Phase extraction mechanism

Table 3.41 Solid-phase extraction mechanisms and phases...

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. 

A soil sample (10 g) was extracted by mechanically shaking with methanol-deionized water. This mixture was filtered and a portion was removed for partitioning into toluene-hexane. A phenyl solid-phase extraction (SPE) cartridge was employed... 

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-... 

Solid phase extraction (SPE) involves the separation of components of samples in solution through their selective interaction with and retention by a solid, particulate sorbent. SPE depends on differences in the affinities of the various components of the sample for the sorbent. The mechanisms of the interactions are virtually identical to the sorption processes that form the basis of liquid chromatographic separations (p. 80). The choice of solvent, the pH and ionic strength of aqueous solutions, and the chemical nature of the sorbent surface, especially its polarity, are all of importance in controlling the selectivity and efficiency of an extraction. 

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... 

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. 

The previous chapters have demonstrated that liquid-liquid extraction is a mass transfer unit operation involving two liquid phases, the raffinate and the extract phase, which have very small mutual solubihty. Let us assume that the raffinate phase is wastewater from a coke plant polluted with phenol. To separate the phenol from the water, there must be close contact with the extract phase, toluene in this case. Water and toluene are not mutually soluble, but toluene is a better solvent for phenol and can extract it from water. Thus, toluene and phenol together are the extract phase. If the solvent reacts with the extracted substance during the extraction, the whole process is called reactive extraction. The reaction is usually used to alter the properties of inorganic cations and anions so they can be extracted from an aqueous solution into the nonpolar organic phase. The mechanisms for these reactions involve ion pah-formation, solvation of an ionic compound, or formation of covalent metal-extractant complexes (see Chapters 3 and 4). Often formation of these new species is a slow process and, in many cases, it is not possible to use columns for this type of extraction mixer-settlers are used instead (Chapter 8). 

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. 

In addition, analysts can create a multimodal SPE column by stacking several discs with differing bonded phase in the disc holder. With the proper chemical design, such multimodal systems can provide customized separation protocols. The stacking order of the discs depends on the particular physicochemical characteristics of the analysis under way. The general rule is to place the disc with the more selective extraction mechanism on top of the less selective adsorbent. 

During recent years, however, the distinction between these two types of methods has become less clear because of the improved methods of sample cleanup that allow selective isolation of groups of compounds. Mixed-mechanism solid-phase extraction procedures and multi-immunoaffinity techniques are clear examples of liquid chromatographic developments that have contributed greatly to the current state of the art within residue analysis. 

Chapuis F et al (2004) Retention mechanism of analytes in the solid-phase extraction process using molecularly imprinted polymers - application to the extraction of triazines from complex matrices. J Chromatogr B Anal Technol Biomed Life Sci 804(1) 93-101... 

In the Starks extraction mechanism, the phase-transfer catalyst moves back and forth across the organic and aqueous phases. The onium salt equilibrates with the inorganic base in the aqueous phase, and extracts hydroxide into the organic phase. 

With regards to the mechanism of the generation of onium anion, the Starks extraction mechanism and interfacial mechanism (Brandstrom-Montanari modification) are suggested (Scheme 1.7). As in the above-described case, the interfacial mechanism seems to be operative in the asymmetric phase-transfer catalysis. 

Elucidation of the phenomenon of TPF by studies of extraction mechanism and structure of metal complexes in the liquid phase and the mesoscopic features of the third-phases formed. Finally, the method of eliminating TPF is to be pursued. 

Several spectroscopic techniques, namely, Ultraviolet-Visible Spectroscopy (UV-Vis), Infrared (IR), Nuclear Magnetic Resonance (NMR), etc., have been used for understanding the mechanism of solvent-extraction processes and identification of extracted species. Berthon et al. reviewed the use of NMR techniques in solvent-extraction studies for monoamides, malonamides, picolinamides, and TBP (116, 117). NMR spectroscopy was used as a tool to identify the structural parameters that control selectivity and efficiency of extraction of metal ions. 13C NMR relaxation-time data were used to determine the distances between the carbon atoms of the monoamide ligands and the actinides centers. The II, 2H, and 13C NMR spectra analysis of the solvent organic phases indicated malonamide dimer formation at low concentrations. However, at higher ligand concentrations, micelle formation was observed. NMR studies were also used to understand nitric acid extraction mechanisms. Before obtaining conformational information from 13C relaxation times, the stoichiometries of the... 

The extraction of a given metallic cation Mn+ into an organic solvent proceeds either through its coextraction with some of the anions initially present in the aqueous feed (two different mechanisms (1) and (2) are distinguished) or through its exchange with proton(s) from the organic solvent to conserve phase neutrality (mechanism (3)) ... 

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