Page 2 - Lesson note 5

The process of converting a precipitate into colloidal solby shaking it with dispersion
medium in the presence of a small amount of electrolyte. The electrolyte used for this
purpose is called peptizing agent. This method is applied, generally, to convert a freshly
prepared precipitate into a colloidal sol.
During peptization, the precipitate adsorbs one of the ions of the electrolyte on its
surface. This causes the development of positive or negative charge on precipitates,
which ultimately break up into smallerparticles of the size of a colloid.
 Purification of colloidal solution :
The process used for reducing the amount of impurities to a requisite minimum is
known as purification of colloidal solution.
 The purification of colloidal solution is carried out by the following mehods:
 Dialysis :
It is a process of removing a dissolved substance from a colloidal solution by
means of diffusion through a suitable membrane.
The apparatus used for this purpose is called dialyser. A bag of
suitablemembrane containing the colloidal solution is suspended
in a vessel through which fresh water is continuously flowing . The
molecules and ions diffuse through membrane into the outer water
and pure colloidal solution is left behind.
 Electro Dialysis :

Ordinarily, the process of dialysis is quite slow. It can be made faster
by applying an electric field if the dissolved substance in the impure
colloidal solution is only an electrolyte. The process is then named
electrodialysis. The colloidal solution is placed in a bag of suitable
membrane while pure water is taken outside. Electrodes are fitted in
the compartment.The ions present in the colloidal solution migrate

out to the oppositely charged electrodes.
 Ultrafiltration: Ultrafiltration is the process of separating the colloidal
particles from the solvent and soluble solutes present in the colloidal
solution by specially prepared filters, which are permeable to all
substances except the colloidal particles. Colloidal particles can pass
through ordinary filter paper because the pores are too large.
However, the pores of filter paper can be reduced in size by
impregnating with collodion solution to stop the flow of colloidal

particles. The usual collodion is a 4% solution of nitrocellulose in a
mixture of alcohol and ether. An ultra-filter paper may be prepared
by soaking the filter paper in a collodion solution, hardening by
formaldehyde and then finally drying it. Thus, by using ultra-filter
paper, the colloidal particles are separated from rest of the