|Optical Properties (Tyndall Effect)
When an intense converging beam of light is passed through a colloidal solution kept in dark, the path of the beam gets illuminated with a bluish light. This phenomenon is called Tyndall effect and the illuminated path is known as Tyndall cone. The phenomenon was first observed by Tyndall in 1869.
The Tyndall effect is due to the scattering of light by colloidal particles. Since the dimensions of colloidal particles are comparable to the wavelength of ultraviolet and visible radiations, they scatter these radiations and get illuminated. Tyndall observed that the zone of scattered light is much larger than the particle itself. This is why colloidal particles look like bright spots when viewed with a microscope at right angles to the beam of light as shown in figure. Thus, Tyndall effect may be defined as the scattering of light by colloidal particles present in a colloidal solution.
Tyndall effect is not exhibited by true solutions. This is because the particles (ions or molecules) present in a true solution are too small to scatter light. Thus, Tyndall effect can be used to distinguish a colloidal solution from a true solution. The phenomenon has also been used to devise an instrument known as ultra microscope. The instrument is used for the detection of the particles of colloidal dimensions. Tyndall effect also establishes the fact that colloidal systems are heterogeneous in nature.
Preparation of Colloidal Solutions
Bredig’s arc method
(ii) Electrical dispersion method (Bredig’s are 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.
(iii) 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.
Some examples of peptization are given below:
(a) 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.
(b) 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.
[B] Condensation Methods (Aggregation Method)
In condensation methods, the smaller particles of the dispersed phase are aggregated to form larger particles of colloidal dimensions. Some important condensation methods are described below.
1. Chemical Methods
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.
(a) 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.
(b) 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.
The gold sol thus obtained is called purple of Cassius.
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
(d) 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
2. Physical Methods
(i) Exchange of solvent:
This method involves the pouring of the true solution to another solvent in which the solute is insoluble but the solvent is completely miscible. An exchange of solvent gives the colloidal solution of the solute. The method may be used for the preparation of the sols of sulphur and phosphorus. For example, sulphur is soluble in alcohol but less soluble in water. When an alcoholic solution of sulphur is poured into water, a colloidal solution of sulphur is obtained.