Hydrogen ions are spontaneously generated in pure water by the dissociation (ionization) of a small percentage of water molecules into equal numbers of hydrogen (H+) ions and hydroxide (OH–) ions. The hydroxide ions remain in solution because of their hydrogen bonds with other water molecules; the hydrogen ions, consisting of naked protons, are immediately attracted to un-ionized water molecules and form hydronium ions (H30+). By convention, scientists refer to hydrogen ions and their concentration as if they were free in this state in liquid water.
The concentration of hydrogen ions dissociating from pure water is 1 × 10-7 moles H+ ions per liter of water. The pH is calculated as the negative of the base 10 logarithm of this concentration:
pH=−log[H+]
The negative log of 1 × 10-7 is equal to 7.0, which is also known as neutral pH. Human cells and blood each maintain near-neutral pH.