11.32

SECTION 11

11.4.3.1

Chemical

Structure of Ion-Exchange Resins.

An ion-exchange resin usually consists

of polystyrene copolymerized with divinylbenzene to build up an inert three-dimensional, cross-

linked matrix of hydrocarbon chains. Protruding from the polymer chains are the ion-exchange sites

distributed statistically throughout the entire resin particle. The ionic sites are balanced by an equiv-

alent number of mobile counter ions. The type and strength of the exchanger is determined by these

active groups. Ion-exchangers are designated anionic or cationic, according to whether they have an

affinity for negative or positive counter ions. Each main

group is further subdivided into strongly

or weakly ionized groups. A selection of commercially available ion-exchange resins is given in

Table 11.16.

The cross-linking of a polystyrene resin is expressed as the proportion by weight percent of

divinylbenzene in the reaction mixture; for example, “

8” for 8 percent cross-linking. As the per-

centage is increased, the ionic groups come into effectively closer proximity, resulting in increased

selectivity. Intermediate cross-linking, in the range of

4 to 8 percent, is usually used. An increase

in cross-linking decreases the diffusion rate in the resin particles; the diffusion rate is the rate-

controlling step in column operations. Decreasing the particle size reduces the time required for

attaining equilibrium, but at the same time decreases the flow rate until it is prohibitively slow unless

pressure is

applied.

In most inorganic chromatography, resins of 100 to 200 mesh size are suitable; difficult sepa-

rations may require 200 to 400 mesh resins. A flow rate of 1 mL · cm

2· min 1 is often satisfactory.

With HPLC columns, the flow rate in long columns of fine adsorbent can be increased by applying

pressure.

11.4.3.1.1

Macroreticular Resins.

Macroreticular resins are an agglomerate of randomly

packed microspheres which extend through the agglomerate in a continuous non-gel pore structure.

The channels throughout the rigid pore structure render the

bead centers accessible even in

non-

11.4.3.1.2

Microreticular Resins.

Microreticular resins, by contrast, are elastic gels that, in

the dry state, avidly absorb water and other polar solvents in which they are immersed. While taking

up solvent, the gel structure expands until the retractile stresses of the distended polymer network

balance the osmotic effect. In nonpolar solvents, little or no swelling occurs and diffusion is impaired.

11.4.3.1.3

Ion-Exchange Membranes.

Ion-exchange membranes are extremely flexible, strong

Sulfonate exchangers

contain the group

9SO ,3 which is strongly acidic and completely disso-

PRACTICAL LABORATORY INFORMATION

11.33

Resin

type and

Minimum wet

Density

nominal

percent

capacity,

(nominal),

cross-linkage

mequiv · mL

1

g · mL 1

Comments

Anion exchange resins

9gel

type

9strongly basic

9quaternary ammonium functionality

Dowex 1-X2

0.6

0.65

Strongly basic anion exchanger with S-DVB

matrix for separation of small peptides,

nucleotides, and large metal complexes.

Molecular weight exclusion is

2700.

Dowex 1-X4

1.0

0.70

Strongly basic anion exchanger with S-DVB

matrix for separation of organic acids,

nucleotides, phosphoinositides, and

other

anions. Molecular weight exclusion is

1400.

Dowex 1-X8

1.2

0.75

Strongly basic anion exchanger with S-DVB

matrix for separation of inorganic and or-

ganic anions with molecular weight ex-

clusion

1000. 100– 200 mesh is stan-

dard for

analytical separations.

Dowex 2-X8

1.2

0.75

Strongly basic (but less basic than Dowex 1

type) anion exchanger with S-DVB ma-

trix for deionization of carbohydrates and

separation of sugars, sugar alcohols, and

glycosides.

Amberlite IRA-400

1.4

1.11

8% cross-linkage. Used for systems essen-

tially free of organic materials.

Amberlite IRA-402

1.3

1.07

Lower cross-linkage than IRA-400; better

diffusion rate with large organic mole-

cules.

Amberlite IRA-410

1.4

1.12

Dimethylethanolamine functionality and

slightly lower basicity than IRA-400.

Amberlite IRA-458

1.2

1.08

Has an acrylic structure rather than S-DVB;

hence more hydrophilic and resistant to

organic

fouling.

Anion exchange

resin– gel type

9intermediate basicity

Bio-Rex 5

2.8

0.70

Intermediate basic anion exchanger with

primarily tertiary amines on a polyalkyle-

neamine matrix for separation of organic

acids.

