Preparation of Colloidal Solutions
The lyophilic sols can be prepared directly by mixing the dispersed phase with the dispersion medium. For example, colloidal solutions of starch, gelatin, gum, etc. are prepared by simply dissolving these substances in hot water. Similarly, a colloidal sol of cellulose nitrate is obtained by dissolving it in alcohol. The resulting solution is called collodion.
However, lyophobic colloids cannot be prepared by direct method. Two types of methods are used for preparing lyophobic colloids:
- Physical methods
- Chemical methods
Physical Methods
These methods are employed for obtaining colloidal solutions of metals like gold, silver, platinum, etc. An electric arc is struck between the two metallic electrodes placed in a container of water. The intense heat of the arc converts the metal into vapours, which are condensed immediately in the cold water bath. This results in the formation of particles of colloidal size.
Peptisation: Peptisation is the process of converting a freshly prepared precipitate into colloidal form by the addition of a suitable electrolyte. The electrolyte is called peptising
agent. For example, when ferric chloride is added to a precipitate of ferric hydroxide, ferric hydroxide gets converted into reddish brown colored colloidal solution. This is due to preferential adsorption of cations of the electrolyte by the precipitate.
Chemical Methods: By oxidation
Sulphur sol is obtained by bubbling H2S gas through the solution of an oxidizing agent like HNO3 or Br2.
Br2 + H2S → S + 2HBr
2HNO3 + H2S → 2H2O + 2NO2 + S