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.
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 anionic 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.