Characterization

Ion exchange capacity is the most appropriate characteristic of organic ion exchangers.

The total capacity indicates the number of fixed acidic or basic groups per specified amount of ion exchanger. It can be described as both weight capacity and volume capacity, having as units mil-liequivalents per gram of dry exchanger (meqg"1) and milliequivalents per cubic centimetre of fully swollen exchanger (meqcm"3), respectively. If not otherwise stated, the capacity should be reported per gram of H form for a cation exchanger or the Cl form for an anion exchanger in the dry state. This capacity is a constant for the material and does not depend on the experimental conditions.

The effective capacity is the number of exchangeable counter-ions per specified amount of exchanger (the same units are used as above). This capacity depends on the experimental conditions and is lower than total capacity.

Another important characteristic is the selectivity which has a major role in the ion exchange processes. The selectivity is the preference of an ion exchanger for a particular counter-ion over the others, when it is

Table 1 Types of ion exchangers and their functional groups

Type

Name offixed functionalgroup

Chemical structure of functional group

Cation exchangers

Strong acid

Aryl sulfonic

-C6H5-SO3H

Weak acid

Carboxylic acid

-COOH

Phenolic hydroxyl

-C6H5-OH

Intermediate acid

Phosphonic

-P(O)(OH)2

Phosphonous

-P(O)H(OH)

Phosphoric

-O-P(O)(OH)2

Anion exchangers

Strong base

Quaternary ammonium

Phosphonium

Sulfonium

— s-""

Weak base

Primary amine Secondary amine Tertiary amine

Weak base

Amphoteric exchangers

Primary amine Secondary amine Tertiary amine

Mixture of acid and base groups

in contact with an electrolyte solution. The selectivity has various physical causes.

An ion exchanger tends to prefer a counter-ion with higher valence, lower solvatation, higher polarizabil-ity, stronger interactions with the fixed groups or the matrix, and less participation in complex formation with the co-ions. The selectivity of an ion exchanger is improved by increasing degree of crosslinking and by decreasing solution concentration and temperature.

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