11.34

SECTION

11

TABLE 11.16

Guide to Ion-Exchange Resins (

Continued

)

Resin type and

Minimum wet

Density

nominal percent

capacity,

(nominal),

cross-linkage

mequiv · mL

1

g · mL

1

Comments

Anion exchange resins

9gel

type

9weakly basic

9polyamine functionality

Dowex 4-X4

1.6

0.70

Weakly basic anion exchanger with tertiary

amines on an acrylic matrix for the

deionization of carbohydrates. Use at pH

7.

Amberlite IRA-68

1.6

1.06

Acrylic-DVB with unusually high capacity

for large

organic molecules.

Cation exchange resins

9gel type

9strongly

acidic

9sulfonic

acid functionality

Dowex 50W-X2

0.6

0.70

Strongly acidic cation exchanger with S-

DVB matrix for separation of peptides,

nucleotides, and cations. Molecular

weight exclusion

2700.

Dowex 50W-X4

1.1

0.80

Strongly acidic cation exchanger with S-

DVB matrix for separation of amino ac-

ids, nucleosides and cations. Molecular

weight exclusion is

1400.

Dowex 50W-X8

1.7

0.80

Strongly acidic cation exchanger with S-

DVB matrix for separation of amino ac-

ids, metal cations, and cations. Molecular

weight exclusion is

1000. 100– 200

mesh is standard for analytical applica-

tions.

Dowex 50W-X12

2.1

0.85

Strongly acidic cation exchanger with S-

DVB matrix used primarily for metal

separations.

Dowex 50W-X16

2.4

0.85

Strongly acidic cation exchanger with S-

DVB matrix and high cross linkage.

Amberlite IR-120

1.9

1.26

8% styrene-DVB type; high physical stabil-

ity.

Amberlite IR-122

2.1

1.32

10% styrene-DVB type; high physical sta-

bility and

high capacity.

Weakly acidic cation exchangers

9gel

type

9carboxylic acid functionality

Duolite C-433

4.5

1.19

Acrylic-DVB type; very high capacity.

Used for metals removal and neutraliza-

tion of alkaline solutions.

Bio-Rex 70

2.4

0.70

Weakly acidic cation exchanger with car-

boxylate groups on a macroreticular

acrylic matrix for separation and fraction-

ation of proteins, peptides, enzymes, and

amines, particularly high

molecular

weight solutes. Does not denature pro-

teins as do styrene-based

resins.

PRACTICAL

LABORATORY

INFORMATION

11.35

TABLE 11.16

Guide to Ion-Exchange Resins (

Continued

)

Resin

type and

Minimum wet

Density

nominal

percent

capacity,

(nominal),

cross-linkage

mequiv · mL

1

g · mL

1

Comments

Selective ion

exchange

resins

Duolite GT-73

1.3

1.30

Removal of Ag, Cd, Cu, Hg, and Pb.

Amberlite IRA-

0.6

1.05

Boron specific ion exchange resin.

743A

Amberlite IRC-718

1.0

1.14

Removal of transition metals.

Chelex

100

0.4

0.65

Weakly acidic chelating resin with S-DVB

matrix for heavy metal concentration.

Anion exchanger

9macroreticular type

9strongly

basic

9quaternary

ammonium functionality

Amberlite IRA-910

1.1

1.09

Dimethylethanolamine styrene-DVB type

which offers slightly less silica removal

than Amberlite IRA resin, but offers im-

proved regeneration efficiency.

Amberlite IRA-938

0.5

1.20

Pore size distribution between 2500 and 23

000 nm; suitable for removal of high mo-

lecular weight organic materials.

Amberlite IRA-958

0.8

Acrylic-DVB; resistant to organic fouling.

AG MP-1

1.0

0.70

Strongly basic macroporous anion ex-

changer with S-DVB matrix for separa-

tion of some enzymes, radioactive anions,

and

other

applications.

Cation exchange

resin

9macroreticular type

9sulfonic

acid

functionality

Amberlite 200

1.7

1.26

Styrene-DVB with 20% DVB by weight;

superior physical stability and greater re-

sistance to oxidation by factor of three

over comparable gel type resin.

AG MP-50

1.5

0.80

Strongly acidic macroporous cation ex-

changer with S-DVB matrix for separa-

tion of radioactive cations and other ap-

plications.

Weak cation exchanger

9macroreticular type

9carboxylic acid or

phenolic functionality

Amberlite DP-1

2.5

1.17

Methacrylic acid-DVB; high resin capacity.

Use

pH

5.

Amberlite IRC-50

3.5

1.25

Methacrylic acid-DVB. Selectivity adsorbs

organic gases such as antibiotics, alka-

loids, peptides, and amino acids. Use pH

5.

