Surface Chemistry Part - 6 | Class - 12th

ü  Preparation of colloids:

Some of the methods used for the preparation of colloids are:

Ø Preparation of lyophilic sols: Prepared by dissolving these substances (e.g. starch, gelatine, glue etc.) in water either in cold or warm. Solutions of colloidal electrolytes (e.g. soaps, dyes) are also prepared in the similar manner.

Ø Preparation of lyophobic sols: To obtain a substance in colloidal form either the substance in bulk is broken down into particles of colloidal dimension (1nm to 1000nm) or the size of molecular particles is increased to colloidal dimensions. In some cases, another substance is added to increase the stability of sol. Such substances are known as stabilizer. Thus, there are two ways by which lyophobic sols can be prepared. 

    a)     Chemical methods: Colloidal solutions can be prepared by chemical reactions like oxidation, reduction, double decomposition, hydrolysis etc.

·       Oxidation: Sulphur sol can be prepared by passing H₂S gas through an aqueous solution of sulphur dioxide. SO₂ + 2H₂S →   3S(sol) + 2H₂O.

·       Reduction: Sols of metals like silver, gold and platinum are obtained by the reduction of their salts with reducing agents like formaldehyde, stannous chloride etc.

2AuCl₃ + 3HCHO + 3H₂O     →    2Au(sol) + 3HCOOH + 6HCl

·       Hydrolysis: Ferric hydroxide sol is obtained when concentrated solution of ferric chloride is added drop-wise to hot water.

FeCl₃ + 3H₂O  →  Fe(OH)₃ (sol) + 3HCl

·       Double Decomposition: A colloidal solution of arsenic sulphide is formed by passing H₂S through a dilute solution of arsenious oxide in water.

As₂O₃ + 3H₂S  →  As₂S₃(sol) + 3H₂O

     b)    Electrical disintegration (Bredig’s arc method): This method is used for the preparation of metal sols like Ag, Au, Pt etc. The metal whose sol is to be prepared is taken in the form of two rods and it is kept in suitable dispersion medium containing small amount of electrolyte. The whole arrangement is kept in an ice bath. When high voltage is passed through the metal, the intense heat produced vapourises the metal, which then condensed to form particles of colloidal dimension.

      c)     Peptisation: It is the process of converting a freshly precipitated substance into colloidal particles by shaking with a suitable electrolyte, e.g., Fe(OH)₃ ppt with FeCl₃ solution, AgI ppt with AgNO₃ or KI solution and Al(OH)₃ ppt . with insufficient quantity of very dil. HCl solution. During peptization, the precipitate adsorbs one of the ion of the electrolyte on its surface resulting in development of positive or negative charge on the precipitate, which ultimately breaks into particles of colloidal dimensions.