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Extraction mechanisms

Solvent selection depends largely on the nature of the analytes and the matrix. Although the discussions in Chapter 2 can be used as a guideline to account for the solvent-analyte interactions, the matrix effects are often unpredictable. There is no single solvent that works universally for all analytes and all matrices. Sometimes, a mixture of water-miscible solvents (such as acetone) with nonmiscible ones (such as hexane or methylene chloride) are used. The water-miscible solvents can penetrate the layer of moisture on the surface of the solid particles, facilitating the extraction of hydrophilic organics. The hydrophobic solvents then extract organic compounds of like polarity. For instance, hexane is efficient in the extraction of nonpolar analytes, and methylene chloride extracts the polar ones. [Pg.141]


Retention, too, is highly tissue-specific. Sometimes, the extraction mechanism is also the retention mechanism, as for Tc-sestamibi, which is retained in mitochondria as long as transmembrane potentials remain intact. Others are separate. F-2-Fluorodeoxyglucose enters the cell by the same pathway as glucose, but is trapped because it is not a substrate for hexokinase, preventing further intracellular metabohsm. [Pg.473]

Stratifying water systems for selective extraction of thiocyanate complexes of platinum metals have been proposed. The extraction degree of mthenium(III) by ethyl and isopropyl alcohols, acetone, polyethylene glycol in optimum conditions amounts to 95-100%. By the help of electronic methods, IR-spectroscopy, equilibrium shift the extractive mechanism has been proposed and stmctures of extractable compounds, which contain single anddouble-chai-ged acidocomplexes [Rh(SCN)J-, [Ru(SCN)J, [Ru(SCN)J -have been determined. Constants of extraction for associates investigated have been calculated. [Pg.257]

In shear layers, large-scale eddies extract mechanical energy from the mean flow. This energy is continuously transferred to smaller and smaller eddies. Such energy transfer continues until energy is dissipated into heat by viscous effects in the smallest eddies of the spectrum. [Pg.48]

Figure 14-18A. Condensing, extraction mechanical drive steam turbine. (Used by permission Lamberson, J. and Moll, R. Technology Report ST 18.6. Dresser-Rand Company. Figure 14-18A. Condensing, extraction mechanical drive steam turbine. (Used by permission Lamberson, J. and Moll, R. Technology Report ST 18.6. Dresser-Rand Company.
The low TTA dependence at 35.0°C probably is attributable to dissolution of TTA in the aqueous phase. Observation of fourth-power dependence on acidity argues against any change in the extraction mechanism (e.g., Pu(IV) reduction or NO3 involvement). An aqueous Pu(TTA)3+ complex has been reported (14, 15) and this possibility has been considered in the error analysis of the Pu(IV)-sulfate stability constants. [Pg.259]

Table 3.41 Solid-phase extraction mechanisms and phases... Table 3.41 Solid-phase extraction mechanisms and phases...
Soxhlet extraction is well established, and generally exhaustively extracts all additives. The selection of extraction solvent can make large differences to the extraction time. The generally long extraction times followed by concentration steps may determine losses of volatile or thermally labile components. Because this form of extraction is one of the oldest and still widely used in industry, it is the standard to which many of the newer extraction technologies (which are likely to determine future applications) are referred. However, it should be realised that extraction mechanisms may be different, and thus comparisons are sometimes irrelevant. [Pg.134]

Crystals of [Tc(tu)6]Cl3 or [TcCl(tu)5]Cl2 are often employed for the synthesis of technetium(III) complexes. However, since the direct reduction of pertechnetate with excess thiourea in a hydrochloric acid solution yields [Tc(tu)6]3+ in high yield [37], direct use of the aqueous solution of the thiourea complex would be preferable for the synthesis of the technetium(III) complex without isolation of the crystals of the thiourea complex. In fact, technetium could be extracted from the aqueous solution of the Tc-thiourea complex with acetylacetone-benzene solution in two steps [38]. More than 95% extraction of technetium was attained using the following procedure [39] First a pertechnetate solution was added to a 0.5 M thiourea solution in 1 M hydrochloric acid. The solution turned red-orange as the Tc(III)-thiourea complex formed. Next, a benzene solution containing a suitable concentration of acetylacetone was added. After the mixture was shaken for a sufficient time (preliminary extraction), the pH of the aqueous phase was adjusted to 4.3 and the aqueous solution was shaken with a freshly prepared acetylacetonebenzene solution (main extraction). The extraction behavior of the technetium complex is shown in Fig. 6. The chemical species extracted into the organic phase seemed to differ from tris(acetylacetonato)technetium(III). Kinetic analysis of the two step extraction mechanism showed that the formation of 4,6-dimethylpyrimidine-... [Pg.268]

