Self-ionization of water

Self-ionization of water is when water (2H₂O) is dissociated into H₃O⁺ and OH^- ions. There is normally a very low percentage of H₃O⁺ and OH^- in water. The equilibrium constant for pure water at 25°C is 10^-14. Most water is somewhat self-ionized. A water deionizer recombines these naturally occurring ions into pure deionized water, albeit temporarily.