ü Purification of Colloids:
The colloidal solution prepared contains excess amount of electrolyte and some other soluble impurities. Even though small amount of electrolyte is required for the stability of colloid, large amount may cause precipitation. The process of reducing the concentration of electrolyte and other impurities is known as purification of colloids. Some methods used for purification are:
Ø  Dialysis: The process of removing the impurities from a sol by means of diffusion through a semi permeable membrane is called dialysis.

Ø  Electrodialysis: When dialysis process is accelerated by the application of a potential difference across the membrane, so ions migrate faster than the colloids.

Ø  Ultra-filtration: Purification of colloidal solution using special filter paper called ultra filters (filter paper which is impregnated with gelatin or collodion followed by hardening in formaldehyde). Colloidion is a 4% nitrocellulose soln in alcohol and ether.

ü  Properties of colloids:
Ø  Heterogeneous nature: Colloids are heterogeneous in nature and consists of two phases, the dispersed phase and the dispersion medium.
Ø  Visibility: The particles are too small to be seen with naked eye but become visible when viewed through ultra-microscope due to scattering of light by them.
Ø  Surface tension and Viscosity: The surface tension and viscosity of lyophobic sols are not very different from that of dispersed medium. But, lyophillic sols show higher viscosity and lower surface tension in comparison to the dispersion medium.
Ø  Tyndall Effect: When a light beam is passed through a colloidal solution, we can see the path of the light beam. This phenomenon is known as Tyndall effect. It is due to the scattering of light beam by the colloidal particles. The visible path is called tyndall cone.


Ø   Brownian movement: Zig-zag movement of colloidal particles due to collision between particles of dispersed phase & dispersion medium, responsible for stability of colloids.