Page 215 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

The formula enables us to visualise a typical cation exchange resin. It consists
of a polymeric skeleton, held together by linkings crossing from one polymer
chain to the next: the ion exchange groups are carried on this skeleton. The
physical properties are largely determined by the degree of cross-linking. This
cannot be determined directly in the resin itself: it is often specified as the moles
per cent of the cross-linking agent in the mixture polymerised. Thus 'polystyrene
sulphonic acid, 5 per cent DVB' refers to a resin containing nominally 1 mole
in 20 of divinylbenzene: the true degree of cross-linking probably differs
somewhat from the nominal value, but the latter is nevertheless useful for grading
resins. Highly cross-linked resins are generally more brittle, harder, and more
impervious than the lightly cross-linked materials; the preference of a resin for
one ion over another is influenced by the degree of cross-linking. The solid
granules of resin swell when placed in water to give a gel structure, but the
swelling is limited by the cross-linking. In the above example the divinylbenzene
units 'weld' the polystyrene chains together and prevent it from swelling
indefinitely and dispersing into solution. The resulting structure is a vast
sponge-like network with negatively charged sulphonate ions attached firmly
to the framework. These fixed negative charges are balanced by an equivalent
number of cations: hydrogen ions in the hydrogen form of the resin and sodium
ions in the sodium form of the resin, etc. These ions move freely within the
water-filled pores and are sometimes called mobile ions; they are the ions which
exchange with other ions. When a cation exchanger containing mobile ions C+
is brought into contact with a solution containing cations B+ the latter diffuse
into the resin structure and cations C+ diffuse out until equilibrium is attained.
The solid and the solution then contain both cations C+ and B+ in numbers
depending upon the position of equilibrium. The same mechanism operates for
the exchange of anions in an anion exchanger.
Anion exchangers are likewise cross-linked, high-molecular-weight polymers.
Their basic character is due to the presence of amino, substituted amino, or
quaternary ammonium groups. The polymers containing quaternary ammonium
groups are strong bases; those with amino or substituted amino groups possess
weak basic properties. A widely used anion exchange resin is prepared by
copolymerisation of styrene and a little divinylbenzene, followed by chloro-
methylation (introduction of the -CH,Cl grouping, Say, in the free para
position) and interaction with a base such as trimethylamine. A hypothetical
formulation of such a polystyrene anion exchange resin is given as (D).
-CH-CH2-CH-CH2-CH-CH2-
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