Anionic extractants

Most hydrated metal ions are more soluble in water than in ILs. The distribution ratios of some metal ions between aqueous and IL phases may be enhanced in the presence of coordinating anions, such as halides or pseudohalides, capable of modifying the metal complex hydrophobicity, increasing partitioning from water [41]. 

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Anionic extractants are commonly based on high molecular weight amines. Metal anions such as MnO or ReO can be exchanged selectively with inorganic anions such as Cl or The equiHbrium for a quaternary onium compound of organic radicals R for two anion species A and B ... 

Table 1. Anion Extraction Constants Eq, of Tetra-n-Butylammonium Ion Pairs Between Water and Chloroform...

Anionic complexes, tungsten, 25 386-387 Anionic copolymerization, 7 624-626 block copolymers, 7 645 Anionic emulsifiers, in VDC emulsion polymerization, 25 722 Anionic extractants, 10 750 Anionic flotation, 24 497 Anionic gels, 9 59-60... 

Of the two extracting systems, the use of an anionic extractant is more common to minimize coextraction of inorganic cations by the organic acids. 

Extraction with 8-quinolinol derivatives, which can act as either a chelating acid or, after protonation, as an anion extractant. It was observed [35] that palladium extraction was favored at low acidity, but platinum was extracted best at high acidity (Fig. 11.16). At about 2 mol dm acid, the two metals were extracted equally. This suggested the following extraction equations ... 

Practically motivated, the aim was to develop methods for recovery and determination of amino acids in the context of analytical chemistry and biotechnology. Amino acids are hydrophilic compounds, which therefore are difficult targets for conventional solvent extraction. Extraction to an organic solvent may be enhanced by the addition of lipophilic cationic or anionic extractants, forming extractable complexes with amino acids, or by the use of macrocyclic compounds, which form stable hydrophobic host-guest complexes. The most popular reagents from the latter group are crown... 

Chelating extractants owe effectiveness to the attraction of adjacent groups on the molecule for the metal. Anionic extractants are commonly based tin high molecular weight amines. Solvating extractants contain one or more electron donor atoms, usually oxygen, which can supplant or partially supplant the water W hich is altached to the metal ions. 

Simple lipophilic cations, like ammonium ions bearing long hydrocarbon chains, allow anion extraction into an organic phase and render liquid membranes permeable to anions by an anion exchange (antiport) process. Such carriers effect, for instance, selective transport of amino acid carboxylates [6.3] against inorganic anions like chloride. 

It is interesting that, in general, the same order of anion extraction is observed as that found with anion-exchange resins, so that previous studies with resins can serve as a useful guide for extractions. 

The currently applied extraction systems are mainly based on classical extractants, as for example cation exchanging acids or chelating agents, solvating ketones, ethers or esters for metal ion extraction and amines or quaternary ammonium salts for anion extraction [16, 18, 19]. 

K. Gloe, B. Antonioli, K. Gloe, L.F. Lindoy, Anion extraction, in [4], online... 

As stated above, anion extraction of the most common type (water to organic layer) is executed by quaternary ammonium or other onium cations or by crown-ether-masked metal cations. To be useful, at least two factors have to be met ... 

Another inorganic anionic extractant, potassium phosphotungstate (K,oP2W 706,), was used in the 18.8% PEG-2000/17.2% (NH4)2SOi (total system composition) ABS for the extraction of f-elements [20,62]. It was observed that anions of unsaturated heteropolytungstates pass into the PEG-rich phase. In PEG-2000/(NH4)2S04 systems without a complex-... 

The anion-specific hydration also plays a fundamental role in determining the selectivity coefficient values. In fact, the increase in concentration of the aqueous NaOH from 15 to 50% always produces a change in anion extractability into the organic phase. Reactions of anions with. fsei(x/ci) > 1 re those most favored by 50% aqueous NaOH due to a higher partition of the reacting in the organic phase, combined with enhanced anion activation derived from its dehydration. On the other hand, the reduced extractability of anions with Xsei(x/ci) < 1 is largely overcome by the dramatic increase in reactivity... 

A comparison between literature data about performances of different catalysts in a particular reaction is not always an easy task to achieve. In fact when the reaction is not run under the same reaction conditions correct comparisons are difficult since various factors are usually simultaneously involved, often counteracting each other. Therefore, evaluation of the important requirements for a PTC process such as catalyst lipophilicity, selective anion extraction, cation-anion separation within the ion pair, and poor anion solvation may be not sufficient for useful predictions. Other factors such as the structure of the substrate or anion hydration, interaction with solvent, or ionic strength of the aqueous phase may assume great importance. 

Multi-crown dendrimers (Fig. 3.7-12) were found to exhibit good solubihty in solvents of low polarity, making them attractive as extractants for separation processes. The combination of protonated tertiary amine groups as anion binding sites together with cation-active crown ether moieties represents an approach for the simultaneous binding of cations and anions. Extraction studies performed with sodium pertechnetate and mercury(II) chloride have shown that the guest molecules are mainly bound in the interior of the protonated polyamine skeleton [18]. 

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