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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 25C is 10-14. Most water is somewhat self-ionized. A water deionizer recombines these naturally occurring ions into pure deionized water, albeit temporarily.