaCl, which modifies the distribution of the species [154]. 

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