Coagulating value
The minimum concentration of an electrolyte in millimoles per litre required to cause precipitation of a sol in two hours is called coagulating value.
The smaller the quantity needed, the higher will be the coagulating power of an ion.
Protective colloids
A protective colloid is a lyophilic colloid that when present in small quantities keeps lyophobic colloids from precipitating under the coagulating action of electrolytes.
When a lyophilic sol is added to the lyophobic sol, the lyophilic particles form a layer around lyophobic particles and thus protect the latter from electrolytes.
Lyophilic colloids used for this purpose are called protective colloids.
Gold Number
The Gold Number is the minimum weight (in milligrams) of a protective colloid required to prevent the coagulation (colour change from Red to Violet) of 10 ml of a standard hydro gold sol when 1 ml of a 10% sodium chloride solution is added to it.
Emulsion
These are liquid-liquid colloidal systems, i.e., the dispersion of finely divided droplets in another liquid.
Stability of Emulsion
Emulsions of oil in water are unstable and sometimes they separate into two layers on standing.
For stabilisation of an emulsion, a third component called emulsifying agent is usually added.
The emulsifying agent forms an interfacial film between suspended particles and the medium.
The principal emulsifying agents for O/W emulsions are proteins, gums, natural and synthetic soaps and for W/O, heavy metal salts of fatty acids, long chain alcohols, lampblack
Colloids in our daily life
Blue colour of the sky
Fog, Mist and Rain
Food articles: Milk, Butter, Halwa, Ice Creams, Fruit Juices
Blood: Alum and Ferric Chloride solution used to coagulation of blood
Soils
Applications of colloids
1. Electrical precipitation of smoke
2. Purification of drinking water
3. Midicines:
a) Argyrol is a silver sol used as an eye lotion.
b) Colloidal antimony is used in curing kalaazar.
c) Colloidal gold is used for intramuscular injection
d) Milk of magnesia, an emulsion, is used for stomach disorders.
4. Tanning
5. Photographic plates and films: Photographic plates or films are prepared by coating an emulsion of the light sensitive silver bromide in gelatin over glass plates or celluloid films.
6. Rubber industry: Latex is a colloidal solution of rubber particles which are negatively charged. Rubber is obtained by coagulation of latex.
7. Industrial products: Paints, inks, synthetic plastics, rubber, graphite lubricants, cement, etc., are all colloidal solutions.