The simplest definition is to use the pH. In this case, "acidity" is related to proton concentration.

pH = - log₁₀[H⁺]

However, the acidic proton may be attached to the acid. The acid equilibrium is

HA « H⁺ + A^-

The acid, HA, looses a proton, H⁺, to become the conjugate base, A^-.

Mathematically, this equilibrium is given by the equilibrium equation

The "acid strength" is characterized by the pK_A given by the equation

pK_A = - log₁₀ K_A

Acids with pK_A less than 7 are said to be acidic (strong relative to water) while those with pK_A greater than 7 are said to be basic (weak relative to water).

Water acidity found by titration is used to determine the total concentration of protons.

If one titrates water with a strong base, the proton may react to form water

OH^- + HA « H₂O + A^-

The pH at the end-point of the titration is related to pK_A by

Titration to a methyl orange indicator end-point (pK_In = 4.3) is used to determine the strong mineral acid content.

Titration to a phenolphthalein indicator end-point (pK_In = 8.2) determines total acids.

In summary, there are three ways to characterize water acidity.