For the case of ionic crystals it is shown to be most straightforward and consistent to define acidity/basicity by the (electro-)chemical potential of the respective ion, in a similar fashion to the way that the Fermi level (i.e.. electrochemical potential of the electron) characterizes the redox state. The isomorphy is explicitly expressed by using the energy-level diagrams introduced for electrons in semiconductor physics. Without having to make further assumptions it is possible 1) to compare acidity/basicity between different solids, 2) to link internal and surface acidity/basicity, and 3) to establish acidity/basicity scales for ionic solids. The point defects are revealed to be the natural acidic and basic elementary centers, and associates between them to be the internal acids/bases exchanging these elementary centers. Even though acidity/basicity is an overall property of the solid, the number of point defects (if dilute) directly represents these properties in the same way as H+ or OH- accomplish this for aqueous solutions.