Preparation methods of colloidal solution:
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:
In this type of preparation method, the larger particles are broken down into particles of the range of colloidal particle size.
Some of the dispersion methods are:
In this method, solid and the liquid dispersion medium are poured into the colloidal mill to form a colloidal sol.
Bredig’s Arc method (Electrodispersion):
Hydrosols of metals e.g., platinum, silver, gold are prepared by this method. The metal electrodes are kept at the two sides of the arc which is enclosed by deionized water. The excessive heat provided by the spark vaporizes some traces of the metal and water gets condensed by vapour. Some amount of KOH is added to water as a stabilizing agent.
In peptization 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 in color.
In this method, small particles are aggregated to form colloidal size particles.
Example: Arsenious oxide is mixed with hydrogen sulfide to form arsenic sulfide sol. The excess amount of hydrogen sulfide is removed by passing stream of hydrogen.
As2O3 + 3H2S → As2S3 (sol) + 3H2O
Example: Noble gases are reacted with organic reducing agents like ethanol, tannic acid, formaldehyde to form their respective sol.
AgNO3 + tannic acid → Ag-sol
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 the particle size ranging from true solutions and suspensions. The range of the diameter of dispersed particle is from 10 Angstrom to 2000 Angstrom. Some of the methods of purification of colloidal solution are as follows:
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, dipped in the water. The ion from the solution diffuses from the permeable membrane. Because of the continuous flow of water, the concentration of electrolyte outside the membrane is neutralized. For e.g.: Ferric hydroxide sol is purified by using this method.
In this process dialysis of colloidal solution is carried under the effect of 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 the non-electrolyte impurities like urea etc.
If filter paper is made with the 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 out 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.
This method can be useful for the purification as well as for the concentration of the sol. When this process is carried 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 of the 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.