Water itself acts as an acid and a base. Water can act as a base by finding a proton to form a hydronium ion or it can act as an acid by releasing a proton to produce the hydroxide ion.

Because water can act as an acid, aqueous solutions always have a concentration of protons. A pure solution of water has 10^-7 moles per liter of protons. Very acidic aqueous solutions such as lemon juice may have concentrations of protons as high as 10^-2 and basic solutions such as milk or magnesium may have concentrations of only 10^-10 protons.

Because of the wide range of possible hydrogen concentrations, Soren Sorensen purposed a scale describing the acidity of solutions by the negative log of the proton concentration. This scale of acidity allows chemists to efficiently describe the proton content of a solution.