Ion-association extraction systems

FIGURE 2.27 Extraction rate profiles of the ion-association extraction of Fe(II) with (a) phen and its (b) dimethyl (DMP) and (c) diphenyl (DPP) derivatives into chloroform in the presence of 0.1 M NaClO. Effect of stirring (4700 rpm) indicates the interfacial adsorption of the complexes. 

Therefore, we have to take into account the adsorption of the extractable complex in the analysis of the extraction kinetic mechanism of these systems. The observed initial rate r showed the relation 

The rate determining step was the 1 1 complex formation in aqueous phase and interface. 

The adsorption of ligand accelerates the extraction (a positive catalytic effect) but the adsorption of the complex apparently suppresses the extraction rate (a negative catalytic effect). 

The effects of anion and solvent on the extraction rate can be explained through the change of the adsorption constant IQ. of Fel Xj. 

---

In the ion-association extraction systems, hydrophobic and interfacially adsorbable ions are encountered very often. Complexes of Fe(II), Cu(II), and Zn(II) with 1,10-phenanthro-line (phen) and its hydrophobic derivatives exhibited remarkable interfacial adsorptivity, although the ligands themselves can hardly adsorb at the interface, except for protonated species [19-21]. Solvent extraction photometry of Fe(II) with phen is widely used for the determination of trace amounts of Fe(II). The extraction rate profiles of Fe(II) with phen and its dimethyl (DMP) and diphenyl (DPP) derivatives into chloroform are shown in Fig.9. In the presence of 0.1 M NaC104, the interfacial adsorption of phen complex is most remarkable. The adsorption of the extractable complex must be considered in the analysis of the extraction kinetic mechanism of these systems. The observed initial rate r° shows the relation... 

Table 4.4 Typical ion-association extraction systems System...

H. Frieser, Relevance of Solubility Parameter in Ion Association Extraction Systems, Anal Chem. 41(10), 1354 (1969). 

H. Watarai and Y. Shibuya, Interfacial adsorption of iron(II)-4,7-diphenyl-l,10-phen-anthroline complex in ion-association extraction systems. Bull. Chem. Soc. Jpn., 62, 3446-3450 (1989). 

Coetzee, C. J., Preiser, H., Anion-responsive electrodes based on ion association extraction systems. Ana/... 

As with chelate systems, ion-association extraction equilibria involve a number of reactions. An example is the extraction of Fe " from HCl solutions into ether. 

Substoichiometric separation is performed by ordinary chemical separation methods such as solvent extraction, ion exchange, precipitation, and electrochemical methods. In recent years, however, the ion exchange and electrochemical methods have not been used very much in substoichiometric separation. The precipitation technique is often used due to its simplicity, while solvent extraction is most widely employed. This is because the procedure for solvent extraction is very simple and an appropriate extraction system can usually be selected from the great number of research papers dealing with solvent extraction of many different elements. Two extraction systems are commonly used chelate extraction of metal ions with chelating agents and ion-association extraction of metal ions with simple negative or positive ions. 

Table 42 Ion association-ion pair extraction systems. (Source G. H. Morrison and H. Freiser, Solvent Extraction in Analytical Chemistry, John Wiley, New York, 1957)...

The present review is devoted to the relation between bulk characteristics (extraction constants, extractant association in the non-aqueous medium) of ion-exchange extraction systems and properties of the monolayers adsorbed at the water-oil interface. 

In the case of inorganic solutes we are concerned largely with samples in aqueous solution so that it is necessary to produce substances, such as neutral metal chelates and ion-association complexes, which are capable of extraction into organic solvents. For organic solutes, however, the extraction system may sometimes involve two immiscible organic solvents rather than the aqueous-organic type of extraction. 

Dagnall and West8 have described the formation and extraction of a blue ternary complex, Ag(I)-l,10-phenanthroline-bromopyrogallol red (BPR), as the basis of a highly sensitive spectrophotometric procedure for the determination of traces of silver (Section 6.16). The reaction mechanism for the formation of the blue complex in aqueous solution was investigated by photometric and potentiometric methods and these studies led to the conclusion that the complex is an ion association system, (Ag(phen)2)2BPR2, i.e. involving a cationic chelate complex of a metal ion (Ag + ) associated with an anionic counter ion derived from the dyestuff (BPR). Ternary complexes have been reviewed by Babko.9... 

