Older equations for electrokinetic phenomena are often based upon the non-rationalized three-quantity electrostatic system. Transition to the internationally accepted rationalized four-quantity system (using S.I. units) is, however, recommended36.
Electrophoresis is the motion of colloidal particles in an electric field. The term cataphoresis should be abandoned.
Electrophoretic velocity is the velocity of a particle during electrophoresis, symbol .
Electrophoretic mobility is the electrophoretic velocity per unit field strength, symbol or ; is positive if the particle moves towards lower potential and negative in the opposite case. According to the first paragraph of this section, the field strength should be expressed in (old: electrostatic c.g.s. units (statvolt) )37.
Electrodeposition (which includes electro-crystallization) is the deposition of dissolved or suspended material by an electric field on an electrode.
Microscopic electrophoresis is the technique in which the electrophoresis of individual particles is observed with the aid of a microscope or ultra-microscope. This has been often referred to as microelectrophoresis, but it is recommended that the latter term be abandoned in view of likely confusion with the following definition.
Microelectrophoresis is the electrophoresis technique involving the movement of a mass of particles on a small scale (e.g. paper electrophoresis).
Electro-osmosis is the motion of a liquid through a membrane (or plug or capillary) as a consequence of the application of an electric field across the membrane. The spelling of electro-osmosis with two o's is preferred to electrosmosis with one o and to the older term electro-endosmosis.
Electro-osmotic velocity per unit field strength ( e.o or ) is the linear velocity of flow.
Electro-osmotic volume flow per unit field strength, , is the volume flow per unit time through the whole plug; and are positive if the flow is in the direction of lower potential.
Electro-osmotic pressure, , is the pressure difference across the membrane, plug, etc., needed just to stop electro-osmotic volume flow. is positive if the higher pressure is on the high potential side.
Streaming potential difference (streaming potential), st or , is the potential difference at zero current caused by the flow of liquid under a pressure gradient through a membrane, plug or capillary. Identical electrodes must be used on both sides of the membrane, plug, etc. is positive if the higher potential is on the high pressure side.
Streaming current, , is the electric current flowing in a streaming cell if the electrodes, which are supposed to be ideally depolarized, are short-circuited. is positive if the current in the membrane, plug, etc., is from high to low pressure side (and in the outside lead from low to high pressure side).
Sedimentation potential difference (sedimentation potential) (also called Dorn effect), sed or , is the potential difference at zero current caused by the sedimentation of particles in the field of gravity or in a centrifuge, between two identical electrodes at different levels (or at different distances from the centre of rotation). is positive if the lower (peripheral) electrode is negative.
Sedimentation field strength, sed, is the potential difference per unit length in a sedimentation or centrifugation cell. As the contributions of the interfacial potential differences at the electrodes are not included in sed this quantity, although theoretically important, is not accessible to measurement.
Surface (excess) conductivity is the excess conductivity in the surface per unit length and width, symbol 38.
Electrokinetic potential (zeta potential), , is the potential drop across the mobile part of the double layer, that is responsible for electrokinetic phenomena. is positive if the potential increases from the bulk of the liquid phase towards the interface. In calculating the electrokinetic potential from electrokinetic phenomena it is often assumed that the liquid adhering to the solid wall and the mobile liquid are separated by a sharp shear plane39. As long as there is no reliable information on the values of the permittivity, , and the viscosity, , in the electrical double-layer close to the interface, the calculation of the electrokinetic potential from electrokinetic experiments remains open to criticism40. It is therefore essential to indicate in all cases which equations have been used in the calculation of . It can be shown, however, that for the same assumptions about and , all electrokinetic phenomena must give the same value for the electrokinetic potential41.
A consistent use of signs requires that the electrophoretic mobility and the streaming potential difference st have the same sign as the electrokinetic potential, but the electro-osmotic velocity e.o. and have a sign opposite to that of .