Circular 79—Design of an apparatus for determining isoelectric point of charge

By R. A. Deju and R. B. Bhappu, 1965, 7 pp., 1 table, 4 figs.

When a potential difference is externally applied to a solution containing suspended particles, it causes a migration of the particles to the pole which carries a charge opposite to that of the particles. If they are small ions, the phenomenon is called ionic conductance; if they are large molecules, such as proteins, it is called electrophoresis; and if they are of colloidal size, it is called cataphoresis.

The zero point of charge for a solid in contact with a solution is determined by the concentration of potential determining ions in solution. At this critical concentration there is no net transfer of free charges, and the electrical double layer is absent. H+ and OH- are potentially determining ions for quartz and some other minerals, but for many other minerals they don't act as such. If the zero charge is a function of H+ and OH- concentration, then zero charge can be expressed in terms of pH. For example, Fuerstenau found the zero charge of quartz to be pH 3.70.

If a mineral particle is placed in an electric field, the particle will move in one direction and the diffuse layer of counter ions will move oppositely. The potential of the moving particle is called zeta(z) potential and causes the diffuse layer to keep reforming. Therefore, the particle continues to appear the same. The moving particle carries with it a very thin liquid film which contains the solvated ions in the Stern plane.

In general, the zeta potential can be regarded as the potential difference in an otherwise practically uniform medium between a point some distance from the surface and a point on the plane of shear. If the applied potential is held constant, the relation between the velocity of the particles and the zeta potential can be given by Henry's equation.