Preparation & Purification Of Colloidal Solutions

Table of Contents

• Preparation methods of colloidal solution
• Purification Of Colloidal Solutions

Preparation methods of colloidal solution:

Lyophilic & Lyophobic Colloids:

Lyophilic colloids can be prepared by just heating the solid with the liquid dispersion medium.

Whereas the lyophobic colloids cannot be prepared just by heating, they need to be prepared by some special methods.

The two methods by which the lyophobic colloids can be prepared are as follows:

1. Dispersion method

2. Aggregation method

Dispersion Method:

In this type of preparation method, the larger particles are broken down into particles of the range of colloidal particle size.

Some dispersion methods are:

• Mechanical Dispersion:

  In this method, solid and liquid dispersion mediums are poured into the colloidal mill to form a colloidal sol.

• Bredig’s Arc method (Electrodispersion):

  Hydrosols of metals e.g., platinum, silver, and gold are prepared by this method. The metal electrodes are kept at the two sides of the arc which is enclosed by deionised water. The excessive heat provided by the spark vaporises some traces of the metal and water gets condensed by vapour. Some amount of KOH is added to water as a stabilising agent.

• Peptisation:
  

  In the peptisation method, some electrolytes are added to form a colloidal sol from a freshly prepared precipitate.
  Example: Ferric chloride is added to a freshly prepared precipitate of ferric hydroxide which converts it into a colloidal sol of reddish brown.

Aggregation methods: 

In this method, small particles are aggregated to form colloidal-size particles.

• Double Decomposition:
  

  Example: Arsenious oxide is mixed with hydrogen sulfide to form arsenic sulfide sol. The excess amount of hydrogen sulfide is removed by passing a stream of hydrogen.

As₂O₃ + 3H₂S → As₂S₃ (sol) + 3H₂O

• Reduction:
  

  Example: Noble gases are reacted with organic reducing agents like ethanol, tannic acid, and formaldehyde to form their respective sol.

AgNO₃ + tannic acid → Ag-sol

• Oxidation:

  When hydrogen sulfide is passed through a solution of sulfur dioxide it forms a sulfur sol.

2 H₂S + SO₂ → 2H₂O + S

Purification Of Colloidal Solutions

The colloidal solution is the solution that has a particle size ranging from true solutions and suspensions. The range of the diameter of the dispersed particle is from 10 Angstrom to 2000 Angstrom.

Some of the methods of purification of a colloidal solution are stated below:

• Dialysis:

  The process in which the ions are removed from the solution by the phenomenon of diffusion through a permeable membrane is known as dialysis. In this process, a sol consisting of ions or molecules is filled in a permeable membrane bag and dipped in the water. The ion from the solution diffuses through the permeable membrane. Because of the continuous flow of water, the concentration of electrolytes outside the membrane is neutralized. For e.g.: Ferric hydroxide sol is purified by using this method.

• Electrodialysis:
  

  In this process dialysis of the colloidal solution is carried out under the effect of the electric field. Some potential is applied between the metal screens that support the membranes. Due to this potential, the speed of the ions moving in the direction of opposite electrodes is increased. Thus, the rate of dialysis is increased. This method is not useful for non-electrolyte impurities like urea etc.

• Ultrafiltration:
  

  If filter paper is made with colloidal or some regenerated cellulose like cellophane, the size of the pores is decreased. This modified form of filter paper is known as ultrafilter. The process in which the particles of the solution are removed from the liquid medium by electrolysis and by using this ultrafilter is known as ultrafiltration. It is a slow process. To speed up the process some external gas pressure has to be applied.

• Electro Decantation:
  

  This method can be useful for purification as well as for the concentration of the sol. When this process is carried out without stirring the solution, then the bottom layer settles down while the top layer consists of pure and concentrated colloids, which can be decantated.

This was just a brief layout of some methods of purification of the colloidal solution. To know more in detail about the purification of suspension and colloids and surface chemistry, please visit BYJU’S.