Nature and Types of Ion Exchange Resins

Ion exchange resins are a common name for everyone who works in laboratories and industries. But despite being general, many people do not know what exactly these components are, how they work, and what their types are. If you are like them too, it is time to increase your knowledge about ion exchange resins. 

To assist you, we have brought here all the details about this component. With this article, you will be able to understand them thoroughly and use them accordingly to the particular use. So let’s begin with the details. 

What Is Ion Exchange Resin?

To understand ion exchange resins, you first need to learn what ion exchange means. It is a reversible chemical reaction. In this, a solution is taken, and its dissolved ions get eliminated to incorporate new ions of the exact same electric charge. This keeps the solution neutral while removing the specific ion from it. 

Ion exchange resin here is a physical medium that aids this ion exchange reaction. It is made from organic polymers that contain a hydrocarbon network, which further includes certain ion exchange sites.

The functional groups of cations (positive charge) or anions (negative charge) get attached to the polymer network on these sites. Finally, these functional groups attract the ions of opposite charge and facilitate the ion exchange reaction. 

This was all about the ion exchange resin mechanism. In simple terms, you can say that an ion exchange resin is a physical channel where the ion exchange reaction takes place readily. 

Ion Exchange Resin Physical Properties

The process from which an ion exchange resin is prepared is called polymerization. Different ion exchange resins incorporate distinct physical properties. However, there is a basic structure followed by most of them, i.e., cross-linking. This particular structure helps in providing the component with a strong and more resilient design. Plus, the capacity by volume also gets improved. 

The resins consist of a resin bed that further includes small and porous microbeads. You can also find sheet-like mesh resins in specific applications, such as electrodialysis. The present resin beads are spherical in shape and tiny in size, as their radius measures only between 0.25 to 1.25 millimeters.

These beads can have a uniform particle size, or their size may differ based on the Gaussian size distribution. This factor depends on individual applications of the resin. In most places, you will find translucent gel resin beads.

They are popular because of their high chemical effectiveness and capacity. However, there are also some macroporous resins. You can identify them from their opaque appearance and white to yellow color.

These variants are highly stable and have excellent chemical resistance. That is why they are mostly kept for certain demanding conditions where the gel beads can not be used. 

But the overall physical properties can further change based on the type of ion exchange resins that are chosen for the individual application.

Ion Exchange Resin Method

As their names implies ion exchange exchange resins find large number of applications in laboratories and industries. Primarily they find use for replacement of charged ions present in solution with those present in the resin.

The most common application is in water softening and treatment plants and several applications are there in liquid chromatographic determination of ionic species in pharmaceuticals, biological fluids, beverages and foods.

Ion exchange resins are capable of exchanging ions present in solutions coming in contact with them. At the same time they are insoluble and do not dissolve in such solutions.

Commonly a cross-linked entity such as polystyrene and divinylbenzene serves as the basic structure. Such cross-linking helps extend the useful life of the resin though slowing down the exchange of ions. In the laboratory they find use in removal of interfering ions for determination of the analytes of interest or to achieve concentration of ionic species of interest.

Types of Ion Exchange Resins

Resins can be classified into two main categories based on their charge and into four main classes depending on their strength. Cationic exchange resins have availability of positively charged ions whereas anionic exchange resins are used for exchange of anions. The four subdivisions are:

• Strong acidic cation exchange resins
• Weak acidic cation exchange resins
• Strong basic anion exchange resins
• Weak basic anion exchange resins

Strong Acidic Cation Exchange Resins

Strong acidic cation exchange resins contain a strong acid group which is available for exchange with metal ions in solution.The common acid used is sulfonic acid which is highly ionized both as acid (and as a salt.

The sodium salt helps exchange of cations such as Ca2+ and Mg 2+ in water when the resin gets exhausted it can be regenerated by allowing acid or NaCl solution to flow through the bed. The exchange capacity remains virtually unaffected over the entire pH range.

Weak Acid Cation Exchange Resins

The caroboxylic acid group replaces the sulfonic acid group in weak acid cation resins. The acidic group is weakly dissociated and the exchange capacity is pH dependent with higher activity in the alkaline range.

Strong Base Anion Exchange Resins

Like strong acid resins the strong base resins are also highly ionized and are effective over the entire pH range. Quaternary anionic groups such as trimethylammonium or hydroxyl groups find regular use.

They react with anions present in water and transform an acidic solution to a neutral solution. Regeneration with a strong alkali solution such as Sodium hydroxide regenerates the exhausted resin.

Weak Base Anion Exchange Resins

The degree of ionization of weak base resins is controlled by the pH of the solution. Above pH 7.0 the exchange capacity begins to decrease rapidly. They absorb acids and are not as effective in breaking down salts. Regeneration is carried out with weak bases such as sodium carbonate , ammonium hydroxide or amines.

It is important to know the common causes of fouling which leads to deterioration of performance of ion exchange resins. The topic will be covered in a subsequent article.