Preparation of colloidal solutions:-

The Lyophoilic sols are prepared simply by mixing the dispersed phase in the dispersion medium.

But for the preparation of Lyophobic sols some special methods are employed. These are:

1. Dispersion or disintegration:- these methods involve the breaking of the bigger particles into smaller ones. Some methods are:

• 1. Mechanical disintegration:- this process is carried out in a machine called colloidal mill. It consists of two steel disks with belts which revolve them in opposite directions. A suspension is added from the above that are broken into colloidal particles.

• Electro disintegration (Bredig’s Method):- and electrical arc is set up between two metallic electrodes suspended in a trough of water. The intense heat of arc converts the metal into vapors which are condensed immediately in ice cold water bath resulting in the formation of colloidal particles. This method is used for metals like Au, Ag, and pt. etc.

• Peptization:- peptization is a process of passing of precipitates into colloidal particles on adding suitable electrolyte. The electrolyte added is called Petizing agent. e.g. Fe(OH)₃ sol. Is prepared by adding freshly prepared FeCl₃ to it, Agl sol. Is prepared by adding AnNO₃ or Kl and a sol. Of Al(OH)₃ prepared by adding a very small amount of dil HCL. It is represented as:-

The cause of peptization is the preferential adsorption of one particular type of ions of electrolyte. Due to having similar charge electrostatic repulsions disperse the particles to give them colloidal size.

 (B) Condensation or Aggregation methods:- These methods involve the joining together of a large number of smaller particles to form the particles of particles of colloidal size. Some methods are:-

1. Chemical methods:-

1. The arsenic sulphide sol is prepared by passing H₂S gas through Arsenic Oxide.

As₂O₃ + 3H₂S  --> As₂S₃ + 3H₂O

                              (colloidal solution)

2. The gold silver, platinum etc sols are obtained by the reduction on very dilute solutions of their salts with suitable reducing agents. e.g. 2AuCl₃ + 3SnCl₂  -->  2Au + 3SnCl₄
3. Sulphur is obtained in the colloidal from by bubbling H₂S gas through Bromine water e.g.
   

   Br₂ + H₂S  -->  S + 2HBr

4. Fe(OH)₃ and Al(OH)₃ sols are obtained by boiling their corresponding chlorides with water. e.g.

FeCl₃ + 3H₂O  à  Fe(OH)₃ + 3HCl

2. By exchange of solvents:- Substances like sulphur and phosphorous are freely soluble in alcohol but less soluble in water. if their solution in alcohol is poured into water colloidal solutions pf phosphorous and sulphur are obtained.

3. By condensing vapours of a substance into the solvent:- colloidal solutions of sulphur and Mercur in water are prepared by passing their vapors in cold water containing a little stabilizing agent like ammonium nutrate.

4.By excessive cooling :- A colloidal solution of organic solvents like ether is obtained by freezing a mixture of solvent and water.