Cation exchanger 18-crown

Inspired by the separation ability of cyclic selectors such as cyclodextrins and crown ethers, Malouk s group studied the synthesis of chiral cyclophanes and their intercalation by cation exchange into a lamellar solid acid, a-zirconium phosphate aiming at the preparation of separation media based on solid inorganic-organic conjugates for simple single-plate batch enantioseparations [77-80]. 

Another material based on the crown ether extractant 4,4 (5 )-bis(t-butyl-cyclohexano)-18 crown-6, marketed under the name Sr-Spec, is useful for separations involving divalent cations including Pb, Ba, and Ra (Horwitz et al. 1991). For Ra analysis by TIMS, Ra-Ba separations are required because the presence of Ba drastically decreases the ionization efficiency of fg Ra samples from 10% to <1%. This material has been widely used for separations of Ra from Ba (e.g., Chabaux et al. 1994 Lundstrom et al. 1998 Rihs et al. 2000 Joannon and Pin 2001 Pietruszka et al. 2002) and is a complement or alternative to cation exchange separations for EDTA complexes of these elements (Volpe et al. 1991 Cohen and O Nions 1991). Sr-Spec material would also be useful for °Pb analysis, since Pb has a greater distribution coefficient than Sr with this extractant. 

Kwon, S.-M., Lee, K.-P., Tanaka, K., and Ohta, K., Simultaneous determination of anions and cations by ion-exclusion chromatography-cation-exchange chromatography with tartaric acid/18-crown-6 as eluent, /. Chromatogr. A, 850, 79, 1999. 

Two new chromogenic crown ether derivatives based on an identical design principle have been synthesized. The lipophilic KBC-002 shown in Figure 23 is a useful new chromo- ionophor for the highly selective calcium determination with cation exchange type optode53. 

Similarly, 39K nmr line-shape analysis has been applied to the study of the interaction of K+ with 18-crown-6 in a range of solvent mixtures (Schmidt Popov, 1983). In most instances, the cation exchange proceeds via a normal dissociative process (as found in water) however, in 1,3-dioxolane, metal exchange switches to a bimolecular exchange mechanism. 

De Jong et al. (1976c, 1977) have studied the rate of decomplexation of t-BuNHjPFj (AM) complexes of crown ethers. The exchange broadening of the t-butyl nmr signal was used to determine the rate of cation exchange between a kinetically stable (X.AM) and a kinetically unstable (Y.AM)... 

For crown ethers in which the H-nmr probes Ha and Hb are diastereotopic in the free crown ether too, the cation exchange process (50) becomes more complicated. For 1 1 complexes the measured free energy of activation also involves conformational inversion components (AG = AG + AG ), whereas exchange between free and complexed crown ether (2 1 ratio of crown ether to salt) only involves the free energy of activation for decomplexation (AG ). Bradshaw et al. (1979b) have recently reported AG -values for cation exchange in RNH3X complexes of crown ethers [257]. The AGf term was estimated to be 0.7, 0.5, and 1.2 kcal mol-1 for compounds [257 n = 1] with... 

Sutherland and co-workers have studied the cation-exchange processes in alkylammonium complexes of crown ethers [258]—[262], Depending on the position of the exchanging H-nmr sites, the measured zlG -values may refer to three different types of cation exchange (0 cation exchange between free and... 

Several papers have reported use of a weak acid/crown ether eluent in ion-exclusion/cation exchange chromatography with conductimetric detection on a weakly acidic cation-exchange column to effect the simultaneous determination of both cations and anions. Resolution was significantly improved when 18-crown-6 was present in the eluent. Detector response was positive for anions and negative for cations. [76-80] A sample chromatogram is shown in Figure 9. 

Ding, M., Tanaka, K., Hu, W., Haddad, P. R., and Miyanaga, A. (2001) Simultaneous Determination of Anions and Cations in Environmental Samples by Using a Weakly Acidic Cation-exchange Column with 2, 6-pyridinedicarboxylic Acid, Sulfosalicylic acid, and 18-crown-6 as Eluent, J. Liq. Chromatogr. Rel. Techn. 24, 3105-3117. 

One of the more interesting possibilities may be the combination of organic-phase cation exchangers with the size-specific crown ethers to produce size-specific or selective synergistic mixtures. 

In addition to strong-acid or weak-acid cation exchangers, crosslinked polymers carrying cyclic polyethers as anchor groups have been used for the separation of alkali and alkaline-earth metals, respectively. The structure of these stationary phases that are also based on silica was described in Section 3.3.1.4. A reasonable separation of alkali metals was obtained by Kimura et al. [38] on silica modified with poly(benzo-15-crown-5) using... 

In contrast to conventional cation exchangers, a reversed elution order is observed with crown ether phases, which is mainly determined by the size ratio between crown ether ring and alkali metal ion. Due to the high affinity of poly(benzo-15-crown-5) toward potassium and rubidium ions, these are more strongly retained than lithium, sodium, and cesium ions, respectively. However, the complexing properties of crown ethers also depend on the counter ion being employed. Thus, in potassium salts, for example, an increase in retention in the order KC1 < KBr < KI is observed with an increasing size of the counter ion. 

Table 14. e-values of sodium isotopes in the system cation exchanger resin/solution of 1 0 M HNO3,0.025 M crown ether, 80%CH3OH (v/v) ... 

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