Preparation Colloidal Solutions

As mentioned earlier, lyophilic colloids have a strong affinity for the dispersion medium and readily form a sol by bringing them into contact or by warming them with the medium. For example, sols of starch, gelatin, gum arabic etc. can be prepared just by warming them with water. Similarly, a colloidal solution of cellulose nitrate can be prepared by dissolving it in an organic solvent such as ethyl alcohol. The product obtained is commercially called collodion. 

Since lyophobic colloids practically have no affinity for the dispersion medium, they do not readily pass into the medium to form a colloidal solution. Hence special methods are required for the preparation of lyophobic sols. The methods used for the preparation of lyophobic sols can broadly be divided into the following two categories.

Dispersion methods   Condensation or aggregation methods

  • Dispersion methods 

  • Condensation or aggregation methods 

Dispersion Methods

In these methods, the bigger particles of a substances are broken down to form smaller particles of colloidal dimensions thus obtained are stabilized by the addition of certain stabilizing agents. Some important dispersion methods are as follows: 

  • Mechanical dispersion method: In this method, the dispersion of the coarse material (whose colloidal solution is to be prepared) is carried out in a machine called colloid mill. It consists of two heavy steel discs separated by a little gap. The gap may be adjusted according to the particle size desired. The two discs rotate at high speed (about 8000 revolutions per minute) in the opposite direction.

    Colloid Mill

    A suspension of the substance in water is introduced into the mill. The coarse particles present in the suspension are grinded to the particles of colloidal dimensions and 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. 

Colloid Mill

  • 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, an electric are is struck between the two electrodes of the metal (whose colloidal solution is to be prepared) immersed in the dispersion medium (say water). The dispersion medium is cooled by surrounding it with a freezing mixture. High temperature of the arc vaporizes some of the metal. The vapour condenses to the particles of colloidal size on cooling. The colloidal particles thus formed get dispersed in the medium to form a sol. of the metal.

  • 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.

  • Peptization:Peptization: In this method, a freshly prepared precipitate of the substance is made to pass into the colloidal state by the addition of a suitable electrolyte. The process of dispersing a freshly prepared precipitate into colloidal form by using a suitable electrolyte is called peptization. The electrolyte added is called peptizing agent. 

    • When a small amount of ferric chloride solution is added to the freshly precipitated ferric hydroxide, a reddish brown coloured colloidal solution of ferric hydroxide is obtained. This occurs due to the adsorption of Fe3+ ions over ferric hydroxide particles which causes them to disperse into the solution due to the electrostatic repulsions between the similarly charged particles.

    • When a freshly prepared precipitate of silver iodide is shaken with a dilute solution of silver nitrate, a colloidal solution of silver iodide is obtained. 

    • Freshly precipitated silver chloride can be converted into a colloidal solution by small amount of hydrochloric acid.

Condensation method

Some chemical reactions may be used to aggregate smaller particles of atomic or ionic sizes to form large particles of colloidal dimensions. These reactions actually involve the formation of the dispersed phase as insoluble reaction products. Some important reactions leading to the formation of hydrophobic sols are as follows

  • By Exchange of Solvents: If a solution of solphur 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: Colloidal solution of sulphur can be prepared by oxidizing an aqueous solution of H2S with a suitable oxidizing agent such as bromine water, nitric acid or SO2.

    oxidation-reaction

  • Reduction: Sols of gold, silver, platinum etc. can be 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. 
    reduction-reaction
    The gold sol thus obtained is called purple of Cassius. 

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

  • 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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