Colloids: Subject-Matter, Preparation and Tests

In this article we will discuss about:- 1. Subject-Matter of Colloids 2. Preparation of different types of Colloidal Systems 3. Tests.

Subject-Matter of Colloids:

The term ‘Colloid’ was first proposed by Thomas Graham to represent the heterogeneous mixture of suspended particles in a dispersion medium.

The particles of colloidal dimension range in size from 1 to 200 mµ. Collodial particles are too small to be seen with light microscope but are large enough to pass through permeable membranes or to scatter light. The colloids may have properties of fluidity (Sols) or without fluidity (Gels).

The different types of colloidal dispersions in liquids may be divided into two classes:

(a) Suspen­sion and

(b) Emulsions.

The former again may be LYOPHILIC (the dispersed phase and the liquid disper­sion medium are attracted to each other), and LYOPHOBIC (two phases actually repel each other). An emulsion is prepared by rigorously shaking two immiscible liquids together. The lyophobic suspension colloid and emulsion are both unstable colloidal systems.

The collodial systems have a number of physical and chemical properties:

(a) Scattering of light (Tyndall Effect);

(b) Brownian movement;

(c) Electrophoresis;

(d) Precipitation;

(e) Adsorption; etc.

The protoplasm of the living system is usually referred to as a collodial complex and exhibits many of the properties attributed to collodial system. Most of the collodial characters to protoplasm are due to the proteins present in it.

Proteins are very large complex molecules, which sometimes reach colloidal dimensions. They are dispersed throughout the ground substance of protoplasm where they are involved in such cell activities as respiration, digestion, secretion etc.

Preparation of Different Types of Colloidal Systems:

(a) Suspension Colloids:

(i) Dilute BaCl₂ solution is slowly poured in dil. H₂SO₄ containing tubes. Precipitation of BaSO₄ formed gradually settles down forming suspension.

(ii) About 1 ml of 30% FeCl₃ solution is slowly mixed with 200 ml of boiled water. An iron suspension sol. is thus formed due to the formation of Fe (OH)₃.

(iii) A small quantity of charcoal powder is added to the following liquids — dist. water, 1% NH₄OH and 0.5% acetic acid, separately. Gradually suspension systems are formed and the charcoal particles settle down.

(iv) Soluble starch is dissolved in water. A colloidal solution of starch in water is formed.

(b) Emulsion Colloids:

(i) A few drops of oil are mixed with water, thoroughly shaken for a few minutes and then allowed to stand. The oil separates out from the emulsion. But if a few drops of dilute alkali or alcohol are added to the mixture, the stability of the emulsion persists for a considerable period.

(ii) Wheat flour is mixed with distilled water and thoroughly shaken, then filtered. The filtrate contains a colloidal emulsion of protein in water.

(c) Sol-Gel Colloid:

Dried agar is dissolved in boiling water. An opalescent solution (Sol) is formed which sets into a gel on cooling. This change is reversible.

Tests for Colloids:

Experiment:

1. Brownian Movement:

(a) A fat taps of latex from a latex bearing are are taken, mounted immediately on a slide and observed under the microscope.

(b) 1ml of olive oil is thoroughly shaken in a test tube with 1 ml of 1% KOH solution and then a drop of this mixture is examined under the microscope.

2. Flocculation:

About 5 ml of arsenious sulphide (As₂S₃) collodial sol is taken in a test tube. Then a few drops of 0.01 M NaCI soln. or 0.01 M CaCI₂ soln. or 0.01 M CaCl₂ solution are added to the sol.

3. Imbibition:

Dry seeds of a measured quantity are placed in water or dil NaCI soln. for 1-2 days. The final weight is taken to denote the amount of water absorbed by the dry seeds.

Observations:

The erratic zigzag movement of the colloidal particles are observed.

The erratic movement of particles is observed.

The sol. is no longer stable and the particles are flocculated.

The rise of seed weight is noticed.

Inference:

The random motion is due to the unbalanced impact of the liquid molecules on the colloidal particles.

The erratic movement of particles is observed.

The collodial solution of As₂S₃has a negatively charged surface. Thus if NaCI, CaCI₂ or AICI₃ soln. is added, the cations are preferentially absorbed by the colloidal particles eventually resulting into a zero potential difference. Finally, the particles are flocculated.

The protoplasmic colloids of the dry seeds imbibe water osmotically and non-osmotically.