Spectrophotometric methods may often be applied directly to the solvent extract utilising the absorption of the extracted species in the ultraviolet or visible region. A typical example is the extraction and determination of nickel as dimethylglyoximate in chloroform by measuring the absorption of the complex at 366 nm. Direct measurement of absorbance may also be made with appropriate ion association complexes, e.g. the ferroin anionic detergent system, but improved results can sometimes be obtained by developing a chelate complex after extraction. An example is the extraction of uranyl nitrate from nitric acid into tributyl phosphate and the subsequent addition of dibenzoylmethane to the solvent to form a soluble coloured chelate. 

The choice of the chromatographic system depends on the chemical character of the extracts being separated. The mobile phase should accomplish all requirements for PLC determined by volatility and low viscosity, because nonvolatile components (e.g., ion association reagents and most buffers) should be avoided. It means that, for PLC of plant extracts, normal phase chromatography is much more preferable than reversed-phase systems. In the latter situation, mixtures such as methanol-ace-tonitrile-water are mostly used. If buffers and acids have to be added to either the... 

Whereas the use of conventional fast atom bombardment (FAB) in the analysis of polymer/additive extracts has been reported (see Section 6.2.4), the need for a glycerol (or other polar) matrix might render FAB-MS analysis of a dissolved polymer/additive system rather unattractive (high chemical background, high level of matrix-, solvent- and polymer-related ions, complicated spectra). Yet, in selected cases the method has proved quite successful. Lay and Miller [53] have developed an alternative method to the use of sample extraction, cleanup, followed by GC in the quantitative analysis of PVC/DEHP with plasticiser levels as typically found in consumer products (ca. 30 %). The method relied on addition of the internal standard didecylphthalate (DDP) to a THF solution of the PVC sample with FAB-MS quantitation based on the relative signal levels of the [MH]+ ions of DEHP and DDP obtained from full-scan spectra, and on the use of a calibration curve (intensity ratio m/z 391/447 vs. mg DEHP/mg DDP). No FAB-matrix was added. No ions associated with the bulk of the PVC polymer were observed. It was... 

These include the simplest ion-association systems in which bulky cations and anions are extracted as pairs or aggregates without further coordination by solvent molecules. An example of this type of system is the extraction of manganese or rhenium as permanganate or perrhenate into chloroform by association with the tetraphenylarsonium cation derived from a halide salt... 

Emulsion liquid membrane extraction of cephalosporins conform to the type II facilitated transport. Here the solute transport is either associated with a cotransport or counter-transport of an anionic species depending on whether ion-pair or ion-exchange extraction is exploited in the ELM system. 

ILs have been used to separate and determine the purity of anthraqui-nones. Rapid and sensitive determination of anthraquinones in Chinese herb using l-butyl-3-methylimidazolium-based IL with p-cyclodextrin (p-CD) as a modifier in CZE was provided by Qi et al. [49]. Successful separation and identification of four anthraquinones of Paedicalyx attopevensis Pierre ex Pitard extracts has been achieved. In the running electrolyte the anthraquinones may associate with the imidazolium ions or with the p-CDs. They may be entirely or partly embedded in the cavity of the p-CDs, so the association with the free imidazolium ions in the bulk solution was weak and those analytes, that were not embedded in the cavity of the p-CDs had rather stronger association with the imidazolium ions in the system. The mechanism of separation is illustrated in Figure 6.6. 

Electrochromism is the property of a material or a system to change its optical properties (color) reversibly if an external potential is applied. It is associated with ion insertion/extraction processes, which can be presented schematically by ... 

Extraction systems may be divided into two classes (1) uncharged covalent species (simple molecules and chelates), and (2) ion associates (ion pairs). 

Another group of ion-association systems is represented by solvated salts (usually nitrates, but also halides and sulphates). Solutions (1-50%) of tri-n-butyl phosphate (TBP, formula 1.11) in hexane, CCI4, and solutions (1-20 %) of tri-n-octylphosphine oxide (TOPO, formula 1.12) in cyclohexane are most often used as the extractants. Solvation with TBP or TOPO (through the strongly basic oxygen atom of the phosphoryl group) enables metal salts. 

---