Monday, November 25, 2013

Preparation Of Colloids

Preparation of lyophilic sols:
         Lyophilic sols prepared by simply warming the solid with liquid dispersion medium.
Example: Starch with water.

Preparation of lyophobic sols:
         Lyophobic sols have to be prepared by special method. These methods fall into two categories.
  • Dispersion methods.
  • Condensation methods.

Dispersion Method:
        Large particles of substances are broken, into smaller particles in the presence of dispersion medium . They are stabilized  by adding some suitable stabilizer. There are 4 ways using dispersion method.

1. Mechanical Dispersion:
        In this method making a colloid by milling a given solid and the powder produced is dispersed into a given dispersing medium.

  • The colloidal mill consists of two steel plates nearly touching to each other and rotating in opposite directions with high speed.
  • The solid particles are turn off to colloidal size and dispersed in liquid to give sol.
  • The colloidal graphite and printing inks are made by this method.

2. Electro-dispersion Method: ( Bredig's arc method )
           In this method, two metallic wires functioning as electrodes are immersed into water. Both the end of the wires gives a strong electric current to be evaporated and then it is dispersed into water to form a metallic sol.
  • This method is suitable for the preparation of colloidal solution of metals like gold, silver, platinum etc.
  • An electric arc is struck between the electrodes of metal immersed in the dispersion medium [ such as water and alkali ].
  • The intense heat of the arc vaporizes some of the metal, which then condenses to form particles of colloidal size.
  • The water is cooled by immersing the container in a cold bath.

         In this method converting a precipitate into a colloidal sol by shaking it with the dispersion medium, in the presence of small amount of electrolyte. The electrolyte used is called as peptizing agent.
( i ) Ferric hydroxide yields a sol by adding ferric chloride.
                Freshly prepared Fe(OH)3 is shaken with aqueous solution of  FeCl( peptising agent ) it adsorbs Fe3+  ions and breaks up into small sized particles of type Fe(OH)3 / Fe3+ .
( ii ) Silver chloride can be converted into a sol by adding hydrochloric acid.

4. Ultra-sonic Dispersion.
         The sound waves of high frequency are usually called ultra-sonic waves. Ultrasonic waves are passed through the solution containing large particles are breaks down to form colloidal solution.

Condensation Method:
           In this method molecules of the dispersed phase appearing first and aggregate to form colloidal particles.

1. Change Of Physical State:
          Colloidal solution of  certain elements such as mercury and sulfur are obtained by passing their vapor through cold water containing a suitable stabilizer such as ammonium salt or citrate.

2. Chemical Methods:
( i ) Double Decomposition:
               A sloe steam of hydrogen sulfide gas is passing through cold solution of aresenious oxide to form an arsenic sulfide sol. This is continued till the yellow color of the sol attains maximum intensity.

                                  As2O3     +     3H2S     →     As2S3     +     3H2O
                                                                         ( Yellow sol )
                                                                                              (Arsenic sulphide)
( ii ) Hydrolysis:
               This method is used to prepare hydroxides and oxides of weakly electropositive metals like Fe, Al, Cr, Sn etc. A colloidal solution of ferric hydroxide is obtained by boiling a dilute solution of ferric chloride.

                                    FeCl3     +     3H2O     →     Fe(OH)3     +     3HCl
                                                                                (Red sol)
                                                                          ( Ferric hydroxide )
( iii ) Oxidation:
                Passing hydrogen sulfide into a solution of sulfur dioxide to form a colloidal solution of sulfur
                                    2H2S     +     SO2      →     2H2O     +     3S

( iv ) Reduction:
                 silver sol can be obtained by treating dilute solution of silver nitrate with organic reducing agents like tannic acid.
                            AgNO3      +      Tannic acid     →     Ag sol.
[ Reducing agents- stannous chloride, tannic acid, formaldehyde, hydrazine. ] Use suitable reducing agents.
Similar to Gold.
                            AuCl3     +     Tannic acid      →     Au sol.

3. Exchange Of Solvent:
               A solution of sulfur in alcohol is poured into water, a colloidal solution of sulfur is obtained due to low solubility in water. similar to phosphorus also.
Reason: Sulfur and phosphorous are soluble in alcohol but insoluble in water. So that first prepare the alcoholic solution and then pouring into the solution in excess of water.

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