The migration of dispersion medium under the influence of an electric potential is called :

The question asks: "The migration of dispersion medium under the influence of an electric potential is called :" with options A. Cataphoresis, B. Electroosmosis, C. Electrophoresis, D. Sedimentation.

First, recall that a colloidal system has two parts: the dispersed phase (the particles) and the dispersion medium (the continuous phase, usually a liquid). The question focuses on the movement of the dispersion medium when an electric potential is applied.

Now, examine each option:

Option A: Cataphoresis - This term is often used interchangeably with electrophoresis, referring to the movement of positively charged particles (cations) towards the cathode. It involves the migration of the dispersed phase, not the dispersion medium. So, this is incorrect.

Option B: Electroosmosis - This phenomenon occurs when an electric field is applied across a porous material or a capillary tube. The dispersion medium (liquid) moves relative to a stationary charged surface. For example, if the surface is negatively charged, the positive ions in the liquid move towards the cathode, carrying the liquid with them. Thus, electroosmosis specifically describes the migration of the dispersion medium under an electric potential.

Option C: Electrophoresis - This involves the movement of charged particles (dispersed phase) through a stationary dispersion medium under an electric field. For instance, in a colloidal solution, charged particles migrate towards the oppositely charged electrode. Here, the dispersed phase moves, not the dispersion medium. Therefore, this is incorrect.

Option D: Sedimentation - This is the settling of particles under gravity without any electric field. It does not involve an electric potential, so it is irrelevant.

Based on the definitions:

• Electrophoresis: Dispersed phase moves, dispersion medium stationary.
• Electroosmosis: Dispersion medium moves, dispersed phase stationary (or fixed in a matrix).

The question specifies the migration of the dispersion medium, which matches electroosmosis.

Hence, the correct answer is Option B.