Silica-Based Cation Exchangers

The prepolymer used in manufacturing this novel cation exchanger consists of a copolymer that is derived from a mixture of butadiene and maleic acid in equal parts  

The structural formula reveals that this polymer contains two different types of carboxyl group which have different dissociation constants. While the first dissociation step is characterized by a pK value of 3.4, the pK value of the second step is about 7.4. Both pK values were determined via titration of the prepolymer with sodium hydroxide solution. The exchange capacity of the finished stationary phase is directly proportional to its polymer content. It may be calculated in advance, since, owing to the chemical composition, the concentration of the exchange groups in the prepolymer is known. 

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. 

Alkaline-earth metal ions, on the other hand, elute from a crown ether phase in the normal elution order (Mg2+ Ca2+ Sr2+ Ba2+). Such a separation is only of pure academic interest, since the resolution between magnesium and calcium is extraordinarily poor due to the low interactions of both ions with crown ethers. 

In contrast, the two 5- rm cation exchangers, Nucleosil 5 SA by Macherey Nagel (Diiren, Germany) with the dimensions of 125 mm X 4 mm i. d., and TSK Gel IC Cation SW from Toyo Soda (Tokyo, Japan) with the dimensions 50 mm X 4.6 mm i.d., are designed for the analysis of divalent cations. All of these 

5 mmol/L ethylenediamine + 50 mL/L acetone, pH 4 flow rate 1.5 mL/min detection direct conductivity injection volume 100 pL solute concentrations  

5 mmol/L tartaric acid other chromatographic conditions see Fig. 4-30. 

Several of the earlier cation exchangers contained groups such as (CH2)3C6H4S03 attached to spherical silica particles, but these no longer find much use in IC. 

Considerable interest has been shown in a novel cation exchanger first developed by Schomburg et. al. [43]. The material consists of a silica substrate of very uniform particle size coated with a poly(butadiene-maleic acid) resin which serves as the cation-exchange moiety. This material, which is now conunercially available, gives good separations of both monovalent and divalent metal ions in a single run. Ordinary eluents such as hydrochloric or methanesulfonic acid, or complexing eluents may be used [44,45]. 

A novel silica-based cation exchanger is functionalized with a combination of car-boxylate and a crown ether functionalities [46]. This stationary phase is more selective toward anunonium, potassium and low-molecular weight amines. Potassium is eluted after ammonium, magnesium and calcium. 

Manufacturer Thermo Fisher Scientific Thermo Fisher Scientific 

Substrate material Ethylvinylbenzene/ divinylbenzene Ethylvinylbenzene/ divinylbenzene 

Stationary phase is directly proportional to its polymer content and can be calculated in advance, because the concentration of the cation-exchange groups in the prepolymer is known on the basis of the chemical composition. 

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Fig. 3-134. Separation of alkaline-earth metals on a silica-based cation exchanger. - Separator column Nucleosil 5 SA eluent 0.0035 mol/L oxalic acid + 0.0025 mol/L ethylenediamine + 50 mL/L acetone, pH 4.0 flow rate 1.5 mL/min detection direct conductivity injection volume 100 pL solute concentrations 2.5 ppm magnesium, 5 ppm calcium, 20 ppm strontium, and 40 ppm barium.

The simultaneous analysis of the most important alkali and alkaline-earth metals was once impossible due to their markedly different retention behavior. However, the inorganic chemists have finally realized their dream this analytical problem no longer poses a problem. One of the two possible solutions is the novel silica-based cation exchanger modified with poly(butadiene-maleic acid), introduced in Section 3.4.I.3. As shown in Fig. 3-135, the most important alkali and alkaline-earth metals and ammonium can be analyzed in a single run via direct conductivity detection using tartaric acid as the eluent. The extremely short time required for such a separation is quite impressive. 

In this equation, Res denotes resin or polymer. Silica-based cation exchangers are generally prepared by reacting silica particles with an appropriate chlorosilane or methoxysilane. A common type of silica catex has the structure ... 

This reaction offered a significant improvement to the synthesis of cation-exchangers in general. The measured exchange capacitie s were 3-S times greater than the best commercial (silica-based) cation-exchange materials (86). 

The actual surface structure is, of course, more complex than represented. A silica-based cation exchanger of this type could be used to separate metal ions such as Ag, Cu, Fe, Ca +, according to Kautsky and Wesslau (546). Ion-exchange surfaces of this type have been described by Yates (547), who prepared colloidal phosphates of Ti +, Zr +, Sn +, Hf, and Ce + and adsorbed them on the silica surface. The advantage of making an exchanger with a silica base rather than entirely of the polyvalent metal phosphate is that silica is easier to form into the wide-pored structure of high surface area needed for efficient performance. 

The selectivity of lonPac CS18 is very similar to those obtained with silica-based cation exchangers (see Section 4.1.2). It exhibits very high resolution of... 

Figure 4.40 Separation of aikaiine-earth metals on a sulfonated silica-based cation exchanger. Separator column Nucleosil 5 SA eluent 3.5 mmol/L oxalic acid -I- 2.5 mmol/L ethylenediamine -l-50mL/L acetone, pH 4 ...

The structural and technical properties of the PBDMA-coated silica-based cation exchangers are summarized in Table 4.5. 

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