Weak base ion exchange

W. Holl and R. Kirch, Regeneration of weak base ion exchange resins, Desalination, 26 (1978), p. 153. 

Weak-acid and weak-base ion exchangers react with strong and weak bases and acids but do noi split neutral salts. 

For demineralization, a set of strong-acid and weak-acid cation-exchange resin columns, and strong-base anion-exchange and weak-base ion-exchange resin columns have been applied [6],... 

W. O. Haag and D. D. Whitehurst, Insoluble Resin-Metal Compound Complex Prepared by Contacting Weak Base Ion Exchange Resin with Solution of Metal-Ligand, US Patent, 4 111 856, 1978. 

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...

Weak acids such as carbonic acid (H2CO3) and silicic acid (H2Si03) are not sufficiently dissociated (strong) to protonate the weakly basic amine grouping, and therefore are not sorbed by a weak base ion exchange resin. This property is manipulated to great effect in such applications as water treatment by ion exchange. 

Traditionally, soil humic substances have been defined by the fact they are soluble in 0.1 N NaOH. Aquatic humic substances, however, are operationally defined as polyelectrolytic acids that can be isolated from water by sorption onto XAD or weak base-ion exchange resins, for example (Thurman 1985). They are nonvolatile, have molecular weights from about 500 to 5000 g/mol, and a molar composition of about 50% C, 4 to 5% H, 35 to 40% O, and 1% N. 

Figure 8.5 Weak-base ion-exchange monomer (tertiary amine on polystyrene)...

When HCl-acidified alumina is contacted with arsenate ions, they strongly displace the chloride ions, especially when the alumina surface remains acidic (pH 5.5-6) This displacement of chloride by arsenate, analogous to weak-base ion exchange, can be written as... 

A crosslinked weak acid or weak base ion exchange resin in microbead form was prepared with an average particle size of about 0.1-10 /xm and about 0.5-10% crosslinking. 

First, when impregnating an acidic ligand (HL) (e.g., organophosphoric [DEHPA] and organothiophosphoric [DEHPTA]) and neutral (TBP, TOPO) derivatives, a polymeric support containing a basic functionality should be selected (e.g., PVP or a weak base ion exchange resins). The acid-base interaction between the extractant and the polymeric support could be described as follows ... 

Although Certify is a mixed mode sorbent with C8 and sulfonic acid moieties, the authors rationalized that the hydrophobic retention on this sorbent is more dominant and caused the nonretention of certain drugs during methanol wash. The weak WCX ion exchanger was also excluded for similar reasons. Both the mixed mode strata-X-C and the ion exchange sorbent SCX were found to be most amenable for the derivatization-based GC/MS analysis and both yielded pure extracts. However, the yields were consistently lower with strata-XC than SCX and the authors hypothesized that this was due to the inability of the 5% ammonia/methanol eluent to completely disrupt the hydrophobic and dipolar interactions between the analytes and XC. 

The magnitude of the diffusion coefficients given in Table I can be compared with a value of 3.3 X 10 5 cm.2/sec. determined experimentally by Stokes (26) for HCl in bulk solution at infinite dilution. The pore diffusion coefficients listed in Table I for HCl vary by a factor of (2 - 4) X 10"2 from that given by Stokes. McNeill and Weiss (15) have indicated that active carbon can be considered as a weak-base anion-exchange sorbent. According to Helfferich (13), diffusion coefficients in such resins can be several orders of magnitude less than the corresponding bulk solution coefficients. The Cl" ion probably limits the rate of diffusion, since its mobility in aqueous solution is much less than that of the H30+ ion. Further evidence to support this conclusion has been obtained in the present work from determinations of pore diffusion... 

Fruit juices can be deacidified with a weak base anion-exchange resin. Removal of compounds which cause a bitter taste is a more popular application (26,27). It is accomplished with resins that have no ion-exchange fimctionality. In essence, they are similar to the copolymer intermediates used by resin manufacturers in the production of macroporous cation and anion exchangers. These products are called polymeric adsorbents. They are excellent for removal of limonin [1180-71-8] and naringin [1023647-2], the principal compounds responsible for bitterness in orange, lemon, and grapefruit juices. The adsorbents are regenerated with steam or alcohol. Decaffeination of coffee (qv) and tea (qv) is practiced with the same polymeric adsorbents (28). 

The two examples illustrative of the section, however, involve alternative procedures for introducing the required substituent into the a-position of the acet-amidomalonate reagent. In the first (Expt 5.183) the hydroxymethyl group is introduced by a simple base-catalysed condensation with formaldehyde subsequent hydrolysis and decarboxylation yields serine (R = CH2OH). In this case, acidic conditions are preferred for the final hydrolytic stage, and the use of a weakly basic ion exchange resin to obtain the halide-free amino acid from a solution of its hydrochloride is described. 

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-... 

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