Solvent extraction theory

If a neutral chelate formed from a ligand such as acetylacetone is sufficiently soluble in water not to precipitate, it may stiH be extracted into an immiscible solvent and thus separated from the other constituents of the water phase. Metal recovery processes (see Mineral recovery and processing), such as from dilute leach dump Hquors, and analytical procedures are based on this phase-transfer process, as with precipitation. Solvent extraction theory and many separation systems have been reviewed (42). 

Section A The following references are recommended for more in-depth readings on absorption and solvent extraction theory, design principles, and practical applications. 

Theory. Conventional anion and cation exchange resins appear to be of limited use for concentrating trace metals from saline solutions such as sea water. The introduction of chelating resins, particularly those based on iminodiacetic acid, makes it possible to concentrate trace metals from brine solutions and separate them from the major components of the solution. Thus the elements cadmium, copper, cobalt, nickel and zinc are selectively retained by the resin Chelex-100 and can be recovered subsequently for determination by atomic absorption spectrophotometry.45 To enhance the sensitivity of the AAS procedure the eluate is evaporated to dryness and the residue dissolved in 90 per cent aqueous acetone. The use of the chelating resin offers the advantage over concentration by solvent extraction that, in principle, there is no limit to the volume of sample which can be used. 

The theory and development of a solvent-extraction scheme for polynuclear aromatic hydrocarbons (PAHs) is described. The use of y-cyclodextrin (CDx) as an aqueous phase modifier makes this scheme unique since it allows for the extraction of PAHs from ether to the aqueous phase. Generally, the extraction of PAHS into water is not feasible due to the low solubility of these compounds in aqueous media. Water-soluble cyclodextrins, which act as hosts in the formation of inclusion complexes, promote this type of extraction by partitioning PAHs into the aqueous phase through the formation of complexes. The stereoselective nature of CDx inclusion-complex formation enhances the separation of different sized PAH molecules present in a mixture. For example, perylene is extracted into the aqueous phase from an organic phase anthracene-perylene mixture in the presence of CDx modifier. Extraction results for a variety of PAHs are presented, and the potential of this method for separation of more complex mixtures is discussed. 

Various types of research are carried out on ITIESs nowadays. These studies are modeled on electrochemical techniques, theories, and systems. Studies of ion transfer across ITIESs are especially interesting and important because these are the only studies on ITIESs. Many complex ion transfers assisted by some chemical reactions have been studied, to say nothing of single ion transfers. In the world of nature, many types of ion transfer play important roles such as selective ion transfer through biological membranes. Therefore, there are quite a few studies that get ideas from those systems, while many interests from analytical applications motivate those too. Since the ion transfer at an ITIES is closely related with the fields of solvent extraction and ion-selective electrodes, these studies mainly deal with facilitated ion transfer by various kinds of ionophores. Since crown ethers as ionophores show interesting selectivity, a lot of derivatives are synthesized and their selectivities are evaluated in solvent extraction, ion-selective systems, etc. Of course electrochemical studies on ITIESs are also suitable for the systems of ion transfer facilitated by crown ethers and have thrown new light on the mechanisms of selectivity exhibited by crown ethers. 

Essentially, extraction of an analyte from one phase into a second phase is dependent upon two main factors solubility and equilibrium. The principle by which solvent extraction is successful is that like dissolves like . To identify which solvent performs best in which system, a number of chemical properties must be considered to determine the efficiency and success of an extraction [77]. Separation of a solute from solid, liquid or gaseous sample by using a suitable solvent is reliant upon the relationship described by Nemst s distribution or partition law. The traditional distribution or partition coefficient is defined as Kn = Cs/C, where Cs is the concentration of the solute in the solid and Ci is the species concentration in the liquid. A small Kd value stands for a more powerful solvent which is more likely to accumulate the target analyte. The shape of the partition isotherm can be used to deduce the behaviour of the solute in the extracting solvent. In theory, partitioning of the analyte between polymer and solvent prevents complete extraction. However, as the quantity of extracting solvent is much larger than that of the polymeric material, and the partition coefficients usually favour the solvent, in practice at equilibrium very low levels in the polymer will result. 

