Extraction liquid cation exchangers

In the examples considered above, use has been made of liquid anion-exchangers for extraction by ion-pair formation. Use can also be made of liquid cation-exchangers for extraction by ion-pair formation and the mechanism of such an extraction is represented below. 

The complex formation between Th(IV) and CF, Br and HF has been studied using liquid-liquid extraction nsing dinonyl naphtalene sulphonic acid (HDNNS), a liquid-cation exchanger solnble in organic solvents but not in water. The experiments have been made at 25°C and an ionic strength/ionic medium given in Table A-43. The... 

The second type of reagent which may be used for uranium(VI) extraction is potentially anionic and is used in a process which has been referred to as liquid cation exchange. Unlike the situation with amine reagents, where the extractant cation does not form an inner sphere complex with UOj, these reagents form complexes by binding directly to the metal. Thus they do not rely on the formation of suitable anionic species such as [UO2 (804)3] or [U02(N03)3] and are effective... 

ABC extractants were found to possess attractive properties with regard to salt extraction. Extraction is efficient and selective. Compared with liquid cation exchangers, ABC extractants are less sensitive to acidity in the aqueous phase and extract alkali and alkaline earth metals better. Both the cation and the anion are extracted therefore, no acid or base addition is required for pH adjustment or for stripping. Extraction is reversible and provides for back-extraction of the extracted salt by water. Several potential applications of ABC extractants in salt extraction were studied, including MgCb recovery from concentrated seawater [2,4,5,7], separation of LiCI [6], removal of Fe from AlCl solutions [14,23], and recovery of ZnSO from zinc electrowinning bleeds [21]. 

On metal salt extraction by ABC extractants the anion A of the water-immiscible acid present in the extractant (HA) binds cationic species, as in the case of liquid cation exchangers, but without releasing a proton to the aqueous solution. The amine cation B" or BH binds the anion of the salt as well as anionic complexes, if formed between the cation and the various anions (including those of the water-immiscible acid). On acid extraction, protons come instead of the metal salt cations. The mechanisms might be quite similar to that of extracting salts of cations not capable of forming... 

Alkylphosphoric acids are acidic extractants sometimes called liquid cation exchangers due to exchange of H+ ions for the extracted cations. They have been used for many metal ions. Examples include mono-(2-ethylhexyl)phosphoric acid (MEHP), dibutylphosphoric acid (DBP), and di-(2-ethylhexyl)phosphoric acid (DEHP). 

Of the acidic extractants applied to lanthanide/actinide separations, the most common are chelating extractants like j8-diketones, pyrazolones and (to a lesser extent) oxines, and the acidic organophosphorus extractants. Some work has been done evaluating the extraction of the subject metal ions by sulfonic acids, but this class of liquid cation exchangers exhibits little selectivity, and they have not proven particularly useful for lanthanide/actinide separation (Khopkar and Narayanankutty 1968). 

Thomas A., Sigmund, G, Guddat. S., et al. (2008) Determination of selected stimulants in urine for sports drug analysis by soUd phase extraction via cation exchange and means of liquid chromatography-tandem mass spectrometry. European Journal of Mass Spectrometry, 14, 135-143. 

Liquid ion-exchangers have been discussed in the section on solvent extraction (p. 65). They can be used in column form by coating them on to a solid support such as cellulose powder or Kel-F (polytrifluorochloroethylene). Tris-n-octylamine (TNOA) and bis(2-ethylhexyl)phosphoric acid (HDEHP) behave as strong-base and strong-acid exchangers for anions and cations respectively. 

The ionic extractants are those chemicals that carry within an ion pair a labile cation or anion, which will exchange with the appropriate metal species in the aqueous phase. Their role may be regarded as a form of liquid ion exchange. Several commercially available amines belong to this class, are shown in Figure 17. 

After hydrolyzate acidification with hydrochloric acid at pH values lower than 1, quinoxaline-2-carboxylic acid is quantitatively extracted into ethyl acetate, chloroform, or dichloromethane, since at these strongly acidic conditions the ionization of their carboxylate moiety is suppressed (pK, 2.88), and then back-extracted into aqueous buffered solutions at pH 6.0 or higher. These liquid-liquid partitioning procedures isolate quinoxaline-2-carboxylic acid from a complex mixture of tissue hydrolysates, and provide an aqueous extract suitable for further purification by solid-phase extraction. This has been accomplished either with the strong cation-exchange resin AG MP-50 (419, 420) or with a polar silica column (422). 

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