CHM 232- Physical Chemistry

In this method, small solute particles are condensed to form a dispersed phase particle.

Examples include the following:

1. Chemical methods:

   a) By oxidation:

Colloidal sulphur can be obtained by passing oxygen gas through a solution of hydrogen sulphides. In this method any oxidising agent like HNO₃, H₃Br₂ can also be used.

ž  2H₂S + O₂ → 2H₂O + 2S (Sulphur sol)

   b) By double decomposition:

A solution of arsenic sulphide is obtained in this method. In this process hydrogen sulphide is passed through Arsenious oxide cold solution in water.

ž  AS₂O₃ + 3H₂S → AS₂S₃ + 3H₂O Arsenic sulphide (sol)

               c) By reduction:

A number of metals such as gold, silver, and platinum are obtained in a colloidal state by reacting the aqueous solution of these salts with suitable reducing agents such as formaldehyde, phenylhydrazine, hydrogen peroxide, stannous chloride etc.

ž  2AuCl₃ + 3SnCl₂ → 3SnCl₄ + 2Au (gold sol)

ž  2AuCl₃ + 3HCHO + 3H₂O → 2Au + 3HCOOH + 6HCl

The gold sol prepared in the reduction of gold chloride solution has a purple colour and is called purple of Cassius.