Applications

Two techniques are employed for practical ion-exchange processing in laboratories: the batch method, and the column method.

Batch Method

Batch processing by ion exchange is favorable when reactions must be performed in a closed system and the inflow of new substances through the resin is technically impossible, such as for catalysis purposes. In this technique, the ion exchange resin and solution are mixed in a batch vessel, the exchange is allowed to come to equilibrium, then the resin is filtrated off from the solution, washed and regenerated in a special system.

Column Method

In both analysis and process, passing a solution through a column containing a bed of exchange resin is analogous to treating the solution in an infinite series of batch vessels. Hence, the separation is equivalent to that achieved in a batch process. The filling of the column is easily done and requires only a few practical steps. First, the ion exchanger is mixed with distilled water in a beaker. In general, two hours are sufficient for swelling. Next, the ion exchanger is slurried into the column to obtain a uniform column packing. Thereby, the ion exchanger must be completely covered by an aqueous layer in order to prevent air bubbles, and excess water should be constantly removed. A cotton ball is placed on the top of the column, and the column is washed several times with distilled water.

Stages of Ion Exchange Working Cycles

• Ion exchange
• Ion exchange column packing wash
• Regenerating or separate elution

Washing steps are necessary in order to remove excess feed solution. Regeneration brings the ion exchanger to its previous ionic form. This means that the resin is flushed free of the newly-exchanged ions and mixed with a solution of the ions to replace them. Alternatively, it is also possible to elute and collect the exchanged ion.

Application Fields

• Chelating ion exchangers for trace enrichment of metal ions
• Determination of total salt content of solutions and water by exchange of hydrogen ion
• Removal of interfering cations or anions
• Chromatographic separation
• Disintegration of compounds difficult to solve
• Ion exchangers as catalysts

Chelating Ion Exchangers for Trace Enrichment of Metal Ions

Especially in the field of inorganic trace analysis, it is possible to concentrate traces of metal ions from strongly diluted solutions. Chelex® -100 is a chelating ion exchanger based on a styrene-divinylbenzene copolymer containing iminodiacetic acid groups. The ion exchanger resin prefers chelating di- and polyvalent cations. Its ability to bind the metal ions is governed by pH value. Optimum results are achieved in a pH range of 4 to 7. After concentrating the metal ions on the ion exchanger, they are eluted from the resin with 5% nitric acid, which protonates the iminodiacetate groups. The column technique is recommended for this chelating ion chromatography.

Determination of Total Salt Content of Solutions and Water by Exchange of Hydrogen Ion

For the determination of total salt content, the salt solution is applied onto a highly acidic cation exchanger and the generated acid in the eluate is then titrated. The pre-requisite is that the cations are quantitatively exchanged for H⁺ ions and the generated acid could be alkalimetrically titrated. This technique can be applied to all solutions that contain chloride, bromide, iodide, nitrate, perchlorate, sulphate, phosphate, bromate, iodate, periodate, borate, acetate, or oxalate ions.

Removal of Interfering Cations or Anions

Occasionally, the presence of cations interferes with the determination of anions. These cations can be removed by means of a strongly acidic cation exchanger. Subsequently, anions can be determined in the eluate.

Quantitative Determination of Sulfate in Water acc. to DIN

Cations are exchanged for H⁺ ions with a strongly acidic cation exchanger. Standard barium chloride solution is added in excess to react with the sulfate, and the non-consumed quantity of barium chloride is complexometrically back-titrated. This test can be used for sulfate determination in drinking, waste and surface water with concentrations > 5 mg sulfate/L. Samples with lower sulfate concentration should be evaporated.

Quantitative Determination of Nitrate in Water acc. to DIN

The colorimetric determination of nitrate with the German sodium salicylate method is interfered by iron. In this case as well, the use of a highly acidic cation exchanger removes the cation prior to analysis.

Photometric Determination of Fluorides in Potable Water Using Lanthanum-alizarin-complexane After Ion Exchange Separation of Interfering Ions

The fluoride ions tend to form stable complexes. Because of this, cations must be removed in tap water prior to analysis. According to a technique from KEMPF, the interfering ions are exchanged with a highly acidic cation exchanger. Subsequently, the fluorides are mixed with the reagent, and the "alizarinfluorine-blue complex" generated is determined photometrically.

