Elution and choice of eluent

Elution is the recovery of sorbed ions from the resin by washing with a displacing electrolyte solution. The reagent used for elution is called the eluent, and the effluent emerging from the column during elution is called the eluate. In principle any electrolyte will displace any ion, even when the selectivity coefficient is unfavourable, provided enough of it is used in 

Sorbed ions that are salts of strong acids or bases can be eluted from strongly acidic and basic resins only with large volumes of eluent. Hydrochloric acid is effective for both. A molar solution is generally used, and 30 or more bed-volumes may be needed (one bed-volume is simply the apparent volume of the resin bed). Much smaller volumes of eluent will be needed if the separation can be done with a weakly acidic or basic resin, because elution with IM hydrochloric acid or 2M ammonia rapidly converts the resin to the free acid or base. Since these are virtually nonionic, the sorbed ions fall off, so to speak, and complete elution can be achieved with five bed-volumes or even less. The eluate then contains all the exchangeable ions that were in the column. Similarly, if the sorbed species are anions of weak acids or cations of weak bases, the same eluents will render them virtually nonionic so that they are no longer bound to the resin. 

Some eluates, e.g. acid eluates containing alkyl sulphate, may need to be neutralised before evaporation of the eluate to dryness to avoid decomposing the surfactant(s) they contain. 

Whatever eluent is used, it is desirable that it should use the same 

Solvent composition is important and can be crucial. MacDonald et al, [3] showed significant variations in eluent efficacy with concentration of methanol in both acid and alkaline eluents. 90% methanol was best for approximately IM hydrochloric acid and 80% methanol was best for 

---

The distinction between strong and weak acids and bases is important when one intends to recover the exchanged ions (see section 4.5.4 on Elution and choice of eluent), and the same concept applies equally to... 

---