Alkali metal ions selective separation

Table 7. Selectivity orders for transport of alkali metal ions into toluene by crown ether carboxylic acids for several separation techniques...

Macrocycllc compounds (some crown ethers and cryptands) are selective reagents for extractive separation of alkali metals [22-27]. These ligands form cationic complexes with alkali metal ions, and these can be extracted as ion-pairs with suitable counter-ions e.g., picrate) [28], most often into chloroform. For potassium, p-nitrophenoxide was used as counter-ion [29]. In cases, where a coloured anionic complex is a counter-ion [30], the extract may serve as a basis for determining the alkali metal. The effect of the structure of the dibenzo-crown ether rings upon the selectivity and effectiveness of isolation of alkali metals has been studied in detail [31]. Chromogenic macrocyclic reagents applied for the isolation and separation of alkali metals have been discussed [32]. 

In this type of separation the analyte cations compete with the eluent cation for ion-exchange sites and move down the eolumn at different rates. The ionic eluent selected depends on the cations to be separated, the type of separation column and on the detector. In many cases an aqueous solution of a strong acid such as hydrochloric, sulfuric or methanesulfonic acid is a satisfactory eluent. Sample cations commonly separated include the following alkali metal ions (Li, Na+, K", Rb, Cs ), ammonium, magnesium, alkaline earths (Ca, Sr +, Ba ), and various organic amine and alkano-lamine cations. Most other metal cations are separated with a weakly complexing eluent. 

Cryptands are cyclic (mainly) polyether molecules with usually three chains linked at nitrogen caps at each end of the molecule (Figure 4.42), much like the sarcophagines but with a different capping atom and different donors. They can, depending on host cavity size, bind metal ions (alkali or alkaline earth ions preferred) or small molecules. A wide range of molecules of this family have been prepared. They can be effective in metal ion selection from a group of ions, useful in both analysis and separation of mixtures. They also help solubilize metal ions in aprotic solvents. 

The utility of Nafion far exceeds application as a membrane material. Yeager59 studied the selectivity of Nafion ion-exchange resins toward mono- and divalent cations. The equilibrium constants X(M+/H+) increase continually with hydrated radius, as was previously found for the polystyrene-sulfonate cation exchangers39,40. The differences between the K values enable a facile chromatographic separation of alkali-metal ions, as seen in Figure 9. 

Examination of theoretical and experimental results shows that principles of ion solvation provide an effective basis for the discrimination of the alkali metal ions by size. No host ligand is necessarily required to achieve high selectivity for Cs" ion in this regard. Rather, the environment offered by certain water-immiscible organic solvents alone can provide a strong size bias that can be exploited to devise effective separation schemes. Certain solvent properties in particular stand out as promoting extraction selectivity... 

Crown ethers have been immobilized within polymer supports and used in metal ion separation, Crown ether moieties at cross-linked polystyrene have been prepared by the reaction of cross-linked chloromethylated polystyrene with hydroxyl-substituted crown ethers [52], The resins 15 formed from the condensation of dibenzo—18-crown-6 and formaldehyde were modified with phosphonate monoethylester and phosphonic acid groups [53]. Resin 15 demonstrates selectivity for K(I) over the other alkali metal ions at pH < 8, and selectivity for Li(I) at pH > 8. A resin 16 prepared from the condensation of a substituted crown ether with formaldehyde is selective for Na(I) over the other alkali metal ions [54],... 

Later research has reafHrmed these early results [23]. A separation of alkali metal ions was first attempted in water alone using the lightly sulfonated macro-porous cation exchanger with aqueous 3 mM methanesulfonic acid as the eluent. Under these conditions the sample cations exhibited very similar retention times. The selectivity of the macroporous resin for alkali metal ions was improved con-... 

The use of 13a in the extraction process or in the transport through supported liquid membranes (SLMs) allows to recover more than 98% of the cesium cation present in solution, making this derivative extremely attractive for declassification of nuclear wastes. Ligand 13a was dso used for the selective detection of cesium in ISEs and ISFETs with very high selectivity and low detection limit. Very recently, we anchored calix[4]arene-crown-5 and -crown-6 derivatives on silica-gel via hydrosilanization and we were able to separate by chromatography potassium or cesium fi"om other alkali metal ions with high efficiency. ... 

Breadmore, M. C., Macka, M., and Haddad, P. R., Manipulation of separation selectivity for alkali metals and ammonium in ion-exchange capillary electrochromatography using a suspension of cation exchange particles in the electrolyte as a pseudo stationary phase, Electrophoresis, 20, 1987, 1999. 

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