Coagulation of Colloidal Solutions
1) A small amount of an electrolyte is necessary for the stability of the colloidal sol. The ions of the electrolytes are adsorbed on the sol. particles and impart them some charge; positive or negative.
2) The charged colloidal particles repel one another and are prevented from coming close together to unite into bigger particles .
3) If somehow the charge is removed the particles will come nearer to each other to form aggregate (or coagulate) and settle down under the force of gravity.
For example: In the presence of a large excess of the electrolyte, the charge on the particles of the dispersed phase is neutralised and as a result, they come closer, grow in size and ultimately form precipitates.
Thus, the phenomenon of precipitation of a colloidal solution by the addition of excess of an electrolyte is called coagulation or flocculation.
Mechanism of Coagulation
When an electrolyte is added to the sol., the colloidal particles take up ions carrying opposite charge from the electrolyte. As a result, their charge gets neutralised and this causes the uncharged particles to come closer and to get coagulated or precipitated.
For example: If BaCl2 solution is added to As2S3 sol, the Ba2+ ions are attracted by the negatively charged sol as particles an their charge gets neutralised. This leads to coagulation.
Hardy Schulze Rule
The coagulation capacity of different electrolytes is different. It depends upon the valency of the active ion or called flocculating ion, which is the ion carrying charge opposite to the charge on the colloidal particles. According to Hardy Schulze rule, greater the valency of the the active ion or flocculating ion, greater will be its coagulating power.
Thus, according to Hardy Schulze rule :
1) 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 fourth power of the valency of the active ions (ions causing coagulation). Greater is the valency of the oppositely charged ion of the electrolyte being added, the faster is the coagulation.
For example: to coagulate negative sol of As2S3 the coagulating power of different cations has been found to decrease in the order as:
Al3+ > Mg2+ > Na+
Similarly, to coagulate a positive sol. such as Fe(OH)3 the coagulating power of different anions has been found to decrease in the order :
[Fe(CN)6]4- > PO43-> SO42-> Cl¯
The minimum concentration of an electrolyte which is required to cause the coagulation or flocculation of a sol is known as Coagulation value or flocculation value.
The coagulating power is inversely proportional to coagulation value or flocculation value.
Methods of Coagulation
Apart from the addition of the electrolyte, the coagulation of a colloidal sol can be affected by the following methods:
1) By Mutual Precipitation
When two- oppositely charged sols are mixed in equimolar proportions, they mutually neutralise their charge and both get coagulated.
For example: if positively charged Fe(OH)3 sol and negatively charged As2S3 sol are mixed, both the sols get coagulated.
2) By Electrophoresis
In the electrophoresis the particles of the dispersed phase move towards the oppositely charged electrodes. If the process is carried for a long time the particles will touch the electrode, lose their charge and get coagulated.
3) By Persistent Dialysis
The stability of colloidal sols is due to the presence of a small amount of electrolyte. If the electrolyte is completely removed by repeated dialysis, the particles left will get coagulated.
4) By heating or cooling
The sols get coagulated on heating.
For example: coagulation of butter
In some cases cooling the sol also results into its coagulation.
For example: coagulation of milk i.e., on cooling milk fats start floating on the surface.
Coagulation of Lyophilic sols
Lyophilic sols are more stable than lyophobic sols. The stability of lyophilic sols is due to two factors :
1) Charged and
2) Solvation of the colloidal particles.
When these two factors are removed, a lyophilic sol can be coagulated.
This can be done
(i) by adding electrolyte and
(i) by adding suitable solvent.
For example: when solvents such as alcohol or acetone are added to hydrophilic sols, it results into dehydration of dispersed phase. Under this condition a small quantity of electrolyte can cause coagulation.
Protection of Colloids
Lyophobic sols such as those of metals like gold, silver etc. can be easily precipitated by the addition of a small amount of electrolytes.
They can be prevented from coagulation by the previous addition of some stable lyophilic colloids like gelatin, albumin, etc. This is because when a lyophilic sol is added to the lyophobic sol, the lyophilic particles form a layer around the lyophobic particles and this protect them from electrolytes.
If a small amount of gelatin is added to gold sol, it is not readily precipitated by the addition of sodium chloride. This process of protecting the lyophobic colloidal solutions from precipitation by the electrolytes due to the previous addition of some lyophilic colloid is called protection. The colloid which is added to prevent coagulation of the colloidal sol is called protecting colloid.
Gold number: The different protecting colloids differ in their protecting powers.
The minimum amount of the protective colloid in milligrams required to just prevent the coagulation of a 10 mL of a given gold sol when 1 mL of a 10% solution of sodium chloride is added to it.
The coagulation of gold sol is indicated by change in colour from red to blue. Smaller the value of the gold number, greater will be protecting power of the protective colloid.
|Egg albumin||0.1- 0.2|
|Gum arabic||0.15- 0.25|
Therefore, reciprocal of gold number is a measure of the protective power of a colloid. Gelatin is the best protective colloid.