Duolite C-464

3.0

1.13

Polyacrylic resin with high capacity and

outstanding resistance to osmotic shock.

11.36

SECTION 11

TABLE 11.16

Guide to Ion-Exchange Resins (

Continued

)

Resin type and

Minimum wet

Density

nominal percent

capacity,

(nominal),

cross-linkage

mequiv · mL

1

g · mL

1

Comments

Weak cation exchanger

9macroreticular type

9carboxylic acid or

phenolic functionality (

continued

)

Duolite A-7

2.2

1.12

Phenolic type resin. High porosity and hy-

drophilic matrix. pH range is 0 to 6.

Duolite A-368

1.7

1.04

Styrene-DVB; pH range is 0 to 9.

Amberlite IRA-35

1.1

Acrylic-DVB; pH range is 0 to 9.

Amberlite IRA-93

1.3

1.04

Styrene-DVB; pH range is 0 to 9. Excellent

resistance to oxidation and organic foul-

ing.

Liquid amines

Amberlite LA-1

A secondary amine containing two highly

branched aliphatic chains of M.W. 351 to

393. Solubility is 15 to 20 mg/mL in wa-

ter. Used as 5 to 40% solutions in hydro-

carbons.

Amberlite LA-2

A secondary amine of M.W. 353 to 395. In-

soluble

in water.

Microcrystalline

exchanger

AMP-1

4.0

Microcrystalline ammonium molybdo-

phosphate with cation exchange capacity

of 1.2 mequiv/g. Selectively adsorbs

larger alkali metal ions from smaller al-

kali metal ions, particularly cesium.

Ion retardation resin

AG 11 A8

0.70

Ion retardation resin containing paired anion

(COO

) and cation (CH

3)3N sites. Selec-

tively

retards ionic

substances.

Source:

J. A. Dean, ed.,

Analytical Chemistry Handbook

, McGraw-Hill, New York, 1995.

Carboxylate

exchangers

contain

9COOH groups

which have weak acidic

properties

and will

only function as cation exchangers when the pH is sufficiently high (pH

6) to permit complete

dissociation

of the

9COOH site. Outside

this range

the

ion

exchanger

can

be used

only at the

cost of reduced capacity.

Quaternary

ammonium exchangers

contain

9R

4N groups

which are strongly

basic and

com-

pletely dissociated in the OH form and the anion form.

Tertiary amine exchangers

possess

9R 3NH

2 groups which

have exchanging properties only in

an acidic medium when a proton is bound to the nitrogen atom.

Aminodiacetate exchangers

have the

9N(CH

2COOH)

2group which has an unusually high pref-

erence for copper,

iron, and

other heavy metal

cations

and, to a lesser extent, for

alkaline earth

PRACTICAL LABORATORY INFORMATION

11.37

11.4.3.3

Ion-Exchange Equilibrium.

Retention differences among cations with an anion ex-

changer,

or among anions with a cation exchanger, are governed by the physical properties of the

solvated ions. The stationary phase will

show these preferences:

To accomplish any separation of two cations (or two anions) of the same net charge, the stationary

phase must show a preference for one more than the other. No variation in the eluant concentration

will improve the separation. However, if the exchange involves ions of different net charges, the

separation factor does depend on the eluant concentration. The more dilute the counterion concen-

tration in the eluant, the more selective the exchange becomes for polyvalent ions.

In the case of an ionized resin, initially in the

H-form and in contact with

a solution

containing

K ions, an equilibrium exists:

resin, H

K N resin, K H

which is characterized by the selectivity coefficient,

k K/H :

k K/H

[K ]r

[H ]

[H ]r

[K ]

where the subscript

r refers to the resin phase. Table 11.17 contains selectivity coefficients for cations

and Table 11.18 for anions. Relative selectivities are of limited use for the prediction of the columnar

TABLE 11.17

Relative Selectivity of Various Counter Cations

Relative

Relative

selectivity for

selectivity for

Counterion

AG 50W-X8 resin

Counterion

AG 50W-X8 resin

H

1.0

Zn

2

2.7

Li

0.86

Co

2

2.8

Na

1.5

Cu

2

2.9

NH 4

1.95

Cd

2

2.95

K

2.5

Ni2

3.0

Rb

2.6

Ca2

3.9

Cs

2.7

Sr2

4.95

Cu

5.3

Hg

2

7.2

Ag

7.6

Pb 2

7.5

Tl

10.7

Ba

2

8.7

Mn

2

2.35

Ce 3

22

Mg

2

2.5

La 3

22

Fe2

2.55