A few important methods for the preparation of colloids are as follows:

(a) `text(Chemical Methods :)` Colloidal solutions can be prepared by chemical reactions leading to formation of molecules by double decomposition, oxidation, reduction or hydrolysis. These molecules then aggregate leading to formation of sols.

`AS_2 O_3 +3H_2S oversettext(Double decompostion)→ As_2S_3 (Sol) +3H_2O`

`SO_2+2H_2S oversettext(Oxidation)→ 3S (Sol)+2H_2O`

`2AuCl_3+3HCHO+3H_2O oversettext(Reduction)→ 2Au (sol)+3HCOOH +6HCl`

`FeCl_2+3H_2O oversettext(Hydrolysis)→ Fe(OH)_3 (sol) +3HCl`

(b) `text(Electrical disintegration or Bredig’s Arc Method :)` This process involves dispersion as well as condensation.

● Colloidal sols of metals such as gold, silver, platinum, etc., can be prepared by this method.

● In this method, electric arc is struck between electrodes of the metal immersed in the dispersion medium.

● The intense heat produced vapourises the metal, which then condenses to form particles of colloidal size.

(c) `text(Peptization :)` Peptization may be defined as the process of converting a precipitate into colloidal sol by 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 smaller particles of the size of a colloid.

`color{purple}(✓✓)color{purple} " DEFINITION ALERT"`
The process of reducing the impurities of the electrolytes to the minimum required level is known as purification of the colloidal solution.

`=>` Colloidal solutions when prepared, generally contain excessive amount of electrolytes and some other soluble impurities.

`=>` While the presence of traces of electrolyte is essential for the stability of the colloidal solution, larger quantities coagulate it.

`=>` Therefore, it is necessary to reduce the concentration of these soluble impurities to a requisite minimum.

`=>` 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 methods :

(i) `text(Dialysis :)`
`color{purple}(✓✓)color{purple} " DEFINITION ALERT"`
It is the process of separating the particles of colloids from those of crystalloids by diffusion of the mixture through a parchment or an animal membrane is called dialysis.

● It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane.

● Since particles (ions or smaller molecules) in a true solution can pass through animal membrane (bladder) or parchment paper or cellophane sheet but not the colloidal particles, the membrane can be used for dialysis.

● The apparatus used for this purpose is called dialyser.

● A bag of suitable membrane containing the colloidal solution is suspended in a vessel through which fresh water is continuously flowing (Fig. 5.9).

● The molecules and ions diffuse through membrane into the outer water and pure colloidal solution is left behind.

• It is used for purification of blood in the artificial kidney machine. The dialysis membrane permits small particles of the excess ions and waste products to pass through whereas colloid-sized particles such as hemoglobin do not pass through membrane.

(ii) `text(Electro-dialysis :)`

● 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 as shown in Fig.

● The ions present in the colloidal solution migrate out to the oppositely charged electrodes.

(iii) `text(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 colloidion solution to stop the flow of colloidal particles.

● The usual colloidion 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 colloidion solution, hardening by formaldehyde and then finally drying it.

● Thus, by using ultra-filter paper, the colloidal particles are separated from rest of the materials.

● Ultrafiltration is a slow process.

● To speed up the process, pressure or suction is applied.

● The colloidal particles left on the ultra-filter paper are then stirred with fresh dispersion medium (solvent) to get a pure colloidal solution.

(iii) `text(Ultra-centrifugation :)` In this method, the impure sol is taken in a tube which is placed in an ultracentrifuge. In this machine, the tube is rotated at a very high speed. Due to this the colloidal particles settle down at the bottom of the tube whereas the crystalloids and other soluble impurities remain in the solution. T he solution is decanted off and the colloid particles are remixed with the dispersion medium so as to give pure colloidal sol.