Ion-exchange and electrodialysis 157 the structure of the solid matrix in equilibrium with an outside phase. The interface between the two phases acts as a semipermeable membrane which allows the passage of any mobile ionic species depending on the Donnan equilibrium. This states that the chemical potential of a salt must be the same inside and outside the ion-exchanger-eg in the simplest case where the only mobile ions present are Na and CI, then at equilibrium, [Na'cI ]Inside phase=[Na][Cl outside phase Thus a certain proportion of co-ions(mobile ions having the same sign- Nat in this example-as the fixed ions) will be present even in the internal phase. Therefore, if an anion exchanger(as in Fig. 6. 1)is in equilibrium with a solution of NaCl, the internal phase contains some Na ions, although the concentration is less than in the external ase because the internal concentration of Cl ions is much larger When an ion-exchanger is contacted with an ionised solution, equilibration betweer the two phases rapidly occurs. Water moves into or out of the internal phase so that equivalent basis. If two or more species of counterion are present in the solution aar? osmotic balance is achieved Counterions also move in and out between the phases on an ill be distributed between the phases according to the proportions of the different present and the relative selectivity of the ion-exchanger for the different ions differential distribution of different counterions which forms the basis of separation by ion-exchange. The relative selectivity for different ionised species results from a range of factors. The overall charge on the ion and the molecular or ionic mass are the primary determining factors, but selectivity is also related to degree of hydration, steric effects and environmental factors such as pH or salt content In the adsorption stage, a negatively charged solute molecule(e.g. a protein P)is attracted to a charged site on the ion-exchanger(r)displacing a counterion(x) R+X-+P→R+P+X In the desorption stage, the anion is displaced from the ion-exchanger by a competing salt ion(S), and hence is eluted RtP+S-→Rts-+P lon-exchangers may be further classified in terms of how their charges vary, with in pH, into weak and strong exchangers. The terms strong or weak do not refer strength of binding of the ions to the exchanger, or the mechanical strength of the matrix but to the ph range over which the materials are effective. Strong ion-exchangers are nised over a wide range, and have a constant capacity within the range, whereas weak exchangers are only ionised over a limited ph range(e. g. weak cation exchangers may lose their charge below pH 6 and weak anion exchangers above pH 9). Thus exchangers may be preferable to strong ones in some situations, for example desorption may be achieved by a relatively small change in pH of the buffer in the region of the pka of the exchange group. Regeneration of weak ion-exchange groups is easier than with strong groups, and therefore has a lower requirement of costly chemicalsIon-exchange and electrodialysis 157 the structure of the solid matrix in equilibrium with an outside phase. The interface between the two phases acts as a semipermeable membrane which allows the passage of any mobile ionic species depending on the Donnan equilibrium. This states that the chemical potential of a salt must be the same inside and outside the ion-exchanger - e.g. in the simplest case where the only mobile ions present are Na' and C1-, then at equilibrium, [Na'] [Cl-lInside phase = iNa'l [C1-lOutside phase Thus a certain proportion of co-ions (mobile ions having the same sign - Na' in this example - as the fixed ions) will be present even in the internal phase. Therefore, if an anion exchanger (as in Fig. 6.1) is in equilibrium with a solution of NaC1, the internal phase contains some Na' ions, although the concentration is less than in the external phase because the internal concentration of C1- ions is much larger. When an ion-exchanger is contacted with an ionised solution, equilibration between the two phases rapidly occurs. Water moves into or out of the internal phase so that osmotic balance is achieved. Counterions also move in and out between the phases on an equivalent basis. If two or more species of counterion are present in the solution, they will be distributed between the phases according to the proportions of the different ions present and the relative selectivity of the ion-exchanger for the different ions. It is this differential distribution of different counterions which forms the basis of separation by ion-exchange. The relative selectivity for different ionised species results from a range of factors. The overall charge on the ion and the molecular or ionic mass are the primary determining factors, but selectivity is also related to degree of hydration, steric effects and environmental factors such as pH or salt content. In the adsorption stage, a negatively charged solute molecule (e.g. a protein P-) is attracted to a charged site on the ion-exchanger (R') displacing a counterion (X-): R+X- + P- -+ R'P- + X￾In the desorption stage, the anion is displaced from the ion-exchanger by a competing salt ion (S), and hence is eluted: R'P- + S- -+ R'S- + P￾Ion-exchangers may be further classified in terms of how their charges vary, with changes in pH, into weak and strong exchangers. The terms strong or weak do not refer to the strength of binding of the ions to the exchanger, or the mechanical strength of the matrix, but to the pH range over which the materials are effective. Strong ion-exchangers are ionised over a wide range, and have a constant capacity within the range, whereas weak exchangers are only ionised over a limited pH range (e.g. weak cation exchangers may lose their charge below pH 6 and weak anion exchangers above pH 9). Thus weak exchangers may be preferable to strong ones in some situations, for example where desorption may be achieved by a relatively small change in pH of the buffer in the region of the pKa of the exchange group. Regeneration of weak ion-exchange groups is easier than with strong groups, and therefore has a lower requirement of costly chemicals