A. Characterization

1. Exchanging Groups

2. Capacity

3. Selectivity

4. Pore Size

5. Moisture Content 

1. Exchanging groups
In analogy to soluble cations and anions, the insoluble ion exchangers can be regarded as strong or weak acids and as strong or weak bases.
Strongly acidic cation exchangers carry a sulfonic acid group and can be used over the entire pH range down to pH 1. Maximum operating temperature is 120°C. Higher temperatures can cause degradation of the functional site especially when the resin is in the H⁺-form.
Weakly acidic cation exchangers are mainly produced by copolymerization of DVB with acrylic or methacrylic monomers. They carry carboxylic groups and differ from the sulfonic acid type ion exchangers mainly in their behavior towards protonation. Below pH 5, the carboxylic groups are no longer ionized and lose their ion exchange properties. Therefore, these resin types can only be used in the neutral and basic pH range. Maximum operating temperature is 120 °C.
A special kind of cation exchangers are chelating resins: they are described in more detail in the Section D:  SERDOLIT^® Chelating Resins.

Strongly basic anion exchangers contain quaternary ammonium groups. Such anion exchangers can therefore be used also in very strongly alkaline media. Type I resins are characterized by a quaternary structure of the following type:-CH₂-N(CH₃)₃^+. Maximum operating temperature is 100 °C for the chloride form and 60°C for the OH-form.
In the hydroxide form such resins tend to eliminate trimethylamine even at room temperature. Already traces of this elimination product have a characteristic fishy smell which disappears upon rinsing.
Type II resins contain a hydroxyethyl group at the nitrogen: -CH₂-N⁺(CH₃)₂-CH₂CH₂-OH.
The elimination products do not smell (or only very little) and such resins can be regenerated more efficiently. Maximum operating temperature is 70 °C for the chloride form and 35 °C for the OH-form.
Weakly basic anion exchangers are characterized by primary, secondary or the especially desirable tertiary amino groups. Anions are bound and exchanged only at neutral or acidic pH-ranges. Such resins are mostly used in acidic solutions, mainly for the neutralization of acids. Since they do not split off amines, they are important in pharmaceutical and food applications. Maximum operating temperature is 100 °C.

2. Capacity
In the same way as the concentration of an acid or base, the capacity of ion exchangers is expressed in equivalents per liter. The equivalent weight is the molecular weight of the ion divided by its valence. For the monovalent sodium ion, the equivalent weight is identical with the molecular weight = 23. For the divalent calcium ion, it is
40 : 2 = 20. Therefore, a cation exchanger with a capacity of 1.7 eq./L can bind
1.7 x 23 = 39.1 g sodium ions per liter and 1.7 x 20 = 34 g calcium ions per liter. Representative capacities for SERDOLIT^® ion exchangers are shown in the following table.

Table 1

These values indicate the maximum quantity which the corresponding ion exchanger can bind under ideal conditions. Due to competing reactions between the exchanged ions, only a part of the theoretical capacity should be used in practice.

3. Selectivity
The selectivity of ion exchangers is often overestimated. Selective separations of many inorganic or organic ions are usually only feasible with elaborate chromatographic techniques and many theoretical plates of the column. In general, the affinity of ions towards the immobilized functional groups increases with valence of the ion. Selectivity also depends on the hydration of the ions and their concentration in solution, furthermore on pH, temperature and degree of crosslinking of the ion exchanger (steric factors which inhibit diffusion of the ions). Last but not least, non-ionic van der Waals or hydrophobic interactions between the ions and the matrix can affect binding. Selectivity also varies between resin types with different functional groups (see below sequence of affinity of alkali metal ions on strongly and weakly acidic cation exchangers).
The selectivity of a resin for ions is expressed by the selectivity coefficient which is the equilibrium constant of the following reaction (for a cation exchanger):
RA + B⁺ = A⁺ + RB
where R is the resin (as cation exchanger) and A⁺ and B⁺ the cations competing for the binding site. The equilibrium constant K is calculated as follows:
K = [A⁺] x [RB] / [B⁺] x [RA]

Symbols in brackets indicate the concentration of the ions in Mol/L or Eq/L in the solution resp. in the resin phase.

