Question

What are the different methods applied for the purification of colloidal solutions?
(A) Dialysis
(B) Ultrafiltration
(C) Normal filtration
(D) None of these

Answer 1

Hint: In Colloidal solution, particles are larger in size compared to the particles in the true solution. Colloidal solution cannot diffuse through fine parchment or membrane. Hence colloidal particles cannot be filtered using normal filter paper.

Complete Solution :
First of all, let us understand what a Colloidal solution is. Colloidal solution is the mixture in which the microscopic particles of a substance are dispersed uniformly in another substance.
The colloidal particles can be purified by both the methods: Dialysis and Ultrafiltration.

- Dialysis:
The process of separation of the colloids from dissolved ions or molecules by the phenomenon of diffusion is called dialysis. The colloidal solution is taken in a semipermeable membrane bag and is dipped in a vessel of pure water. Due to the continuous flow of water the dissolved ions or molecules move through the tiny pores of the membrane. The colloidal particles remain in the bag because of its large size.

- Ultrafiltration:
The size of the pores of an ordinary filter paper are very large enough to allow the passage of both impurity particles as well as colloidal particles. Hence we have to reduce the pore size of the filter paper. The pores can be made smaller by soaking this filter paper in the solution of gelatin or collodion and hardening them by soaking the filter paper in the formaldehyde solution. This treatment decreases the pore size and the colloidal particles can be retained in the filter paper. This modified filter paper is called ultrafilters. Filtration done through the ultrafilters is called Ultra filtration.

Additional Information:
- Colloidal solution is a heterogeneous mixture.
- The main property of the colloidal solution is that particles do not settle at the bottom of the container when left undisturbed.
- The size of the colloidal particles ranges from $1nm$ to $1\mu m$.
- The colloidal particles exhibit Tyndall effect. Tyndall effect is the phenomenon in which the colloidal particles scatter the beam of light passing through them.

Note: Other than dialysis and ultrafiltration, we can also purify the colloidal solution by electrodialysis, electro decantation and ultracentrifugation methods which are also effective in this process.