Ion Exchange Resins: Cation exchange resins and anion exchange resins

Ion exchange resins are porous polymers containing charge groups which are capable of holding positive or negative ions. It is defined as “the reversible exchange of ions between a solid phase (ion-exchange material) and a liquid phase in which there is no permanent change in structure. In general word, ion exchange resin is a polymeric material that functions as an ion exchange medium. These possess an open permeable molecular structure that allows ions and solvent molecules to pass freely in and out and are usually insoluble in water.

The free mobile ions of a solid called an ion exchanger are exchanged for different ions of similar charges present in the solution. The exchanging power of an ion increases with an increase in the charge on the ion and vice versa. Mostly, ion exchange resins are used for separation and purification. Water softening, water purification, etc. are some of the applications of ion exchangers.

Ion exchange method is the most successful and most rapid method for the separation of lanthanides.

Properties of Ion exchange resins (Ion Exchangers)

Some of the common properties and requirements of ion exchangers are:

  • They are insoluble in water and organic solvents like ether, carbon tetrachloride (CCl4), benzene, etc.
  • They are complex, porous, and polymeric in nature.
  • They should have a property to exchange ions actively with the ion present in the solution without any change in the given structure.
  • They should be able to easily escape and permit the ions.

How does ion exchange resin works?

For the separation of metals, a solution of lanthanides is run down a column of synthetic ion-exchange resin bearing -SO3H groups. The Ln3+ ions replace the H+ ions in the -SO3H groups. As a result, Ln3+ ions get attached to the resin and H+ ions get eluted through the column.

ion exchange resins

The resin is then passed through a buffered solution of citric acid/ammonium citrate (eluent) at PH 5.5. The Ln3+ ions are gradually removed from the resin by this elution. As with their citrate complexes, the Ln3+ ions are complicated. As a result, the lighter members are eluted last and the heavier ones first. Hence, these smaller ions come out from the column first, one by one.

Because the ions leave the column at various rates, separation is feasible. Each container receives a portion of the solution that is concentrated in a certain metal as it flows out of the column in various portions and is collected in various containers. The metal ion precipitates as its oxalate upon addition of ammonium oxalate solution to each solution component, which upon heating provides the oxalate.

ion exchange resin

Types of Ion exchange resins

Based on the charge groups of polymeric materials, ion exchange resins are of two types:

  1. Cation exchange resins
  2. Anion exchange resins

In cation exchange resins, the active ions are cations, whereas, in anion exchange resins, they are anions. As a result, cation exchange resin has active cations and a polymeric anion, whereas anion exchange resin has active anions and a polymeric cation.

Cation exchange resins (Cation Exchanger)

Cation exchange resins are high molecular weight polymers containing active cations. The polymers usually contain functional groups like sulphonic, carboxylic, phenolic groups, etc.

A cation exchange resin is indicated by (Res. A)B+, where Res is the resin’s basic polymer and A and B+ are its attached anion and mobile cation, respectively. The mobile or active cation of a cation exchange resin exchanges with cations in the solution when it is added to a solution.

cation exchange resins

The cation C+ from the solution is totally fixed on the cation exchanger and equilibrium is completely moved from left to right according to the experimental parameters. The exchanger may exhibit various affinities for the metal ions, allowing separation in solutions that comprise salts of two or more metal ions.

Example: Displacement of mobile ions from Dowex-50 by calcium ions

The calcium ion may be eliminated from the resin and its original sodium form may be regenerated by passing a solution containing sodium ions.

sulphonated polystyrene
Figure: Structure of sulphonated polystyrene cation exchange resin

One of the common applications of cation exchangers is the separation of lanthanide ions via cation exchange chromatography.

Anion exchange resins (Anion Exchanger)

Anion exchange resins are cross-linked, high-molecular-weight polymers containing active anions. It contains basic groups such as amino, substituted amino, or quaternary ammonium groups as an essential component of the polymer lattice and an equivalent quantity of anions chloride, hydroxyl, or sulfate ions.

Anion exchange resin is indicated by (Res. B+)A. It contains free anions and can be exchanged for anions in solution.

In terms of action, strongly basic anion exchange resins resemble strongly sulphonate cation exchange resins, such as quaternary ammonium polystyrene. Both salt and hydroxide ions are ionized in anion exchange resins.

structure of anion exchange resin
Figure: Structure of anion exchange resin (quaternary ammonium polystyrene)

How to regenerate ion exchange resin?

Ion exchange resin can be carried out in the following steps:

  • Co-flow regeneration to remove suspended solids and decompact the bed.
  • Inject diluted regenerant into suitable water. The contact time ranges from 20 to 40 minutes due to the low flow rate of the injection.
  • Use dilution water to replace the regenerant at the same flow rate.
  • In order to achieve the appropriate treated water quality, rinse the bed with feed water at service flow rates.

Note: These are valid for ion exchange columns.

Applications of Ion-exchange method

In recent years, ion exchange methods have become quite popular and have numerous applications in analytical chemistry as well as in industry. Some of the applications are:

  • Used for softening or deionizing water.
  • For the separation of a mixture of alkali and alkaline earth metals using EDTA solution.
  • Cation exchange resin is used to separate lanthanides.
  • It offers easy separation from the products and has the ability for re-use.

Ion Exchange resins Video


How does ion exchange resin soften water?

In order to cover the surface of the sulfonated polystyrene beads in the softener’s microporous exchange resin, the beads are typically supersaturated with sodium. Calcium and magnesium ions bind to the resin beads when water flows through them, and the sodium that has been trapped in the resin but is not tightly bound is released into the water.

Ion exchange resin softener

Ion-exchange softeners involve the exchange of one ion from the water being treated for another ion in the resin.

Dowex ion exchange resin

Dowex ion exchange resins are the polymers that permit the exchange of ions with ions in a solution.

What is ion exchange resin?

Ion exchange resin is the reversible exchange of ions between a solid phase (ion-exchange material) and a liquid phase in which there is no permanent change in structure.


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