Alkaline earth cations, separation

Subsequent to any decomposition, but also in the case of liquid samples such as water and urine, the analytes of interest are generally present in dilute solution together with a large excess of foreign ions (e.g.. alkali-metal and alkaline-earth cations). Separation and concentration of the analytes may be necessary to improve the limit of detection and exclude interference. Useful techniques in this regard include liquid-liquid extraction. solid-phase extraction, special precipitation reactions, and electrolytic deposition. 

Figure 4.20 Separation of comon anions and alkaline earth cations by ion chronatography using conductivity detection.

Nair, L. M., Saari-Nordhaus, R., and Anderson, Jr., J. M., Simultaneous separation of alkali and alkaline-earth cations on polybudaiene-maleic acid-coated stationary phase by mineral acid eluents, /. Chromatogr., 640, 41,1993. 

The selective cation binding properties ol crown ethers and cryptands have obvious commercial applications in the separation of metal ions and these have recently been reviewed (B-78MI52103.79MI52102, B-81MI52103). Many liquid-liquid extraction systems have been developed for alkali and alkaline earth metal separations. Since the hardness of the counterion is inversely proportional to the extraction coefficient, large, soft anions, such as picrate, are usually used. 

Organic ligands that bind to alkali metal and alkaline earth cations are known as ionophores, literally ion carrying compounds. Valinomycin, a cyclic natural antibiotic consisting of alternating d- and L-valine separated by D-hydroxyisovalerate and L-lactate groups shown in Fig. 5.2, binds K+ one thousand times better than... 

The immobilization of metal complex catalysts on polymers and inorganic oxides has received considerable attention as a means of combining the best advantages of homogeneous and hetereo-geneous catalysis (1-6). The swelling layer lattice silicates known as smectite clay minerals have added an important new dimension to metal complex Immobilization. These compounds have mica-type structures in which two-dimensional silicate sheets are separated by monolayers of alkali metal or alkaline earth cations (7). The structure of a typical smectite, hectorite, is illustrated in Figure 1. 

The eluent amines listed in Table 7.2 form 1-h cations with the exception of the ami-nopyridines which can form 2+ cations. A t5rpical separation of alkali metal cations employs approximately -0.1 mM protonated phenylethylamine or 4-methylbenzyla-mine. A higher concentration (approximately -10 mM) is used for separation of the divalent magnesium and alkaline earth cations. 

The chromatogram from the analysis of a mixture of alkali and alkaline earth cations at levels of a few parts per million is shown in figure 7.14. The cations lithium, sodium, ammonium, potassium, magnesium and calcium were present in the original mixture at concentrations of 1,4, 10, 10, 5 and 10 ppm respectively. The separation obtained is shown in figure 7.14. A proprietary ion exchange column, IonPacCS12, was used and the mobile phase consisted of a 20 nM methanesulfonic acid solution in water. A flow rate of 1 ml/min was employed and the sample volume was 25 pi. 

Separation of cations can be influenced by their interaction with crown ethers, which depends on the sizes of the cation and the crown ether cavity. The concentration of a crown ether in the running buffer also plays a role. The best results have been obtained with 18-crown-6-ether where the selectivity changes were largest. The use of crown ethers makes it possible to separate, e.g., potassium from ammonium. Electrolyte containing 4mmoll 18-crown-6, 4 mmol 1 copper sulfate, and 4 mmol 1 formic acid was successfully applied to complete separation of all alkali and alkaline earth cations including ammonium (Figure 2). 

Figure 1 illustrates the dilute solution behavior of a series of HTP in toluene at 25°C. Compared to the relative viscosity (Hrel) curve of the non-neutralized PBD (Hycar CTB), that of the neutralized polymer increases more or less abruptly with concentration. Ultimately a gelation phenomenon occurs, which is the obvious consequence of the intermolecular interactions of the ion pairs that the metal carboxylate end-groups form in toluene at 25°C. In the alkaline-earth cations series (Ba, Ca, Mg), the sharp increase of the relative viscosity appears at decreasing concentrations as the cation size decreases (Figure 1). As already mentioned, the smallest alkaline earth cation (Be) exhibits systematically a phase separation at high dilution which prevents significant measurements in that range with a capillary viscometer. These results can be explained by the equation (eq. 2) relating the attractive force between anion and cation to their charge (e and eQ, respectively), the square of their distance (r) and the dielectric constant (c) of the medium, respectively.

Immobilization of polyaza as well as mixed-donor (O, N) and (O, S) macrocycles to a silica gel matrix was described. These phases were used to investigate their selectivity toward preconcentration and separation of transition metal ions. A 17-membered 02N3-donor macrocycle exhibited a remarkable selectivity toward Cu(II) retention in presence of Ni(II), Zn(II), and Cd(II). The polyaza-macrocycles l,4,7,10-tetraaza-18-crown-6, pentaaza-15-crown-5, and hexaaza-18-crown-6 were synthesized with macrocycles covalently bonded to silica gel via one of the nitrogen donor atoms. These bound macrocycles were found to have strong and selective interaction with soft heavy metal ions. Separation of ppb levels of these cations from concentrated mixtures of other cations such as alkali and alkaline earth cations were accomplished (Scheme 13). ... 

The similarity of the log K values for the bound macrocycles to those involving the unbound macrocycles suggests that prediction of metal separations using silica gel-bound macrocyclic ligands should be possible. Thus, one has a powerful means to predict separations using available data compilations [5]. Such separations have been studied. For example, 18-crown-6 gel material 2 was used to separate 0.001 M concentrations of the alkaline earth cations [14]. The log A values for the association of unbound 18-crown-6 with Sr and Ba in water are 2.72 and 3.87, respectively [5]. The log K value for 18-crown-6-Ca interaction should be between 0.9 and 1.9 as described in Section 3. Magnesium ions do not complex with 18-crown-6. All the heavier alkaline earth cations were separated from Mg " by their being retained on the gel 2 column. Sr and Ba were selectively retained on the column over Ca by factors of 54 and 339, respectively, and Ba was retained... 

From the available literature on SMB processes, the 7r-complexation sorbents have not been nsed. 5A zeolite is used for the separation of n-paraffins from branched and cyclo-paraffins, and the separation is accomplished by molecnlar size exclnsion (of the branched and cyclo-paraffins). All other separations rely on alkaline-earth forms of zeolites. The interactions of the tt-electrons of the aromatic or olefinic componnds with the alkaline earth cations are much weaker than those with the -block metal cations such as Cu+ and Ag+. As a result, the separation factors on the jr-complexation sorbents are significantly higher. 

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