Info

(^mol dm~3)

Silicone tubing

1 ± 0.4

Latex tubing

16 ± 4.7

PVC tubing

14 ± 1.0

Polyethylene tubing

60 ± 2.0

Polyethylene test-tube

70 ± 3.8

Polyethylene stopper

77 ± 5.3

Plexiglass chippings

74 ± 4.3

Teflon chippings

81 ± 4.4

Polystyrene test-tube

89 ± 1.2

a Reproduced from G. Krieglstein et al., Arzneim. Forsch., 22, 1538

8.5.2 Ion-exchange resins

Synthetic organic polymers comprising a hydrocarbon crosslinked network to which ionisable groups are attached have the ability to exchange ions attracted to their ionised groups with ions of the same charge present in solution (Fig. 8.26). These substances, usually prepared in the form of beads, are ion-exchange resins and are insoluble in water, the aqueous phase diffusing into the porous resin beads. Because ions must diffuse into and out of the resin for exchange to occur, ions larger than a given size may be excluded from reaction by altering the nature of the crosslinks in the polymer. The resins may be either cation exchangers in which the resin ionisable group is acidic, for example, sulfonic, carboxylic (XVI) or phenolic groups, or anion exchangers in which the ionisable group is basic, either amine or quaternary ammonium groups.

The equations describing the equilibria involved are

Cation-exchange resin

Anion-exchange resin

The equilibrium constant for the cation exchange resin is

However, application of equation (8.31) is impossible because of the inaccessibility of the terms [POL-B+] and [POL-A+]. Some estimation of a resin's affinity for ions can be made using a standard ion such as lithium for cation e

Figure 8.26 Schematic diagram of a cation-exchange resin framework with fixed exchange sites prior to and following an exchange reaction: (a) initial state prior to exchange reaction with cation B +; (b) equilibrium state after exchange reaction with cation B+.

Modified from W. J. Weber, Physicochemical Processes for Water Quality Control, Wiley, New York, 1972.

Figure 8.26 Schematic diagram of a cation-exchange resin framework with fixed exchange sites prior to and following an exchange reaction: (a) initial state prior to exchange reaction with cation B +; (b) equilibrium state after exchange reaction with cation B+.

Modified from W. J. Weber, Physicochemical Processes for Water Quality Control, Wiley, New York, 1972.

Sulphonic acid ion-exchange resin

— CH —CH2—CH — CH2 — CH —CH2—CH —CH2

Sulphonic acid ion-exchange resin

— CH —CH2—CH — CH2 — CH —CH2—CH —CH2

Carboxylic acid ion-exchange resin

Carboxylic acid ion-exchange resin

CH3 CH3 CH3 I I I -C-CH2-CH — CH2— C — CH2— C-CH2-

CH3 CH3 CH3

— CH2—CH — C — CH2— CH — CH2— C — CH2— C — CH2—

CH3 CH3

(COO^ (COOH) Structure XVI Sulfonic acid and carboxylic acid ion-exchange resins exchange resins. A selectivity coefficient, k, may be defined as

resin

solution k=-

solution

Even here there is a problem arising from the difficulty in the determination of the activity of the ions in the resin (because of the complexity of the environment) and the overall concentration of ion is generally used instead.

The ability of a resin to exchange one ion for another depends on its affinity for the ion and the concentration of ions in solution. Cation-exchange resins tend to have affinity in decreasing order for calcium, potassium, sodium, ammonium and hydrogen ions. Administered orally, cation-exchange resins effect changes in the electrolyte balance of the plasma by exchanging cations with those in the gut lumen. In the ammonium form, cation-exchange resins are used in the treatment of retention oedema and for the control of sodium retention in pregnancy. Depletion of plasma potassium can be prevented by including a proportion of resin in the form of the potassium salt. These resins are also used (as calcium and sodium forms) to treat hyperkalaemia.

Anion-exchange resins such as polyamine methylene resin and polyaminostyrene have been used as antacids.

Some pharmaceutical ion exchangers are listed in Table 8.10.

Apart from these medical uses, ionexchange resins are used in the removal of

Table 8.10 Ion exchangers used

in pharmacy

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