where the change in aHS
+ 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