Dispersion Methods

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.