Ion exchange is a process in which ions are exchanged between a solution and an ion exchanger, an insoluble solid or gel. Ion exchangers are either cation exchangers for positively charged cations or anion exchangers for negatively charged anions. Ion exchange is a reversible process and the ion exchanger can be regenerated or loaded by washing with an excess of the ions to be exchanged. Cation exchange membrane consists of negatively charged groups chemically attached to the polymer chains. Ions with a charge opposite to the fixed charge are freely exchanged at these sites. The concentration of counter ions cations is relatively high therefore cations carry most of the electric current through the membrane. The fixed charges attached to the polymer chains repel ions of the same charge. Since their concentration is very low, anions carry only a small fraction of the electric current through a cation exchange membrane.