Weak-Acid Ion Exchangers

The preparation and use of two new experimental resins was described by Morris and Fritz [16]. Both resins were prepared by simple Friedel-Crafts addition reactions. One has the carboxyl group attached to the benzene ring of a spherical polystyrene-divinylbenzene (PS-DVB) resin via a spacer arm of three carbon atoms. The carboxyl group is attached directly to the benzene ring in the other resin. Excellent ion chromatographic separations of metal ions are possible using these resins in conjunction with any of several complexing eluents. 

A separation of several metal ions with an eluent containing 2,6-pyridinedicar-boxylic acid (PDA) is shown in Fig. 7.13. The elution order is unusual. Zinc(II) is eluted even before Na owing to its strong complexation by PDA (see Table 7.7). Magnesium(ll) is only weakly complexed by PDA and elutes after Ca, Sr and Ba. In other systems, Mg almost always elutes before the alkaline earths. 

Another example of a metal ion separation on a weak-acid column is shown in Fig. 7.14. Here, spectrophotometric detection at 530 nm was used after post-column addition of pyridylazo resorcinol (PAR) as a complexing reagent. At highly alkaline pH values forms colored complexes with metal ions that are more stable than the eluent complexing reagent (Chapter 4). 

Column lonPac (4 x 250 mm) and CG5A (4 X 50 mm) Eluent MetPac PDCA Eluent Flow Rate 1.2 mUmin Detection Absorbance, 530 nm with PAR in MetPac Postcolumn Reagent OHuent Peaks  

Separations of metal cations are best carried out with a carboxyhc acid stationary phase. The mobile phase contains a chelating reagent, such as citric, ojaUc, or 2,6-pyridinedicarboxylic acid (PDA). The concentration and pH of the mobile phase are adjusted so that metal ions in the sample will be in an equilibrium between the cationic form, M, and a neutral or anionic metal ligand complex form. The rate at which a metal analyte moves down the column wiU depend on the fraction that remains in the free cationic form. A greater degree of complexa-tion results in a shorter retention time. 

Coluntn lonPac (4 x 250 mm) and CGSA (4 X SO mm) Eluent MetPac PDCA Eluent 

Row Rate 1.2 mUrnin Detection Absorbance, 530 nm with PAR in MeiPac Postcotumn Reagent Diluent Peaks  

The cations are eluted in the order of decreasing complexation of the sample ions  

In the absence of a complexing mobile phase, magnesium would elute before the alkaUne earth cations. The sample cations are detected with reasonable sensitivity by non-suppressed conductivity. 

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The polymer may behave as a weak acid ion exchange resin. Branched copolymers have also been proposed as an approach to achieving "smoother" release patterns for polypeptide systems (40). 

Acrylic This resin is manufactured from a copolymer of acrylic or methacrylic acid with divinylbenzene (Figure 4.10). The result is a weak acid ion-exchange resin with -COOH groups that have very little salt splitting capacity, but are very effective in removing Ca2+ and similar ions under alkaline conditions. 

W. Holl and H. Sontheimer, Ion-exchange kinetics of the protonation of weak acid ion exchange resins, Chem. Engrg. Sci., 32 (1977), p. 755. 

W. Holl and G. Geiselhart, Kinetics of the neutralization of weak acid ion exchange resins with different solutions, Desalination, 25 (1978), p. 217. 

Suppose you have a weakly acidic ion-exchange column (negatively charged), also at pH 6.1. Predict the order of elution of the amino acids from this column. Propose a strategy for separating the amino... 

Resins with cation-exchange functionality are available either as strongly or weakly acidic ion exchangers. The strong-acid resins have sulfonic acid (—S03H) functional groups, whereas the... 

Snyder DW, Snoeyink VL, Pfeffer JL. 1986. Weak-acid ion exchange for removing barium, radium, and hardness. Am Water Works Assoc J 78 98-104. 

Which catalyst should be chosen for a given reaction will depend upon chemical, steric, and mechanistic factors. The application of Pearson s soft and hard acid-base (SHAB) principle has often proved a valuable qualitative guide as to suitable surface sites for a particular reactant. In fact, certain solids actually owe their catalytic power to attached Bronsted or Lewis acid and base groups as exemplified by weak acid ion exchange resins (Sect. 2.3), alumina (Sect. 3.2), and sometimes charcoals. Steric aspects can be con-... 

In one paper [51] examining cation-exchanger treatment of water a mbcture of equal amounts of strong- and weak-acid ion exchangers was employed. 

Schemes using only weak-acid and weak-base ion exchangers have been considered as well [52,53]. In the first column illustrating this (Fig. 23) water is treated with a weak-base anion exchanger in the bicarbonate form. All anions are replaced by bicarbonate ion. In the second column the bicarbonate solution obtained is treated with weak-acid ion exchanger in the H form. Because carbon dioxide is formed all the reactions of the type...

For an ideally reversible exchange reaction the plot of log D, n+ vs. log[mH+] should resolve a straight line with a slope of -n, the valency of the exchanged metal ion (Fig. 1). Much attention has been paid as to whether the assumptions of Eq. (6) are applicable for the situation where [mH+]" [m n+] with weak-acid ion exchangers. 

L. Moscou Generally, weakly acidic ion exchange resins have been used in order to prevent loss of zeolite crystallinity. This method enables one to exchange at least 75% of the sodium in Y-type zeolites. Thermal stability of these H-Na-Y zeolites is between 700° and 800°C. 

Weak-acid ion exchange resins adsorb hydrogen ions most strongly. A slight excess of hydrogen ions in the solution can displace all other cations from the resin. Weak-acid resins are particularly selective for multivalent metal ions. 

Figure S3 Weak-acid ion-exchange monomer (polyacrylic acid with DVB...

Weakly acidic ion exchange resins containing carboxylic groups have been much less studied as catalysts. An interesting example is that of polymer 15, a linear poly(N-isopropylacrylamide), which is soluble in cold water but insoluble in water heated above the critical solution temperature, facilitating its recovery and reuse. Such materials have been tested as catalysts for the hydrolysis of ketals [79],... 

It is customary to use a weak acid ion-exchange resin in the sodium form (e.g,. a polyacrylic catboxylic reain) and regenerate with hydrochloric acid. Neomycin, a related antibiotic, is also recovered with a carboxylic acid ion exchanger. However, since Ihe basic amine groups in neomycin are considerably weaker... 

As just mentioned, strong or weak acidic ion exchangers contain sulfonic acid or carboxylic acid groups in the H or alkali metal salt form and consist of 1,4-divinylbenzene cross-linked polyst3Tene or poly(acrylic acid) as shown in formulae 7a and 7b. Another example is the sulfonated phenol-formaldehyde condensation polymer 8. The preparation of ion-exchange resins and the determination of their capacities are described in Section 5.4, Experiments 5-1 and 5-2. 

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