Peptisation Bredig’s arc method, Tyndall effect


Peptisation Bredig’s arc method, Tyndall effect- Class 12 Peptisation Bredig’s arc method, Tyndall effect easy notes


Peptisation is a process  of passing of a precipitate into colloidal particles on adding suitable electrolyte. The electrolyte added is called peptising agent.

Bredig’s arc method or Electro-disintegration method

Colloidal solution of Pt, Au, etc are prepared by this method the intense heat of the electric arc set up between two metallic electrodes is utilized for making colloidal solution.

Brownian Movement – The continuous rapid zig-zag motion of the colloidal particles in the dispersion medium is called Brownian Movement. It is due to the unbalanced bombardment of the particles of the dispersed phase by the molecules of dispersion medium. This stabilises the sol.

Tyndall Effect

If a strong converging beam of light is passed through a colloidal solution placed in a dark room, the path of beam gets illuminated with a bluish light, this phenomenon is known as Tyndall Effect.

Electrophoresis– The moment of colloidal particles towards positive or negative electrode in an electric field is called electrophoresis. The process of aggregating together the colloidal particles is called coagulation or precipitation of the sol.

Hardy – Schulze Rule – According to the rule, greater the valency of the oppositely charged ion of the electrolyte being added, the faster is the coagulation.

E.g. For positively charged sol Fe(OH)3 the effectiveness of negative charged ions in coagulation is                              [Fe(CN)_{ 6 }]^{ 4 } >{ PO }_{ 4 }^{ 3- }>{ SO }_{ 4 }^{ 2- }>{ Cl }^{ - }

For negatively charged sol As2S3 the coagulating powers of the cations are in the order.

{Al}^{3+} > {Ba}^{2+} > {Na}^{+}

Gold Number– The minimum weight of a protective colloid in milligrams which must be added to 10ml of a standard red gold sol, so that no coagulation of the gold sol takes place when 1mL of 10% sodium chloride solution is rapidly added to it is known as gold number. Smaller the gold number of a protective colloid grater is its protective action.

{Protective  power}  \alpha \frac {1}{Gold  Number}


  • Emulsion is dispersion is of one liquid into another which are immisible.
  • Emulsion are formed when both dispersed phase and dispersed medium are liquids in a colloidal system.
  • Emulsion also show Brownian movement and Tyndall effect.
  • If a mixture of two immisible or partially miscible liquid is shaken, coarse dispersion of one liquid on the other is obtained, which is known as emulsion.
  • The process of making emulsion is called emulsification.

Application of colloids-

  • Purification of drinking water
  • Medicines
  • Tanning
  • Electrical precipitation of smoke
  • Cleansing action of soaps and detergents, etc
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