Weak base anion-exchangers

Weak Base. Weak base anion-exchange resins may have primary, secondary, or tertiary amines as the functional group. The tertiary amine -N(CH2)2 is most common. Weak base resins are frequentiy preferred over strong base resins for removal of strong acids in order to take advantage of the greater ease in regeneration. 

Most weak base anion exchangers adsorbweak organic acids such as formic acid [64-18-6] and acetic acid [64-19-7] but do not remove weak organic acids such as carbonic acid [463-79-6] or siHcic acid [7669-41 ]. 

Weak Base Anion Exchangers. Both styreoic and acryHc copolymers can be converted to weak base anion-exchange resias, but differeat syathetic routes are aecessary. Styreae—DVB copolymers are chloromethylated and aminated ia a two-step process. Chloromethyl groups are attached to the aromatic rings (5) by reactioa of chloromethyl methyl ether [107-30-2] CH2OCH2CI, with the copolymer ia the preseace of a Friedel-Crafts catalyst such as aluminum chloride [7446-70-0], AlCl, iroa(III) chloride [7705-08-0], FeCl, or ziac chloride [7646-85-7], ZaCl. ... 

Strong Base Anion Exchangers. As ia the synthesis of weak base anion exchangers, strong base resias are manufactured from styrenic as well as acryhc copolymers. Those based on copolymers of styrene and divinylben2ene are chloromethylated and then aminated. These reactions are the same as for the styrenic weakbase resias. The esseatial differeace is the amine used for amination. Trimethyl amine [75-50-3] N(CH2)3, and /V, /V- dim ethyl eth a n ol amine [108-01 -0] (CH2)2NCH2CH20H, are most commonly used. Both form quaternary ammonium functional groups similar to (8). 

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.

For an amine (weak base) anion-exchange column in its chloride form, the following order has been observed ... 

Strong-acid cation exchanger weak base anion exchangers... 

Weak base anion exchangers Duolite A303 Duolite A378 Amberlite 45 Amberlite 68 Amberlite 93 AG3-X4A... 

When passage of a sugar solution at 90°C through a weak-base anion exchange resin was interrupted, an explosion occurred. This was attributed to an exothermic Maillard reaction (interaction of an amino acid with a glycosidic OH group) under the poor heat transfer conditions in a particulate bed without fluid flow. 

Aliphatic amine-type weak base anion exchangers Amberlites IR-45 and IRA-67 Dowex 3-x4A Permutit E Permutit A 240A (Rohm and Haas, USA) (Dow Chemical Co, USA) (Permutit AG, Germany) (Phillips and Pain-Vermorel, France)... 

Adsorption chromatography is an efficient way to isolate organic acids from large volumes of water. The nonionic, macroporous, Amberlite XAD-8 and the weak-base anion-exchange resin Duolite A-7 are two resins well suited for this purpose. These resins have been successfully used to extract organic acids from natural waters at sites where it was necessary to process thousands of gallons of sample. 

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). 

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