The concentration of water is constant in aqueous solutions at 55.5M and does not appear in the equilibrium expression for water. When dealing with an acid, the concentration of hydrogen ions (or hydronium ions) is relevant. The measurement of this quantity, called the pH value, is defined as
Likewise, the measure of the concentration of hydroxide ions is relevant when a base is involved in a reaction. The pOH value is defined as the negative log of the concentration of the hydroxide ions.
