Page 220 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

7 ION EXCHANCE
manner to that indicated for cations. In dilute solution multicharged
anions are generally absorbed preferentially.
(d) When a cation in solution is being exchanged for an ion of different
charge the relative affinity of the ion of higher charge increases in direct
proportion to the dilution. Thus to exchange an ion of higher charge on
the exchanger for one of lower charge in solution, exchange will be
favoured by increasing the concentration, while if the ion of lower charge
is in the exchanger and the ion of higher charge is in solution, exchange
will be favoured by high dilutions.
2. Nature of ion exchange resin. The absorption* of ions will depend upon the
nature of the functional groups in the resin. It will also depend upon the
degree of cross-linking: as the degree of cross-linking is increased, resins
become more selective towards ions of different sizes (the volume of the ion
is assumed to include the water of hydration); the ion with the smaller
hydrated volume will usually be absorbed preferentially.

Exchange of organic ions. Although similar principles apply to the exchange
of organic ions, the following features must also be taken into consideration.
1. The sizes of organic ions differ to a much greater extent than is the case for
inorganic ions and may exceed 100-fold or even 1000-fold the average size
of inorganic ions.
2. Many organic compounds are only slightly soluble in water so that
non-aqueous ion exchange has an important role in operations with organic
substance^.^'
Clearly the application of macroreticular (macroporous) ion exchange resins
will be often advantageous in the separation of organic species.
Ion exchange capacity. The total ion exchange capacity of a resin is dependent
upon the total number of ion-active groups per unit weight of material, and the
greater the number of ions, the greater will be the capacity. The total ion
exchange capacity is usually expressed as millimoles per gram of exchanger.
The capacities of the weakly acidic and weakly basic ion exchangers are
functions of pH, the former reaching moderately constant values at pH above
about 9 and the latter at pH below about 5. Values for the total exchange
capacities, expressed as mm01 g-' of dry resin, for a few typical resins are:
Duolite C225 (Na' form), 4.5-5; Zerolit 226 (H' form), 9-10; Duolite A113
(Cl- form), 4.0; Amberlite IR-45, 5.0. The total exchange capacity expressed as
mm01 ml - ' of the wet resin is about 3 - 3 of the mm01 g - ' of the dry resin.
These figures are useful in estimating very approximately the quantity of resin
required in a determination: an adequate excess must be employed, since the
'break-through' capacity is often much less than the total capacity of the resin.
In most cases a 100 per cent excess is satisfactory.
The exchange capacity of a cation exchange resin may be measured in the
laboratory by determining the number of milligram moles of sodium ion which
are absorbed by 1 g of the dry resin in the hydrogen form. Similarly, the exchange
capacity of a strongly basic anion exchange resin is evaluated by measuring the
amount of chloride ion taken up by 1 g of dry resin in the hydroxide form.

*The term absorption is used whenever ions or other solutes are taken up by an ion exchanger. It
does not imply any specific types of forces responsible for this uptake.