where the change in a H +S is expressed in its corresponding pHs, and the resulting change in σ0 in the change in the net number B of basic groups. By definition, ∂B/∂pHs represents the buffer capacity βint of the oxide surface. It is called intrinsic in this case, because it is only capable of buffering small changes in the surface pH pHs) and not in the bulk pH (pHb), under normal conditions. For reasons of charge neutrality the surface charge σ0 is balanced by an equal but opposite charge, σdl, in the electrolyte. The position of this charge density σdl defines the so-called double layer and the two opposite charges, σ0 and σdl, parallel to each other form the integral double-layer capacitance Cdl,i. The potential difference over the capacitance Cdl,i is equal to the difference between the potential of the oxide surface and the bulk solution, as defined in eqn. 18: ψ0=ψS- ψB. The relation between σ0, σdl, Cdl,i and ψ0 is given by