Origin of charge on surfaces

A great variety of processes occur to produce a surface charge, and these are summarised below. 

These are ions that have such a high affinity for the surface of the particles that they may be taken as part of the surface, e.g. Ag+ and 1 for Agl. For Agl in a solution of KNO3, the surface charge co is given by the expression 

Similarly, for an oxide such as silica or alumina in KNO3, FI+ and OH may be taken as part of the surface, 

The ions that determine the charge on the surface are termed potentialdetermining ions. 

The charge depends on the pH of the solution Below a certain pH the surface is positive and above a certain pH it is negative. At a specific pH (Fh = Foh) surface is uncharged this is referred to as the point of zero charge (p.z.c.). 

---