Chromatographic Separations

It is possible to chromatographically separate dissolved cations or anions by means of ion exchange resins. The separation principle is determined by the affinity of the ions to the ion exchanger. This selectivity depends on type of charge, charge, size and form of the ions to be exchanged. Elution takes place by a step-wise gradient of acidic or basic eluents. Very often, to improve separation efficiency, complexing agents, such as ethylene diamino acetic acid (Titriplex III), tartaric or citric acid, are used as eluents.

Ion Exchange Resins for Disintegration of Salts Which Are Difficult to Dissolve

Aqueous slurries of hardly soluble salts can be dissolved with solid ion exchangers in batch mode if the solubility product of the salts are not too low. Due to the higher reaction speed, cation exchangers supplied in H⁺ ions are particularly suitable for this purpose. During the ion exchange process, protons are constantly generated, thus increasing the solubility. This technique is applicable for the dissolution of the phosphates of calcium, strontium, barium, manganese, zinc, cobalt and nickel, as well as for the sulfates of calcium, strontium, barium and lead at elevated temperatures.

Ion Exchangers as Acid/Base Catalysts

It is well known that acids or bases are used as catalysts for many organic reactions, such as esterifications, hydrolyses, condensations, polymerizations, dehydrations, cyclizations, and rearrangements. Strongly acidic resins supplied as H⁺ ions are frequently used as strong acid catalysts instead of soluble acids. They show similar catalytic activity to sulfuric acid in esterifications, epoxidations, hydrolyses, phenol alkylations and other acid catalyzed reactions. Weakly acidic cation exchangers are not applicable for catalytic purposes because the functional ionic site is not highly dissociated. For base-catalyzed reactions, strongly basic and medium basic anion exchangers supplied as OH- ions can be applied.

Advantages of Exchange Resins Versus Catalysts Dissolved in the Homogeneous Reaction Phase

• Easy separation of the catalyst from the reaction mixture by simple filtration from the reaction product/mixture. Resin catalysts used in column mode, facilitate the use of a more continuous process.
• Very often, resin catalysts may directly be reused repeatedly without regeneration.
• Greater selectivity of the reaction direction is often possible using ion exchange resins as catalysts. Side reactions are sometimes reduced or eliminated. In some cases, it is possible to isolate reaction intermediates not obtainable with soluble catalysts.
• In general, resin catalysts prevent the contamination of the reaction product with impurity ions. Furthermore, they prevent interfering side reactions so that the reaction products exhibit a unique purity. They increase the yield in comparison to a reaction with soluble catalysts in a homogeneous phase.
• Ion exchange resins are easier to handle than their soluble counterpart.
• Waste water contamination is also drastically reduced.

Most Important Ion Exchanger Catalyzed Reactions

• Esterifications
• Condensations
• Aldol condensations
• Oligomerization
• Cyanhydrin synthesis
• Nitration
• Inversion of sugars
• Hydrolysis
• Hydrations
• Polymerizations 
• Alkylations
• Formation of acetates
• Epoxidations
• Rearrangement reactions

Product Portfolio

• Ion exchanger I
• Ion exchanger II
• Ion exchanger III
• Ion exchanger IV
• Ion exchanger V
• Ion exchanger Amberjet® 4200
• Ion exchanger Amberlite® IR-120 (H+ form)
• Ion exchanger Amberlite® IR-120 (Na+ form)
• Ion exchanger Amberlite® IRA-67
• Ion exchanger Amberlite® IRA-402
• Ion exchanger Amberlite® IRA-410
• Ion exchanger Amberlite® IRN-150
• Ion exchanger Amberlite® MB-3
• Ion exchanger Amberlite® MB-6113
• Ion exchanger Amberlyst® 15
• Ion exchanger Dowex® 50 WX 4
• Ion exchanger Dowex® 50W-X8
• Ion exchanger Dowex® 1-X8
• Adsorber resin Amberlite® XAD-4