Interfacial potential differences can only be interpreted in terms of a physical reality between phases of identical chemical composition. Many substances acquire a charge when immersed in water and migrate under an applied
electric field. This is usually interpreted in terms of an ionic double layer at the surface and the zeta potential is defined as the potential at the plain of shear. It is doubtful if the concept of the zeta potential is of assistance in describing electrokinetic phenomena as interfacial potentials are only physically meaningful in a few carefully proscribed instances. Instead, in electrophoresis, for example, all the necessary observed experimental data to define the system should be recorded and a comparison made of mobilities, other parameters being kept constant.