Resins exchange sites

Structures of styrene, divinylbenzene, and a styrene-divinylbenzene co-polymer modified for use as an ion-exchange resin. The ion-exchange sites, indicated by R, are mostly in the para position and are not necessarily bound to all styrene units. 

Strong"Base. Strong base anion-exchange resins have quaternary ammonium groups, - NR OH , where R is usually CH, as the functional exchange sites (see Quaternary ammonium compounds). These resins are used most frequentiy in the hydroxide form for acidity reduction. 

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 ... 

Iron fouling is caused by both forms of iron ions the insoluble form will coat the resin bead surface and the soluble form can exchange and attach to exchange sites on the resin bead. These exchanged ions can be oxidized by subsequent cycles and precipitate ferric oxide within the bead interior. 

Organic fouling also affects cation resins, albeit to a lesser degree. The fouling primarily stems from the attachment to the anion exchange site of a variety of low-level organic matter of variable molecular weight that is present in the raw water source. 

In this scheme, a counter ion Y", attached to the R+ group on the resin, is exchanged for another anion X" from solution. To exchange cations, we need a structure with exchange sites that are negatively charged, and associated positive counter ions ... 

Diffusion of ions within the resin particle to the exchange sites... 

For our purposes here, the problem with resins formed by co-polymerization of the macrocycle is that they are often soft and unable to endure the pressures needed for HPLC applications. Furthermore, adsorption kinetics are slow and band broadening more significant when exchange sites are distributed throughout the polymer, rather than on the surface. However, these resins present good candidates for sample preparation applications where separation efficiency is not critical. 

Following the injection of a small volume (lOOpL) of sample, rapid exchange and separation of the anions is accomplished with a specially prepared low capacity resin (containing quaternary ammonium exchange sites existing as a thin film on the surface to facilitate fast equilibrium). 

Strongly acidic solutions obstruct exchange with the acid exchange sites of typical cation exchange resins. 

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