Electrolphoresis, Electro-osmosis, Protection and Coagulation of colloidal solution

Electrophoresis

The migration of electrically charged colloidal particles under the influence of an electrical potential is called electrophoresis. This phenomenon can be studied by using the experimental arrangement which consists of a U tube containing the sol. A small quantity of dispersion medium is added to either of the two limbs. Distinct boundaries are formed between the colloidal solution and the added dispersion medium. With the help of the two electrodes, potential difference is now applied. It is seen that the boundary starts moving towards one of the electrodes. The movement of boundary occurs due to the movement of the colloidal particles. under the influence of applied potential difference. If the boundary moves towards the cathode, it may be concluded that the sol is positively charged and the if it moves towards the anode, it is negatively charged. The distance moved by the particle in one second and under a potential gradient of one volt /cm is called electrophoretic mobility. Electrophoretic mobility is related to zeta potential.

Some of the positively charged and negatively charged sols are given below:

Positively charged sol : Ferric hydroxide, almunium hydroxide, chromium hydoxide, zirconium hydroxide, etc.

Negatively charged sol : Gold, silver, platinum, sulphur, selenium, arsenoius sulphide, copper sulphide, lead sulphide, silicic acid, etc.

Applications of electrophoresis

i) In determining the charge : The sign of thr charge of the sol particles can be ascertained by observing directions in which the particles migrate. It is always opposite to the charge on the sol particles.

ii) In the removal of dirt from sewage : The sewage contains dirt particles suspended in water forming a negative sol. The negatively charged particles coagulate on anode.

iii) In removal of carbon particles from smoke : Smoke is a negative sol consisting of carbon particles dispersed in the air. Thus, these particles are removed by passing through a chamber provided with highly positive charged metallic knob.

iv) Electro-deposition of rubber : Rubber is an emulsion of negatively charged particles dispersed in water. It can be deposited on articles taken as anode in the process of electrophoresis.

Electro-osmosis

The colloidal particles move under the influence of an electric potential. However, if the colloidal particles are prevented from moving, the solvent moves under the influence of the electric potential. The migration of the solvent under the influence of electric potential is called electro-osmosis. In this process, the colloidal solution is sandwitched between the dispersion medium and the level of the level of the liquid in the vertical glass tubes is noted. An electrical potential difference is applied across the colloidal system with the help of two electrodes. It is observed at the level of the solvent rises in one of the tube due to the migration of the solvent. A difference of pressure steadily built up till the movement of solvent due to electro-osmosis is exactly balanced by a flow in the opposite direction due to the developed pressure difference. The difference of pressure biult in this way is called electro-osmotic pressure.

Coagulation of Colloidal Solutions

Presence of small concentration of appropriate electrolytes is neccessary to stabilize the colloidal solutions. However, if the elecctrolytes are present in higher concentration, then the ions of the electrolyte neutralize the charge of colloidal particles. Now, these colloidal particles may unite together to form bigger particles which are then precipitated. The precipitation of colloids through induced aggregation by the addition of siutable electrolyte is called coagulation or flocculation.

The coagulation of colloidal solution by an electrolyte does not take place until the added electrolyte has certain minimum concentration in the solution. The minimum concentration of electrolyte in millimoles that must be added to one litre of the sol so as to bring about complete coagulation is called coagulation or flocculation value of the electrolyte for the sol.

Different electrolytes have different coagulation values. Smaller the coagulation value of the electrolyte, the larger is its coagulating or precipitating power. The coagulation behaviour of various electrolytes was studied in details by Hardy and Schulze. They observed that

i) The ions carrying charge opposite to that of sol particles are effective in causing the coagulation of the sol.

ii) Coagulating power of the electrolyte is directly proportional to the valency of the active ions i.e ions causing coagulation.

Thus, for the coagulation of sols carrying negative charge (like As₂S₃sol), Al⁺³ ions are more effective than Ba⁺² or Na⁺ ions. Similarly, for the coagulation of sols carrying positive charge, such as Fe(OH)₃sol, PO_^3-4ions are more effective than SO^3-₄or Cl^- ions. These two observations are collectively called as Hardy-Schulze rule.

Coagulation of colloidal solids can be achieved by following methods:

i) By mutual precipitation : When two oppositely charged sols such as Fe(OH)₃ and As₂S₃ are mixed in equi-molar proportions, they neutralize each other and may get coagulated. Sometimes, the sols may get coagulated due to the mutual destruction of stabilizing agents.

ii) Electro-phoresis : We know that during electro-phoresis the sol particles move towards the oppositely charged elecctrodes. If the process is carried for a long time, the particles will touch the electrodes, lose their charge and get coagulated.

iii) By repeated dialysis : The stability of colliodal sols is due the presence of a small amount of electrolyte, lose their charge and get coagulated.

iv) By heating : The sol may be coagulated even by simple heating.

Protection of Colloids

Lyophobic sols such as those of metals ilke 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. For example 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 electrolytes due to the previous addition of some lyophobic colloids is called protection. The colloid which is added to prevent coagulation of colloidal sol is called protecting colloid.

The protecting power of different protective ( lyophilic) colloids is expressed in terms of gold number.

Gold Number

The different protecting colloids differ in their protecting powers. Zsigmondy produced a term called gold number to describe the protective colloids. This is defined as the minimum amount of the protective colloids in milligrams required to just prevent the coagulation of a 10ml of a given sol when 1ml of 10% solution of sodium chloride is added to it.

The coagulation of gold sol is indicated by change in colour from red to blue. The gold number of few protective colloids are as follows:

Gelatin : 0.005-0.01

Haemoglobin : 0.03-0.07

Egg albumin : 0.1-0.2

Gum arabic : 0.15- 0.25

Starch : 20-25

Dextrin : 6-20

It may be noted that smaller the value of the gold number , greater will be protecting power of the protective colloid. Therefore, reciprocal of gold number is the measure of the protective power of a colloid. Thus, out of the list gelatin is the best protecctive colloid.

Reference :

Glasstone, Samuel. Textbook of Physical Chemistry. New Delhi: Macmillan India Ltd, 1996.

Kapoor, L.,. Textbook of Physical Chemistry, Vol I and Vol II. Macmillan India Ltd, 1992.

Kundu, N. and Jain, S.K.,Physical Chemistry, Ist Edition. Physical Chemistry. New Delhi: S. Chand and Company (Pvt) Ltd, 1996.

Maron, S.H. and Prutton C.F.,. Principles of Physical Chemistry. Oxford and IBH publication Company, 1992.