Question

Ion Exchange Chromatography

Explain how the Cu²⁺ ions interact with H⁺ ions in the resin.

Explain how the Mg²⁺ ions elute the Cu²⁺ ions from the resin.

Answer 1

Ion chromatography (IC) is an analytical technique for the separation and determination of ionic solutes in water.

The mechanism of interaction of the solutes with the stationary phase determines the classification of the mode of liquid chromatography. In ion chromatography the basic interaction is ionic. The stationary phase is charged due to fixed anions or cations, which are neutralized by counter ions of the corresponding opposite charge. The counter ions can be exchanged by other ions either from the mobile phase or from the sample, hence the name ion-exchange chromatography.

Ion-exchangers are characterized both by the nature of the matrix used as a support and the nature of the ionic functional groups on the surface.

Cation-exchangers functional groups can function as such only when they are ionized, therefore they are classified into strong acid and weak acid types accordingly. The strong acidic functional groups are ionized over a wide pH range.

As a rule, an increase of the charge-density (charge / solvated size) of the solute ion increases, its affinity for the stationary phase. Higher charge with smaller solvated ion radius result in higher retention due to higher coulombic interactions.

So The Cu²⁺ ion will displace the H⁺ ion in the stationary phase due to the higher charge and Mg²⁺ (ionic size 86 pm) will be able to displace Cu²⁺ (ionic size 87 pm) due to it ionic size being smaller than that of Cu²⁺