Weak acid cation-exchange

Weakly acidic cation exchangers—gel type—carboxylic acid functionality... 

Bio-Rex 70 2.4 0.70 Weakly acidic cation exchanger with car-boxylate groups on a macroreticular acrylic matrix for separation and fractionation of proteins, peptides, enzymes, and amines, particularly high molecular weight solutes. Does not denature proteins as do styrene-based resins. 

Weak acid cation exchangers have essentially no abiUty to spHt neutral salts such as sodium chloride [7647-14-5]. On the other hand, an exchange is favorable when the electrolyte is a salt of a strong base and a weak acid. 

The acryHc weak base resias are syathesized from copolymers similar to those used for the manufacture of weak acid cation-exchange resias. For example, uader appropriate temperature and pressure conditions, a weak acid resia reacts with a polyfuactioaal amine, such as dimethylaminopropylamine [109-55-7] (7) to give a weak base resia with a tertiary amine fuactioaaHty. 

Another alternative involves the use of a weak acid cation exchanger in the hydrogen form. This resin is not capable of removing aH cations. It removes only the amount equivalent to the bicarbonate in the influent water. The acidity in the effluent stream is carbonic acid [463-79-6] which can be eliminated by installing a degasifter. 

Fig. 1. Demineializei systems consist of various unit processes arranged to meet the system needs. I Lstrong acid cation exchanger I I Strong ha anion exchanger 0 Degasifier I Mixed bed I Weak acid cation exchanger 1 1 Weak base anion exchanger and I IConnterflow cation.

Ion exchange resins are also useful for demineralising biochemical preparations such as proteins. Removal of metal ions from protein solutions using polystyrene-based resins, however, may lead to protein denaturation. This difficulty may be avoided by using a weakly acidic cation exchanger such as Bio-Rex 70. 

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 ... 

Weak acid cation exchangers Duolite C433 Duolite C464 Amberlite 84 Amberlite 50 Bio-Rex 70 ... 

Weakly acidic cation exchangers (e.g. polymethylacrylic acid resins). These resins (Zerolit 226, Amberlite 50, etc.) are usually supplied in the hydrogen form. They are readily changed into the sodium form by treatment with 1M sodium hydroxide an increase in volume of 80-100 per cent may be expected. The swelling is reversible and does not appear to cause any damage to the bead structure. Below a pH of about 3.5, the hydrogen form exists almost entirely in the little ionised carboxylic acid form. Exchange with metal ions will occur in solution only when these are associated in solution with anions of weak acids, i.e. pH values above about 4. 

Simultaneous determination of both cations and anions in acid rain has been achieved using a portable conductimetric ion-exclusion cation-exchange chromatographic analyzer.14 This system utilized the poly(meth-ylmethacrylate)-based weak acid cation exchange resin TSK-Gel OA-PAK-A, (Tosoh , Tokyo, Japan) with an eluent of tartaric acid-methanol-water. All of the desired species, 3 anions and 5 cations, were separated in less than 30 minutes detection limits were on the order of 10 ppb. Simultaneous determination of nitrate, phosphate, and ammonium ions in wastewater has been reported utilizing isocratic IEC followed by sequential flow injection analysis.9 The ammonium cations were detected by colorimetry, while the anions were measured by conductivity. These determinations could be done with a single injection and the run time was under 9 minutes. 

Tanaka, K., Chikara, H., Hu, W., and Hasebe, K., Separation of carboxylic acids on a weakly acidic cation-exchange resin by ion-exclusion chromatography, /. Chromatogr. A, 850, 187, 1999. 

One fact to keep in mind with such phases is that weak acid cation-exchange materials based on carboxylic acid functional groups are subject to esterification in the presence of alcohol containing eluents. Even thongh typical eluent conditions (i.e., weakly acidic aqneous eluents containing alcohol) do not favor ester formation, such stationary phases typically exhibit slowly declining capacity when operated in the presence of alcohol-containing eluents. Consequently, such columns are normally operated with acetonitrile, tetrahydrofuran or acetone rather than with methanol, in order to avoid this problem. 

A good general-purpose weak acid cation-exchange column for non-suppressed applications is the Metrosep Cation C 2 column from Metrohm. As the selection of eluents used with this column is quite varied, the reader is referred to Metrohm column literature for specific eluent recommendations. The Metrosep Cation C 2 column is prepared via encapsulation of a porous silica substrate maliec acid copolymer. 

D. K. Hale D. Reichenberg Development of carboxylic addition polymers as weak acid cation exchangers. 1949-1956... 

---