Preparation Colloidal Solutions

• Dispersion methods and

• Condensation or aggregation methods 

?Refer to the following video for preparation of colloidal solutions

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Dispersion Methods

In dispersion methods, the bigger particles of any substance are broken down into small particles of colloidal size. The small particles thus obtained are stabilized by adding some stabilizing agents.

Some of the dispersion methods are discussed below :

• Mechanical dispersion method: In this method, the large particles of the material whose sol is to be prepared are broken down in a machine called colloid mill. Colloid mill consists of two heavy steel discs separated by a little gap. The can be adjusted according to the particle size desired. The two discs rotate at high speed (about 8000 revolutions per minute) in the opposite direction.
  A suspension of the substance in water is added into the mill. The particles present in the suspension are grinded to form the particles of colloidal size and als get dispersed in water to form a sol.

  Finer dispersion can be obtained by adding an inert diluents which prevents the colloidal particles to grow in size. For example, glucose is used as diluents in the preparation of sulphur sol. 

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• Electrical dispersion method (Bredig’s arc method): This method is used for the preparation of sols metals such as gold, silver, platinum etc.
  In this method, two electrodes of the metals whose sol is to be prepared are immersed into the dispersion medium and an electric arc is produced bas passing high voltage current. The dispersion medium is cooled by surrounding it with a freezing mixture.
  Due the heat produced from electric arc , the metal starts vaporizing and the vapour condense into the cold dispersion medium to form to the particles of colloidal size on cooling.

• Ultrasonic dispersion: The sound waves of high frequency are usually called ultrasonic waves.
  These waves can be produced when quartz crystal discs are connected with high frequency generator.
  The application of ultrasonic waves for the preparation of colloidal solutions was first introduced bt wood and Loomis, in 1927.
  Various substances like oils, mercury, sulphides and oxides of metals can be dispersed into colloidal state easily with the help of ultrasonic waves.

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  Peptization: Peptization is the method of  breaking down the freshly prepared precipitate particles into the particles of colloidal size. This is done by adding suitable electrolytes.  The electrolyte added is called peptizing agent. 

  • Brown coloured colloidal solution is obtained by addition of Ferric chloride solution into freshly precipitated ferric hydroxide .This happened due to adsorption of ferric ions on the surface of precipitate. Accumulation of positively chafed ferric ions on the surface causes repulsion between them because of which the large precipitate particles break down into small particles of colloidal size.

Condensation method

Some chemical reactions may be used to aggregate smaller particles of atomic or ionic sizes to form large particles of colloidal dimensions.The dispersed phase are fromed as insoluble reaction product in these reactions. Here are some important reactions used forthe formation of hydrophobic sols

• By Exchange of Solvents: If a solution of sulphur or phosphorus prepared in alcohol is poured into water, a colloidal solution of sulphur or phosphorus is obtained due to low solubility of sulphur or phosphorus in water. Thus, there are number of substances whose colloidal solutions can be prepared by taking a solution of the substance in one solvent and pouring it into another solvent in which the substance is relatively less soluble.

• By change of physical state: Colloidal solution of certain elements such as mercury and sulphur are obtained by passing their vapours through cold water containing a stabilizer ( an ammonium salt or a citrate).

• Oxidation: Sol sulphur is prepared by oxidizing an aqueous solution of H₂S with  bromine water or nitric acid or SO₂ or any other suitable oxidising agent.
  

• Reduction: Sols of gold, silver, platinum etc. are obtained by the reduction of dilute solutions of their salts with a suitable reducing agent. For example, gold sol can be obtained by reducing a dilute aqueous solution of gold with stannous chloride. 
  
  The gold sol thus obtained is called purple of Cassius. 

• Hydrolysis:  Sols of ferric hydroxide and aluminium hydroxide are prepared at boiling the aqueous solution of the corresponding chlorides., 
  FeCI₃    +   3H₂S          →       Fe(OH)₃    +   3HCI             

• Double decomposition: The sols of inorganic insoluble salts such as arsenous sulphide, silver halides etc. may be prepared by using double decomposition reaction. For example, arsenous sulphide sol can be prepared by passing H₂S gas through a dilute aqueous solution of arsenous oxide.
  As₂O₃      +       3H₂S     →          As₂S₃(OH)₃     +     3H₂O
  Colloidal arsenous sulphide?

Question 1: Which of the following alternatives does not represents a condensation method for preparation of colloids?

a. Oxidation

b. Reducation

c. Exchange of solvents

d. Electrical dispersion

Question 2: Electrical dispersion method is not used for the preparation of sols of  

a. gold

b. silver

c. copper

d. platinum

Question 3: Glucose is used as diluents in the preparation of 

a. sulphur sol. 

b. benzosols

c. aquasols

d. micelles

Question 4: Sols of ferric hydroxide and aluminium hydroxide can be prepared boiling the aqueous solution of the corresponding 

a. oxides

b. chlorides

c. bromides

d. sulphides

Q.1	Q.2	Q.3	Q.4
d	c	a	b

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