Purification Of Colloids: ( Sol )

( a ) Dialysis.
( b ) Electro-dialysis.
( c ) Ultra-filtration.
Dialysis:
     Dialysis is the removal of soluble impurities from sols by the use of semipermeable membrane. The ordinary process of dialysis is slow. The apparatus used for this purpose is called Dialyser.
      Cellophane bag is filled with impure colloidal solution and placed in fresh water ( distilled water ). The cellophane bag is not allow the colloidal particles to pass through, but particles of true solution can pass through it. The molecules and ions diffuse through the membrane into the outer water and pure colloidal solution is left behind. The water is renewed frequently to avoid accumulation of the crystalloid as otherwise they may start diffusing back into the bag.

Electro-dialysis:
       Electro dialysis is the removal of soluble impurities from sols by the use of semipermeable membrane in the presence of electric field.
       Cellophane bag is filled with impure colloidal solution and placed in fresh water ( distilled water ). The cellophane bag is not allow the colloidal particles to pass through, but particles of true solution can pass through it. Now the electric field is applied. The soluble particles passed through the membrane and migrate towards the oppositely charged electrodes. Leaving behind the pure colloidal solution. The most important application of dialysis process in purification of the artificial kidney mechaine, used for the purification of blood.

Ultra filtration:
       In this method, colloidal solutions are purified by carrying out filtration through special type of filters called ultra-filters. Sols pass through ordinary filter paper, So that use special filter paper. The filters are permeable to all substances except the colloidal particles. This process is very slow. Pressure are suction is applied to speed it up.

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.

3.Peptisation:

         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.

Example:

( i ) Ferric hydroxide yields a sol by adding ferric chloride.

                Freshly prepared Fe(OH)₃ is shaken with aqueous solution of  FeCl₃ ( 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.

Lyophobic and Lyophilic Colloids

Lyophobic Colloids:

• Colloidal solution in which the dispersed phase has very little affinity for the dispersion medium are termed as lyophobic colloids. 
• Irreversible in nature and once precipitation can not easily pass into colloidal state because particles are solvent hating.
• They are unstable and the particles are aggregates of thousands of molecules.
• Low concentration of dispersed phase are possible. Example. Sulphur in water.

Lyophilic Colloids:

• Colloidal solution in which are dispersed phase has considerable affinity for the dispersion medium are called lyophilic colloids.
• Reversible in nature and once precipitated can readily pass into colloidal state because particles are solvent loving.
• They are stable and self stabilized.
• Particles are true molecule and bigger size.
• High concentration of dispersed phase are possible. Example: Gelatin, Starch and protein.

Classification Of Colloids

• Hydrophilic colloids.
• Hydrophobic colloids.

Hydrophilic colloids:

         Hydrophilic colloids are water loving colloids. The colloid particles are attracted towards water. This type of colloids are also called reversible sols.

Hydrophobic colloids:

          Hydrophobic colloids are opposite in nature to hydrophilic colloids. The colloid particles repelled by water. The colloid particles are further away from the water. This type of colloids are called irreversible sols. 

Types Of Colloids

      A colloidal system is made up of two phases.

• Dispersed phase.
• Dispersion medium.

Dispersed phase:

          The substances distributed as the colloidal particles is called the dispersed phase. ( the substance found in the lesser extent).

Dispersion Medium:

           The colloidal particles are dispersed in that phase is called dispersion medium. ( the substance in a colloid found in the greater extent).

Colloid

             A colloid is a substance microscopically dispersed throughout another substance. This field of study was introduced in 1861 by Thomas Graham. Certain substances such as sugar, salt, and acids diffused readily through a parchment membrane, other substances such as gelatin, albumen and glue diffused at very slow rate. The substances belonging to the second category were given the name colloids.
            A type of homogeneous mixture in which the dispersed particles do not settle out.
Example: Butter, milk, fog, smoke, paint.

Catalytic Poison

             A substance which destroy the activity of catalyst this process is called Catalytic poisoning.