Separation of Alkali Metal Ions

A separation of alkali-metal ions was first attempted in water alone using the lightly sulfonated macroporous cation exchanger with aqueous 3 mM methanesulfo-nic acid as the eluent. Under these conditions the sample cations exhibited very similar retention times. 

When the macroporous resin column was used with the same acidic eluent in 100 % methanol, the chromatographic separation was improved considerably. Now the alkali-metal ions are solvated with methanol and the resin matrix is probably coated with a thin layer of methanol, which makes the ions and the resin surface more compatible with one another. 

Although several factors may influence the selectivity of cation-exchange resins for 

The results of these studies [12] are summarized in Table 7.5. The use of non-aque-ous solvents with macroporous cation-exchange resin permit several separations that are very difficult with aqueous eluents. Methanol was found to be the most favorable solvent due to the best combination of resolution and peak shape. Acetonitrile and ethanol, although producing broader peaks, are useful for separating ions that usually elute close together, Li /Na and K /NH4 respectively. Elution order in acetonitrile 

Separation of Metal Ions with a Compiexing Eluent 7.5.1 

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T. Ikeshoji, Separation of alkali-metal ions by intercalation into a Prussian blue electrode. J. Electrochem. Soc. 133, 2108-2109 (1986). 

Kimura, K., Hayata, E., and Shono, T. (1984) Convenient, efficient crown ether-containing stationary phases for chromatographic separation of alkali metal ions dynamic coating of highly lipophilic crown ethers on octadecylsilanized silica, Chem. Commun., 271-272. 

Fig. 3-136. Separation of alkali metal ions on silica modified with poly(benzo-15-crown-5). - Eluent water flow rate 1 mL/min detection direct conductivity injection volume 1 pL solute concentrations 13.6 g/L LiBr, 14.4 g/L NaBr, 26.2 g/L KBr, 36.4 g/L RbBr, and 34 g/L CsBr (taken from [38]).

Figure 4. Chromatographic separation of alkali metal ions using 1200 EW Nafion...

FIGURE 9. Chromatographic separation of alkali metal ions using 1200 EW Nafion at 25 °C. Reprinted with permission from H. L. Yeager, in Perfluorinated Ionomer Membranes (Eds. A. Eisenberg and H. L. Yeager), Chap. 3,... 

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. 

Used for complexing and extraction-separation of alkali metal ions. Liq. Misc. H2O. 

Used as 1,2-dichloroethane soln. for extraction separation of alkali metal ions. Orange cryst. (CHCI3). Sol. CHCI3, 1,2-dichloroethane, dioxan. Mp 207.0-207.7°. 

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

Except for sensor applications, the intercalation of alkali metal ions in metal hexacyanoferrates was used for adsorption and separation of cesium ions from different aqueous solutions with Prussian blue [43,44] and cupric hexacyanoferrate [45,46],... 

The complexes of alkali metal ions and of their salts described in paras. II—V may also be considered as lattice compounds because they do not necessarily persist in solution. Where the charge on the cation is neutralised by a small anion to give a salt, the solid may contain ion pairs coordinated by the additional ligand molecules, or the ions may be separated by the ligands, which usually form hydrogen bonds to the anion. When the cation is neutralised by a polydentate anion, the co-... 

As the carbon black structure may be reduced by the presence of alkali metal ions in the reaction zone [4.11], alkali metal salts, preferably aqueous solutions of potassium hydroxide or potassium chloride, are often added to the make oil in the oil injector. Alternatively, the additives may be sprayed separately into the combustion chamber. In special cases, other additives, e.g., alkaline-earth metal compounds which increase the specific surface area are introduced in a similar manner. 

Chikui, S. Electrometric determination in chromatographic development. III. Determination of alkaline earth metals during ascending development on paper, and their separation from alkali metal ions. Jap, Analyst 20, 167 (1971) Anal. Abstr. 23, 93 (1972)... 

Fig. 3-130. Separation of alkali metals on a surface-sulfonated polyvinyl resin. - Separator column ION 200 eluent 0.002 mol/ L picolinic acid, pH 2.0 flow rate 2.6 mL/min detection direct conductivity.