Table 2 lists selectivity coefficients of alkali, alkaline earth and some transition metal ions on 2 DOWEX^® resins with different degrees of crosslinking. The larger the selectivity coefficient the stronger the ion is bound by the exchanger. Furthermore, it shows the effect of crosslinking on binding of ions. Weakly bound ions like hydrogen or sodium can preferently be  displaced by the stronger binding magnesium and calcium ions, which is used practically in softening of hard water.

Table 2
Selectivity coefficients of various cations (compared with the hydrogen ion) on sulfonated polystyrene DOWEX^® cation exchange resins of different crosslinkage

 Provided by Courtesy of The Dow Chemical Company

The binding strength for alkali metal cations on sulphonated resins follows the degree of their hydration in reversed order : Li⁺, the smallest ion, is the most strongly hydrated but the most weakly bound. The largest ion, Cs⁺, is the least hydrated and bound most strongly.  Similar effects are observed with alkaline earth cations.

Some cations have very similar selectivity coefficients, e.g. cesium and rubidium. When they are bound to a cation exchanger, it will be very difficult if not impossible to separate them from each other by elution with a stronger binding cation.
The same holds true for the transition metals Zn, Co, Cu and Ni. To separate them from each other
and alkaline earth ions,  it is more convenient to use chelating resins (see table 7 and 8). By addition of complex-forming agents, e.g. chloride ions, negatively charged complexes are formed (e.g. [FeCl₄]^-) which can be bound by a strongly basic anion exchanger. By this method, heavy metal ions can be recovered from waste water containing also alkali metal and alkaline earth ions, which do not form complexes.
On resins with carboxylic groups, alkali ions show a reversed sequence of affinity with lithium being held most strongly and cesium the least. Below pH 4, the exchange stops and the carboxylic ion exchanger simply represents an insoluble, undissociated polymer acid.

Table 3
Selectivity Coefficients of various anions (compared with the hydroxyl ion) on functionalized Styrene-DVB DOWEX^® anion exchange resins of different base strength

Provided by Courtesy of the Dow Chemical Company

The differences in binding strength for halide ions on a type I anion exchanger are much greater than the differences for alkali metal or alkaline earth ions on a strong cation exchanger. In fact, the complete regeneration of a type I anion exchanger in iodide form with sodium hydroxide is a tedious process.
Weakly basic anion exchangers show similar sequences with the exception of OH- which shifts completely to high affinity region. Above pH 10, weakly basic anion exchangers are formally uncharged polyamines.

4. Pore Size
The percentage of DVB in the polymer is called crosslinkage: usually it is 2 %, 4 %,
6 % or 8 %, e.g. DOWEX^® 50 WX8 contains 8 % crosslinker and DOWEX^® 50 WX2
2 %. Higher crosslinking means narrower pores. Consequently, the polymer gains higher chemical and mechanical stability and retains less residual water. Therefore more ionized groups are available per volume unit, leading to higher ion exchange capacity in the moist state. Since narrow pores are closer to the size of the ions to be exchanged, steric factors become important in the equilibration step.

These types of resins are called gel-type or microporous ion exchangers.
If polymerization is performed in the presence of an inert solvent, macroporous (also called macroreticular) resins are formed. They are characterized by a rigid structure with large pores and channels. Due to these properties, ions can penetrate faster into the resin and ion exchange process is less inhibited than in microporous resins.

5. Moisture Content
Ion exchangers are very hygroscopic and lose the incorporated water only after exhaustive drying at 120 °C. There is a relation between moisture content and crosslinking as shown for some DOWEX^® resins in the table below. The structure of gel-type  ion exchangers tends to collapse irreversibly when all the water is removed, especially when crosslinking is below 8 %. The porous structure becomes nearly impermeable and hence ion exchange in non-aqueous solvents can only be performed with macroporous resins.

Table 4

 1) These data are no absolute values , but are very much dependent on the application conditions (e.g. pH, ionic strength etc) and may be useful for the selection of the right resin
Provided by Courtesy of the Dow Chemical Company

Table 5: Conversion of mesh size in millimeter