Ion exchange resins are polymers containing positive or negative ions within their structure. Most of them are in the form of beads and are capable of exchanging, without physical modification or alteration, their mobile ions with ions of the same sign contained in the same solution with which they are brought into contact. Ion exchange resins are classified in two categories according to the nature of the ions that will be exchanged. If it exchanges a cation, it is called a cationic resin and if it exchanges an anion, it is called an anionic resin. Each resin is characterized by an exchange capacity indicating the number of active ion sites available for exchange per unit volume. Once the exchange capacity is reached, the resin becomes saturated and must be regenerated to return to its original state.