Mon December 26, 2011 By: Akankshya Sahu

In Bredig's arc method, what is meant by dispersion and condensation?please explain elaborately.

Expert Reply
Wed December 28, 2011

Lyophobic colloids are prepared by mainly two types of methods: Condensation method and dispersion method.

(i) Condensation method: In this method, smaller particles of dispersed phase are condensed suitably to be of colloidal size. This is done by the following methods.

(a) By oxidation: A colloidal solution of sulphur can be obtained by bubbling oxygen (or any other oxidising agent like HNO3, Br2 etc.) through a solution of hydrogen sulphide in water.

 (b) By reduction : Metals such as silver, gold can be obtained in colloidal state by treating the aqueous solution of their salts with a suitable reducing agent such as formaldehyde, phenyl hydrazine, hydrogen peroxide, stannous chloride etc.

2AuCl3 + 3SnCl2 ? 3SnCl + 2Au

                                   Gold sol

 (c) By hydrolysis: Many salt solutions are rapidly hydrolysed by boiling dilute solutions of their salts. For example, ferric hydroxide and aluminium hydroxide sols are obtained by boiling solutions of the corresponding chlorides.

FeCl3 + 3H2O ? Fe(OH)3 + 3HCl

                        Colloidal sol

 (d) By double decomposition: A sol of arsenic sulphide is obtained by passing hydrogen sulphide through a cold solution of arsenious oxide in water.

As2O3 + 3H2S ? As2S3 + 3H2O

(e)By excessive cooling: A colloidal solution of ice in an organic solvent like ether or chloroform can be prepared by freezing a solution of water in the solvent. The molecules of water which can no longer be held in solution separately combine to form particles of colloidal size.

 (f) By exchange of solvent : Colloidal solution of some substances like sulphur, phosphorus, which are soluble in alcohol but insoluble in water can be prepared by pouring their alcoholic solution in excess of water. For example, alcoholic solution of sulphur on pouring into water gives milky colloidal solution of sulphur.

(g) By change of physical state: Sols of substances like mercury and sulphur are prepared by passing their vapour’s through a cold water containing a suitable stabilizer such as ammonium salt or citrate.

(ii) 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 and 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 which nearly touch each other and rotate in opposite directions at a very high speed. 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 generated 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 peptization. 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. Freshly prepared ferric hydroxide can be converted into colloidal state by shaking it with water containing Fe3+ or OH– ions, viz. FeCl3 or NH4OH respectively.

   Fe(OH)3 + FeCl        ? [Fe(OH)3Fe]3+ + 3Cl–

Precipitate electrolyte      Colloidal sol

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