Question

What is lyophilic colloids coagulation?

Answer 1

Colloids occupy an intermediate place between suspension and solutions. They bridge the microscopic and the macroscopic, they possess some of the properties of both which makes colloids highly adaptable to specific uses and functions.

Complete answer:
“The phenomenon of the precipitation of a colloidal solution by the addition of the excess of an electrolyte is called coagulation or flocculation.”
Lyophilic sols are more stable than the lyophobic sols. The stability of lyophilic sols is due to the solvation i.e. a protective layer is created around the colloidal particles by the dispersion medium in which they are dispersed.
Coagulation of lyophilic sols, there are two factors responsible for the stability of lyophilic sols. These factors are the charge and solvation of the colloidal particles. When these two factors are removed, a lyophilic sol can be coagulated.
$(1)$ By adding electrolyte- When an excess of an electrolyte is added to a sol, oppositely charged ions of electrolyte removes the charged on sol particles. Neutral sol particles aggregate to form a precipitate. Any negatively charged sol is coagulated by the cations of electrolyte and vice-versa. The ion carrying the charge responsible for coagulation Is called a flocculating ion.
Example- if $BaC{l_2}$ is added to the $A{s_2}{S_3}$ sol which is negative sol, $B{a^{ + 2}}$ ions causes coagulation of sol.
The minimum concentration of an electrolyte in millimoles per litre required to cause precipitation of sol in $2$ hours is called flocculating value.
$(2)$ By the prolonged electrophoresis, the ions are combining and neutralized coagulate. Under the influence of an electric field, sol particle move to the electrode where they get discharged as a neutral particle. Neutral sol particles aggregate to form a precipitate.
Example- Rubber from latex is coagulated by electrophoresis on cloth or metal.
Now, we need to know what is Flocculating power or coagulating power.
According to Hardy Schulze rule, greater the valency of the action ion or flocculating ion, greater will be its coagulating power. Thus, Hardy Schulze law state:
$2$ The ions carrying the charge opposite to that of sol particles are effective in causing coagulation of the sol.
$(2)$ Coagulating power of an electrolyte is directly proportional to the valency of the active ions (ions causing coagulation.)
For example- To coagulate negative sol of $A{s_2}{S_3}$as $Fe{(OH)_3}$, the coagulating power of different anions has been found to decrease in the order: ${[Fe(C{N_6})]^{4 - }} > P{O_4}^{3 - } > S{O_4}^{2 - } > C{l^ - }$
Coagulation or flocculation value: The minimum concentration of an electrolyte which is required to cause the coagulation or flocculation of a sol is known as flocculation value.
OR
The number of millimoles of an electrolyte required to bring about the coagulation of one litre of a colloidal solution is called its flocculation value.
Coagulation value or flocculating value $\propto \dfrac{1}{{CP}}$ (CP is coagulating power).

Note:
A sol is a colloid made out of very small solid particles in a continuous liquid medium. When solvent such as alcohol and acetone are added to hydrophilic sols the dehydration of dispersed phase occurs. Under this condition a small quantity of electrolyte can bring about coagulation.
The power of protection of lyophilic sols is measured in terms of gold number. The number of milligrams of lyophilic colloid that will just prevent the precipitation of $10ml$ of gold sol on the addition of $1ml$ of $10\%$ sodium chloride solution is a gold number.