Typically, monovalent cations such as alkali metals are separated using a dilute mineral acid as the eluent. Examples for the separation of alkali metals are displayed in Figs. 3-129, 3-130 (Section 3.4.1.1), and 3-132 (Section 3.4.1.2). These figures reveal that the retention of the alkali metals increases with increasing ionic radius. Compared to conventional instrumental analysis methods, the advantage of ion chromatography is the simultaneousness of the method. Without any doubt, the key ion in this chromatogram is ammonium which elutes between sodium and potassium, Its sensitive detection by other methods is very difficult. 

The parameter that determines retention in the separation of alkali metals is solely the concentration of the acid used as the eluent. As seen in Fig. 3-137, a linear relationship is obtained for the different solute ions when the logarithm of the capacity factors is plotted as a function of the ionic strength. 

An example of crown ether applied in selective separation of alkali metals is dibenzo-18-crown-6 (formula 1.15). Extractive separations of metal ions are also performed with macrocyclic ligands containing nitrogen or oxygen atoms, as well as macrocycles with combinations of oxygen, nitrogen, and sulphur atoms (N-0, S-0, N-S) [45,48]. A macrocyclic compound with only nitrogen hetero-atoms (formula 1.16) is selective for copper. 

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]. 

Modern ion chromatography was in a sense bom in 1975 when Small, Stevens and Bauman [3] devised a new system that made it possible to use a conductivity detector. In this paper, 0.01 to 0.02 M hydrochloric acid was used as the eluent in the separation of alkali metal cations. A stripper column (later called a suppressor column) below the separator column, containing an anion-exchange resin in the -OH form, was used to convert the H Cl" to water. 

Lactate has the same a-hydroxycarboxylate complexing group as tartrate and HIBA, but it is a smaller molecule and forms somewhat weaker complexes than tartrate with most metal ions. Shi and Fritz found that a lactate system gave excellent separations for divalent metal ions and for trivalent lanthanides. A brief optimization was first carried out to establish the best concentrations of lactate and UV probe ion and the best pH. Excellent separations were obtained for all thirteen lanthanides, alkali metal ions, magnesium and the alkaline earths, and several divalent transition metal ions. All of these except copper(II) eluted before the lanthanides. An excellent separation of 27 metal ions was obtained in a single run that required only 6 min (Fig. 10.13). 

An alternative approach which allows the separation of an excess of alkali metal ions from other cations uses a chelating ion-exchange resin. This type of resin forms chelates with the metal ions. The most common of these is Chelex-100 . This resin contains iminodiacetic acid functional groups which behave in a similar way to ethylenediaminetetraacetic acid (EDTA). It has been found that Chelex-100 , in acetate buffer at pH 5-6, can retain Al, Bi, Cd, Co, Cu, Fe, Ni, Pb, Mn, Mo, Sc, Sn, Th, U, V, W, Zn and Y, plus various rare-earth metals, while at the same time it does not retain alkali metals (e.g. Li, Na, Rb and Cs), alkali-earth metals (Be, Ca, Mg, Sr and Ba) and anions (F-, Cl-, Br- and I-). 

Relatively little is known about competitive solvation in mixtures of nonaqueous solvents. Complex formation between Na+ and THF in solutions of Na+[AlBu4] in hexane at molar ratios 1 1 (solvated contact ion pair) and 1 4 (solvent-separated ion pair) was reported by Schaschel and Day with proton NMR as well as IR and conductivity measurements (83, 84). Preferential solvation of alkali metal ions by DMSO in 1-pentanole and by acetone in nitromethane was observed by Popov et al. 69, 85). 

The rate of tetraethylammonium transfer has been measured in both directions at the water-DCE interface consistent values of ko ( 2 cm s ) were obtained in both cases, although the sum of the a values (Eqs. (30) and (31)) exceeded unity [107]. The facilitated transfer of alkali metal ions by the ligand dibenzo-18-crown-6 has been investigated using the micropipette approach at the water-DCE interface k0 values around 1.0 cm s 1 have been measured for the potassium ion [19,104,107], whereas a separate study reported ko values increasing in the series Li+ (0.3 cm s-1) < Na+ (0.9 cm s 1) < K+(1.7 cm s-1) [108]. 

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