Itagaki, M. Fukushima, H. Inoue, H. Watanabe, K. Electrochemical impedance spectroscopy study on the solvent extraction mechanism of Ni(II) at the water 1,2-dichloroethane interface. J. Electroanal. Chem. 2001, 504, 96-103. [Pg.803]

The mechanism of phase transfer catalysis is still a matter of discussion and remains a subject of some controversy. However, it will be roughly classified into two the extraction mechanism proposed by Starks121 and the interfadal mechanism by Makosza1111... [Pg.124]

The thermodynamics of the extraction mechanism is extremely complex. In the initial equilibration of the ion pairs (Scheme 1.6) account has to be taken not only of the relative stabilities of the ion-pairs but also of the relative hydration of the anionic species. Assuming the complete non-solvation of the ion-pairs, the formation of the ion-pair [Q+Y] will generally be favoured when the relative hydration of X- is greater than that of Y. However, in many cases, the anion of the ion-pair is hydrated [8-11] (Table 1.1) and this has a significant effect both on equilibrium between the ion-pairs in the aqueous phase and the relative values of the partition coefficients of the two ion-pairs [Q+X ] and [Q+Y ] between the two phases. [Pg.9]

The extraction mechanism accommodates a large number of anionic reactions and provides a rationale for general absence of catalytic activity by hydrophilic ammonium salts, e.g. tetramethylammonium halides [24, 25], which have negligible solubility in organic solvents. Similarly, owing to the highly hydrophilic nature of the hydroxide anion, the mechanism is also untenable as a rationalization for the majority of base-catalysed reactions. [Pg.11]

The interfacial mechanism probably competes to some extent with the extraction mechanism, particularly with the less lipophilic catalysts. The dependence of the rate of many nucleophilic substitution reactions on the stirring rate up to 250-300 rpm and the independence of the reaction rate at higher stirring rates has been taken as evidence for a change over from a predominant interfacial mechanism to an extraction process. The interfacial mechanism is also particularly relevant to base-initiated reactions. [Pg.12]

For each experiment, we measured the variation of log D = f(log) [HA]). Independent of the conditions, the distribution coefficient D was always found to have a third-power dependence on TTA concentration. The results are consistent with an extraction mechanism based on the reaction ... [Pg.14]

Many extractants reach a constant interfacial concentration at bulk organic concentrations far below the practical concentrations that are generally used to perform extraction kinetic studies. This means that when writing a rate law for an extraction mechanism that is based on interfacial chemical reactions, the interfacial concentrations can often be incorporated into the apparent rate constants. This leads to simplifications in the rate laws and to ambiguities in their interpretation, which are discussed in later sections. [Pg.225]

Comparison of Eqs. (5.34), (5.47), and (5.56) shows that if [BH] is sufficiently small to allow introduction of the approximation k [BH] k- into the denominator of Eq. (5.56) (i.e., for a low and restricted concentration range of the extractant), the three rate equations have exactly the same dependence on [M ], [BH], and [H ], although the extraction mechanisms are characterized by different rate-determining steps ... [Pg.238]

A tertiary amine (Alamine 336) is used as a selective extractant of Cr(VI). The extraction mechanism involves ion pairing between the amine and the dichromate anion, HCr04. After extraction, the Cr(VI)-loaded extractant is stripped with alkali to produce a strip liquor, containing 2% Cr(VI). This solution can be recycled. [Pg.627]

Other aspects such as the effects of Hgand concentration and free hgand addition to aqueous phase [162,215], the effect of pH and/or ionic strength [162, 215,216], and the extraction mechanism and the effect of the tail length of affinity cosuxfactant [219,220] are also of interest. [Pg.162]

In this paper an overview of the developments in liquid membrane extraction of cephalosporin antibiotics has been presented. The principle of reactive extraction via the so-called liquid-liquid ion exchange extraction mechanism can be exploited to develop liquid membrane processes for extraction of cephalosporin antibiotics. The mathematical models that have been used to simulate experimental data have been discussed. Emulsion liquid membrane and supported liquid membrane could provide high extraction flux for cephalosporins, but stability problems need to be fully resolved for process application. Non-dispersive extraction in hollow fib er membrane is likely to offer an attractive alternative in this respect. The applicability of the liquid membrane process has been discussed from process engineering and design considerations. [Pg.209]

Naturally, the consideration above concerns only that particular set of ILs. In general, the potential of variations in lUs nature is so high that one may expect much greater variations in extraction efficiency and selectivity, as well as in extraction mechanisms, particularly if novel classes of ILs are synthesized and used. [Pg.253]


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