In liquid water there is some self-ionization giving hydronium ions and hydroxide ions.
The equilibrium constant for this reaction, known as the ionic product of water, , has a value of about  at 25 °C. At neutral pH, the concentration of the hydroxide ion () equals that of the (solvated) hydrogen ion (), with a value close to 10 mol L at 25 °C. See data page for values at other temperatures.
The thermodynamic equilibrium constant is a quotient of thermodynamic activities of all products and reactants including water:
However, for dilute solutions, the activity of a solute such as HO or OH is approximated by its concentration, and the activity of the solvent HO is approximated by 1, so that we obtain the simple ionic product