8.1 Protective colloids

2.1 Characteristics of Adsorption
2.2 Types of Adsorption

4.1 Freundlich adsorption isotherm
4.2 The Langmuir adsorption isotherm

7.1 Different Colloidal Systems
7.2 Classification of Colloidal System
7.3 Preparation of Colloidal System
7.4 Purification of Colloidal System
7.5 Properties of colloidal system

8.1 Protective colloids

9.1 Foams
9.2 Applications of colloids and emulsions

10.1 Positive and negative catalysis
10.2 Types of catalysis

12.1 Characteristics of Enzymes
12.2 Mechanism of enzyme catalysis
12.3 Autocatalysis
12.4 Induced catalysis

Lyophilic colloids are much more stable than the lyophobic colloids. The extra stability of the former is due to the fact that they are extensively solvated.

The extra stability of lyophilic colloids imparts them the property of protecting lyophobic colloids from precipitation by electrolytes. When a lyophilic sol is added to a lyophobic sol, the former particles provide envelopes around the latter particles and thus protect the latter from the action of electrolytes. Lyophilic colloids used for such purposes are known as protective colloids. Thus, the addition of gelatin to a gold sol protects the latter by the action of limited amount of sodium chloride.

The lyophilic colloids differ in their protective powers. Zsigmondy introduced the term gold number to measure the protective power of different colloids. He defined the gold number as the weight in milligrams of a protective colloid which checks the coagulation of $10\ mL$ of a given gold solution on adding $1\ mL$ of a $10\%$ solution of sodium chloride. Thus, smaller the gold number of a lyophilic colloid, greater is its protective power.

Ostwald introduced another term congo rubin number which is defined as the amount of protective colloid in milligrams which prevents colour change in $100\ mL$ of $0.01\%$ congo rubin dye to which $0.16\ gm$ equivalent of $KCl$ is added.

Question 2. Given the gold number of the following colloids.

1. Gelatine

2. Casein

3. Egg albumin

4. Gum arabic

Solution: The gold number of the following are:

1. Gelatine - $0.005-0.01$

2. Casein - $0.01-0.02$

3. Egg albumin - $0.1-0.2$

4. Gum arabic - $0.15-0.25$