Preparation of Colloids

Chemical methods: 

Colloidal solutions can be prepared by chemical reactions which form molecules by double displacement, oxidation, reduction or hydrolysis. These molecules then aggregate to form sols.

As₂O₃ + 3H₂S →  As₂S₃ (sol) + 3H₂O

SO₂ + 2H₂S → 3S (sol) + 2H₂O

2AuCl₃ + 3 HCHO + 3H₂O → 2Au (sol) + 3HCOOH + 6HCl

FeCl₃ + 3H₂O → Fe (OH)₃ (sol) + 3HCl

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.

Peptization: 

Peptization is 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.

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.