In these methods, larger particles of a substance (suspensions) are broken into smaller particles. The following methods are employed.
(a) Mechanical dispersion
• In this method, the substance is first ground to coarse particles.
• It is then mixed with the dispersion medium to get a suspension.
• The suspension is then grinded in colloidal mill.
• It consists of two metallic discs nearly touching each other and rotating in opposite directions at a very high speed about 7000 revolution per minute.
• The space between the discs of the mill is so adjusted that coarse suspension is subjected to great shearing force giving rise to particles of colloidal size.
• Colloidal solutions of black ink, paints, varnishes, dyes etc. are obtained by this method.
(b) By electrical dispersion or Bredig’s arc method
• This method is used to prepare sols of platinum, silver,
copper or gold.
• The metal whose sol is to be prepared is made as two electrodes which immerged in dispersion medium such as water etc.
• The dispersion medium is kept cooled by ice.
• An electric arc is struck between the electrodes.
• The tremendous heat generate by this method and give colloidal solution.
• The colloidal solution prepared is stabilised by adding a small amount of
KOH to it.
(c) By peptisation
• The process of converting a freshly prepared precipitate into colloidal form by the addition of suitable electrolyte is called peptisation.
• The electrolyte is used for this purpose is called peptizing agent or stabilizing agent.
• Cause of peptisation is the adsorption of the ions of the electrolyte by the particles of the precipitate.
• Important peptizing agents are sugar, gum, gelatin and electrolytes.
• Freshly prepared ferric hydroxide can be converted into colloidal state by shaking it with water containing or ions, viz. or respectively.