Solvation solvent extraction

A process developed in Israel (263) uses solvent extraction using a higher alcohol or other solvating solvent. This removes phosphoric acid and some hydrochloric acid from the system driving the equiHbrium of equation 42 to the right. The same principle can be appHed in other salt—acid reactions of the form... 

Accelerated solvent extraction (ASE) is a technique which attempts to merge the beneficial solvation properties of SFE with traditional organic solvents. Specifically, the sample is placed in an extraction vessel which can withstand high pressures while being maintained at a constant temperature. Extraction is carried out by pumping the extraction solvent through the samples for a limited time. As an example of the use of ASE, Richter and Covino extracted PCBs from a 10-g fish tissue sample with hexane... 

In almost all theoretical studies of AGf , it is postulated or tacitly understood that when an ion is transferred across the 0/W interface, it strips off solvated molecules completely, and hence the crystal ionic radius is usually employed for the calculation of AGfr°. Although Abraham and Liszi [17], in considering the transfer between mutually saturated solvents, were aware of the effects of hydration of ions in organic solvents in which water is quite soluble (e.g., 1-octanol, 1-pentanol, and methylisobutyl ketone), they concluded that in solvents such as NB andl,2-DCE, the solubility of water is rather small and most ions in the water-saturated solvent exist as unhydrated entities. However, even a water-immiscible organic solvent such as NB dissolves a considerable amount of water (e.g., ca. 170mM H2O in NB). In such a medium, hydrophilic ions such as Li, Na, Ca, Ba, CH, and Br are selectively solvated by water. This phenomenon has become apparent since at least 1968 by solvent extraction studies with the Karl-Fischer method [35 5]. Rais et al. [35] and Iwachido and coworkers [36-39] determined hydration numbers, i.e., the number of coextracted water molecules, for alkali and alkaline earth metal... 

In solvent extraction with TBP, the fluoride complexes form solvated complexes with TBP. The stabilities of such complexes depend on the acidity/concentration of hydrofluoric... 

A single SFE/ESE instrument may perform (i) pressurised C02 (SFE), (ii) pressurised C02/modifier and (iii) pressurised modifier (i.e. ASE /ESE , organic solvent) extractions. The division between SFE and ASE /ESE blurs when high percentages of modifier are used. Each method has its own unique advantages and applications. ESE is a viable method to conduct matrix/analyte extraction provided a solvent with good solvating power for the analyte is selected. Sample clean-up is necessary for certain matrix/analyte combinations. In some circumstances studied [498], SFE may offer a better choice since recoveries are comparable but the clean-up step is not necessary. 

The development of more benign alternatives to cyanide for gold-leaching (see Section 9.17.3.1) such as thiourea, thiocyanate, or thiosulfate, which form stable complexes in water has prompted research to identify suitable solvent extractants from these media. Cyanex 301, 302, 272, Ionquest 801, LIX 26, MEHPA, DEHPA, Alamine 300 (Table 5) have been evaluated as extractants for gold or silver from acidic thiourea solutions.347 Whilst the efficacy of Cyanex 301 and 302 was unaffected by the presence of thiourea in the aqueous feed, the loading of the other extractants is severely depressed. Formation of solvated complexes of gold and of an inner-sphere complex of silver has been proposed.347... 

Mooiman, M. B. Miller, J. D. The chemistry of gold solvent extraction from alkaline cyanide solution by solvating extractants. Hydrometallurgy 1991, 27, 29-46. 

Marcus, Y. Yamir, E. Kertes, A. S., Compilers, Part III "Compound Forming Extractants, Solvating Solvents, and Inert Solvents" in "Equilibrium Constants of Liquid-Liquid Distribution Reactions" IUPAC Chemical Data Series No. 15, Pergamon Press, Oxford, 1977. 

In many practical solvent extraction systems, one of the two liquids between which the solute distributes is an aqueous solution that contains one or more electrolytes. The distributing solute itself may be an electrolyte. An electrolyte is a substance that is capable of ionic dissociation, and does dissociate at least partly to ions in solution. These ions are likely to be solvated by the solvent (or, in water, to be hydrated) [5]. In addition to ion-solvent interactions, the ions will also interact with one another repulsively, if of the same charge sign, attractively, if of the opposite sign. However, ion-ion interactions may be negligible if the solution is extremely dilute. The electrolyte is made up of... 

The important role of thermodynamics in complex formation, ionic medium effects, hydration, solvation, Lewis acid-base interactions, and chelation has been presented in this chapter. Knowledge of these factors are of great value in understanding solvent extraction and designing new and better extraction systems. 

In this section, we describe three simple cases of rates and mechanisms that have been found suitable for the interpretation of extraction kinetic processes in kinetic regimes. These simple cases deal with the exuaction reaction of a monovalent metal cation (solvation water molecules are omitted in the notation) with a weakly acidic solvent extraction reagent, BH. The overall extraction reaction is... 

Various instruments of theoretical chemistry have been widely to describe separate steps of solvent extraction of metal ions. Because of the complexity of solvent extraction systems, there is still no unified theory and no successful approach aimed at merging the extraction steps. It has already been pointed out that the challenging problem for theoreticians dealing with solvent extraction of metals, in particular with thermodynamic calculations, is to evaluate correctly solvent effects by the use of the most accurate explicit solvation models and QM calculations. However, such calculations on extremely large sets consisting of hundreds or even thousands of molecules, necessary to model all aspects of the extraction systems, are still impossible due to both hardware and software limitations. 

A number of solvent extraction experiments have demonstrated that individual crown ethers in combination with a lipophilic sulfonic acid (such as didodecylnaphthalene sulfonic acid) are efficient, synergistic phase transfer agents for manganese(II) from aqueous solution into an organic phase." The X-ray structure of the manganese di-t-butylnaphthalenesul-fonate with cyclohexano-15-crown-5 ether as its toluene solvate has been reported." ... 

The distribution ratios (D s) for crown-ether-based extraction processes using conventional solvents depend on two major factors (1) the thermodynamic driving force for cafion complexation by a crown efher and (2) the solvation of fhe cafion and counfer anion by the organic solvent [1,4] the former factor is usually thermodynamically favored (see Equation 10.3). Difficulties in increasing the solvent-extraction efficiency of conventional solvent-extraction systems using crown ethers as extractants lie in the... 

From a thermodynamic perspective, the solvation of ionic species (see Equations 10.6 and 10.7), such as crown-ether complexes, NO3, and SO 4, in the ILs, should be much more favored thermodynamically than those of conventional solvent extractions (Equations 10.1 and 10.2). This is one of the key advantages of using ILs in separations involving ionic species. In this case, cationic crown-ether complexes and their counter anions are not expected to form ion pairs, but to be solvated separately by ionic species from the ILs. Therefore, the extraction process using crown ethers in ILs may not be an ion-pair extraction process. 

Solvent extraction of base metals by solvating extractants 810... 

Organic extractants facilitate the transfer of the metal ions from the aqueous phase to the organic phase in solvent extraction. Based on the nature of the organic extractant, the metal ion, and the diluent, effective separation methods can be devised. Uranium extraction into diethyl ether from nitrate medium by salting out is perhaps one of the first uses of solvent extraction for large-scale actinide processing (9). In this case, ether not only acts as the diluent, it also acts as the extractant, which works according to the solvation mechanism (discussed below). 

The term Third-Phase Formation in solvent extraction refers to a phenomenon in which the organic phase splits into two phases (126). One of the two phases is diluent rich, whereas the other is rich in extractant and also contains the metal solvate. Third-... 

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