The unidirectional migration of sol particles or dispersed phase particles or colloidal particles towards the oppositely charged electrode under the influence of the applied electric field is called electrophoresis or cataphoresis.
Cause Of Electrophoresis:
All sol particle (colloidal particles) carry the same electric charge either positive or negative. If an electric potential is applied across two platinum electrodes dipping in a sol, the sol particles move towards oppositely charged electrodes.
Consider a sol of As2S3 is taken in a ‘U’ shaped glass tube. The sol particles of the sol are negatively charged.
Now the dispersion medium with little quantity of electrolyte is introduced over the colloidal solution. There should be a sharp boundary between the sol and the dispersion medium.
Electric potential is applied across the two platinum electrodes dipped in a sol in two limbs, it is observed that the level of sol drops at the negative electrode and rises at the positive electrode side.
This shows that sol particles have migrated to the positive electrode, indicating that the particles are negatively charged.
If colloidal particles are allowed to reach the electrode, their charges are neutralised and coagulation takes place.
Applications of Electrophoresis:
Electrophoresis is used to detect the nature of charge on colloidal particles.
It is used in the removal of carbon particles from chimney gases.
It is used in electro-deposition of rubber on metal, wood or cloth surfaces from latex.
It is used to bring about coagulation of sol.
The migration of the dispersion medium of a colloidal solution under the influence of the electric field when the movements of colloidal particles are prevented is called as electro-osmosis.
Cause of Electro-osmosis:
Since the sol as a whole is electrically neutral, dispersion medium has an opposite electric charge as compared with that of the sol particles. If the dispersed phase has a positive charge we say that the dispersion medium has a negative charge.
A sol of As2S3 is filled in a glass tube. The sol particles of the sol are negatively charged. Hence the dispersion medium (water) is positively charged.
The colloidal solution and pure dispersion medium in a glass tube are separated by a semipermeable membrane.
When an electric potential is applied across the platinum electrodes dipping in each arm, sol particles cannot pass through the semipermeable membrane but dispersion medium (water) move to the negative electrode through the semipermeable membrane. The level of sol drops at the +ve electrode and rises at -ve electrode.
This movement of dispersion medium towards -ve electrode shows that the charge on the dispersion medium is positive.
Applications of Electro-osmosis:
Electro-osmosis is used in dewatering of moist clay