with HB
+ representing the protons in the bulk of the solution. From these chemical
reactions it is clear that an originally neutral surface hydroxyl site can bind a proton
from the bulk solution, becoming a positive site as well as donate a proton to the
solution, leaving a negative site on the oxide surface. For this reason it is called an
amphoteric site.
For these equilibrium reactions, the following set of thermodynamic equations is
valid: