The one pK and two pK models of the metal oxide-water interface were examined by using chemical thermodynamics. The concentration units, the standard states, and the reference states in both models were compared. It was shown that the chemical thermodynamics can give a unified description of protonation and deprotonation of surface of hydrous oxides and that a proper understanding of the choices of different standard states is at the heart of the comparison between one pK and two pK models. The inner meaning of intrinsic acidity constants in the two pK model was discussed.