Acid Dissociation Constant of Acetic Acid

Acetic acid is the standard weak acid: at 25 °C only about 0.42 % of the molecules in a 0.10 M solution have given up their proton, which is why that solution sits at pH 2.87 instead of the pH 1.00 a strong acid would give. The constant K_a = 1.75 × 10⁻⁵ is what encodes that reluctance, and combined with its conjugate base acetate it gives the acetate buffer that holds pH between roughly 3.8 and 5.8 — the working range for protein purification, electrophoresis and countless enzyme assays.

Household vinegar is 5 % acetic acid, about 0.83 M, with a pH near 2.4. Note that K_a is temperature-dependent like every equilibrium constant, though weak acids drift far less than water's autoionisation: acetic acid's K_a actually passes through a maximum near 22.5 °C and falls slightly in both directions, a consequence of an enthalpy of ionisation that is close to zero.