Watch the animation explaining the theory of solvent extraction at www. brightredbooks.net... 

Diffusion is a complex phenomenon. A complete physical description involves conceptual and mathematical difficulties associated with the need to involve theories of molecular interactions and to solve complicated differential equations [3-6]. Here and in sections 5.8 and 5.9, we present only a simplified picture of the diffusional processes, which is valid for hmiting conditions. The objective is to make the reader aware of the importance of this phenomenon in connection with solvent extraction kinetics. 

The theory of solvent extraction was considered in Chapter 1, and Chapter 7 covered the application of liquid-liquid extraction in industry. The principles underlying the design of industrial applications are addressed in this chapter. 

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. 

The lack of satisfactory solvent recovery methods prior to 1930 prevented the use of selective solvents more suitable for lubricating oils. The major part of any solvent extraction plant is its complex solvent recovery system. Chemical engineering s contributions to distillation theory and process design resulted in the development of efficient solvent recovery techniques. In 1933, as illustrated by Figure 3, large commercial plants were... 

The complexity of the problem and the diversity of operating conditions in saline water conversion make it unlikely that any process based on one principle or phenomenon will provide the most efficient conversion in all operating situations encountered. The art of saline water conversion has now reached a level at which one can begin to take stock with respect to the particular advantages of the many different processes in any given situation. The final selection of a process will only be possible after careful consideration of process operation data as applied to the conversion problem at hand. Since such data are available on but very few processes at the present time, it is only possible to project on the basis of theory and experience those points which set apart one process from another. The purpose of this paper is to present information now available which may help to locate the solvent extraction process in its rightful position in the saline water conversion field. 

Giergielewicz-Mozajska, H., L. Dqbrowski, and J. Namiesnik. 2001. Accelerated solvent extraction (ASE) in the analysis of environmental solid samples—some aspects of theory and practice. Crit. Rev. Anal. Chem. 31 149-165. 

This study is by no means comprehensive and covers only a narrow range of variables. However, it does demonstrate the influence of entrainer in the improvement of separation over a single supercritical solvent. The increase in selectivity (1.4 to 1.8) for butene/butadiene mixtures is compared with the value of 1.63 obtained with liquid ammonia for the same binary system(l). Moreover, it has been demonstrated that a mixture of a pure solvent and an entrainer permits an improvement in the separation at temperatures and pressures lower than would have been otherwise predicted with a single gas solvent(20). For mixtures containing a highly polar component, such as ammonia, molecular size alone cannot account for the large selectivities observed in these experiments. At present, all theories are inadequate in explaining the chemical interactions between the entrainer and the mixture. The state of the art is comparable to liquid phase solvent extraction. 

Later Bjerrum s theory was supported by the work of Kraus [138], who showed importance of the dielectric constant, and Atherton [139], who demonstrated the existence of ion pairs using electron spin resonance spectroscopy. The formation of ion pairs may be studied by various methods conductance studies, UV-visible spectrometry, IR spectrophotometry, partition, distribution, or solvent extraction. The lifetime of ion pairs was determined to be at least 10 sec, which is equivalent to about 10 molecular vibrations, demonstrating that ion pairs can be considered as independent species [140]. Today, the ion-pair formation as independent species is widely accepted. 

Extraction chromatography (reversed phase partition chromatography) has been used in analytical and biochemistry to effect chemical separations. It is a method which combines the simplicity of ion exchange and the selectivity of solvent extraction. Ion exchange theory may be used to calculate the number of theoretical plates in the column and the enrichment coefficient. Extraction chromatography as a separation method has been recently reviewed by Cerrai (J) and Katykhin (7). 

The theory of membrane-based solvent extraction suggests that overall mass transfer of treated solute consists of several steps diffusion of the solute through the aqueous layer from the bulk source aqueous solution to the phases interface (nonequibbrium process), interaction of the solute with extractant and formation of the solute-extractant complex (as a rule, the... 

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