Electrical Properties of Colloidal Solution (Electrophoresis and Electro-Osmosis)

(I).    Electrophoresis

                When a potential difference (electric field) is applied across two platinum electrodes immersed in a colloidal solution, the particles of dispersed phase move towards either the positive or negative electrode. This observation was first discovered by Rauss in 1807 and was investigated later by Linder and Picton.

The movement of colloidal particles under the action of electric field is known as Electrophoresis.
If the colloidal particles move towards the positive electrode (Anode) they carry negative charge. On the other hand if the sol particles migrate towards negative electrode (Cathode), they are positively charged. From the direction of movement of colloidal particles it is possible to find out the charge on colloidals.

Demonstration of Electrophoresis
                 The demonstration of electrophoresis is as follows:-
Take a colloidal sol say AS2S3 sol in a U – tube. Place an electrolyte, having density less than that of solution (say distilled water). The electrolyte provides distinct boundary between electrolyte and colloidal sol.
Place two platinum electrodes in two arms of U – tube such that they dip in the colloidal sol. When a high potential difference of about 100 volts is applied across the two platinum electrodes, it is observed that the level or Boundary of colloidal solution falls on the negative electrode side and rises up on positive electrode side. On reaching the positive electrode, the colloidal particles get discharged. As a result of neutralisation of charge, the colloidal particles aggregate and settle down at the bottom.

Electro-Osmosis
A colloidal solution as a whole is electrically neutral in nature i.e., dispersion medium carries an equal and opposite charge to that of the particles of dispersed phase. When the movement of dispersed phase of colloidal solution is prevented by suitable means, the dispersion medium can be made to move under the influence of an applied electric field or potential. This phenomenon is referred to as Electro-Osmosis. Thus electro-osmosis may be defined as the movement of the dispersion medium under the influence of an applied electric field when the particles of dispersed phase are prevented from moving.

Demonstration of Electro-Osmosis

The phenomenon of electro-osmosis can be demonstrated experimentally as follows:-
The demonstration of electro-osmosis is carried out in a specially designed apparatus. The apparatus consists of a bigger tube having two side tubes T and T/ attached to its ends. The bigger tube is divided into three compartments A, B and C by means of two semi-permeable membranes. A tube carrying a stop-cock is attached to the central compartment A. Two platinum electrodes are inserted in the outer compartments B and C.
A colloidal dispersion is placed in the central compartment A and the outer compartments B and C are filled with water. The water in compartments B and C also extends to the side tube T and T/. The function of membrane is to prevent the movement of colloidal particles. Therefore, when a potential difference is applied across the electrodes held close to the membranes in the compartment B and C, dispersion medium begins to move.

If the particles carry positive charge, the dispersion medium would start moving towards the anode and the level of water in the side tube T would be seen to rise, indicating the presence of negative charge on the dispersion medium. If the particles carry negative charge, the dispersion medium would be seen to move towards cathode and water in the side tube T would start rising.
Electro osmosis is utilizing for dewatering moist clay and drying of dye pastes.