Weak acid resins

Weakly acidic cation-exchange resins have carboxylic groups (COOH) as the exchange sites. When operated on the hydrogen cycle, the weakly acidic resins are capable of removing only those cations equivalent to the amount of alkalinity present in the water, and most efficiently the hardness (calcium and magnesium) associated with alkalinity, according to these reactions ... 

Weakly acidic resins are used primarily for softening and dealkalization, frequently in conjunction with a strongly acidic polishing resin. Systems which use both resins profit from the regeneration economy of the weakly acidic resin and produce treated water of quality comparable to that available with a strongly acidic resin. 

Weak base resins are like weak acid resins, in that the degree of ionization is strongly influenced by pH. Consequently, weak base resins exhibit minimum exchange capacity above a pH of 7.0. These resins merely sorb strong acids they caimot split salts. 

The absolute order depends on the individual ion exchanger, but deviations from the above order are usually only slight for different cation and anion exchangers. For weak-acid resins, H+ is preferred over any common cation, while weak-base resins prefer OH over any of the common anions. 

Weak base anion resins Just like for weak acid resins, the operation of weak base anion resins is greatly affected by pH. They exhibit their maximum exchange capacity in the pH range up to 7.0. They hardly adsorb any strong acids they cannot split salts. 

A simple but effective means of preparing supported metal ion catalysts is to employ ion exchange resins. For example, a cobalt-exchanged H-type resin (Dowex 50) was shown43 to be an effective solid catalyst for the autoxidation of acetaldehyde to acetic acid at 20°C. No leaching of cobalt ions from the resin was observed and the catalyst was used repeatedly (5x) without any significant loss of activity. More recently the use of weak acid resins exchanged with cobalt ions as catalysts for the autoxidation of cyclohexane... 

Finally, one of the first continuous ion-exchange plants installed used a weak-acid resin to recover copper from rayon-fibre spinning solutions. In the Bemberg or copper(II) ammonium process,357 the spinning takes place in an addic copper sulfate solution, and the fibre is then washed in ammonia solution. The wash water contains as much as 30% of the copper required for the spinning operation and its recovery is important in economic and environmental terms. The copper is extracted as the cationic amine complex by the weak-acid resin, and is then stripped from the resin with the acidic spinning solution. Zinc is recovered in a similar manner from vicrose rayon-spinning operations. 

The acrylic weak base resins are synthesized from copolymers similar to those used for the manufacture of weak acid cation-exchange resins. For example, under appropriate temperature and pressure conditions, a weak acid resin reacts with a polyfunctional amine, such as dimethylaminopropylamine [109-55-7] (7) to give a weak base resin with a tertiary amine functionality. 

Polymers with chelated Co have also been used as catalysts for alkane autoxidation. Kulkarni et al. (166) employed a tyrosine-based polymer for autoxidation in pure cyclohexane, but very different conditions were used by Shen and Weng (167,168) in the autoxidation of cyclohexane or cyclohexanone. The latter authors, used glacial acetic acid as a solvent and a Co-exchanged weak acid resin as the catalyst. At high conversions, adipic acid is formed ... 

Horst, J., W. H. Holl, and S. H. Eberle. 1990. Application of the surface complex formation model to exchange equilibria on ion exchange resins. Part I. Weak-acid resins. React. Polym. 13 209-231. 

The research effort in perfecting a resin system capable of efficient performance in a thermally regenerable ion-exchange process can be divided into two distinct phases (j) selection of mixtures of weakly basic and weakly acidic resins as... 

Products of Permutit Company, London. De-Acidite G weak-base resin in free form Zeo-Karb 226 weak-acid resin in free form... 

The rate of exchange of ions with a resin is usually regulated by the diffusion rate through the resin or the thin film of solution surrounding the resin bead. Weak-acid resins in the hydrogen form often have a slow rate of exchange. For ion-exchange particles of the order of 0.05 mm in diameter, flow rates of about 1 ml/ cm -min are commonly used. 

Dissociation of the hydroxide form is very weak since any significant concentration of hydroxide ions would immediately convert the resin back to the undissociated free base form. For the same reason ion exchange with a neutral anion as represented by equation 4.15 cannot substantially occur because of the implied liberation of free hydroxide ions, and an immediate reversal of exchange. In other words the hydroxide ion is so greatly preferred by the resin that ion exchange in strong alkali solutions is totally unfavourable, and the exchanger remains in the free base form cf. weak acid resins). 

Sometimes slight acidity in the treated water may be observed very early in the service cycle which is due to most weak acid resins... 

Weak-acid resins generally extract metal ions only at a pH value of 3 or more. At high pH values, where the carboxylic acid functional group is predominantly ionized, the metal-extraction behaviour of a weak-acid resin parallels that of a strong-acid resin. The weak-acid resins tend to be more selective than strong-acid resins, however, and practical separations of metals are possible. In certain interactions between metal ions and weak-acid resins it is difficult to determine whether the interaction is purely electrostatic or whether chemical bonds are formed. In particular, there is strong evidence that Ag" and Cu + form complexes with carboxylic acid groups. Copper, in... 

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