Precipitation Of Colloids

Precipitation of colloids

To cause the precipitation of colloids,we have to remove the surface charge present in the colloidal particles.The removal of surface charge only may not cause the precipitation of lyophilic sol though it decreases its stability.To cause the precipitation of lyophilic sol,the solvent sheath also should be removed but in the case of lyophobic sol,due to the absence of solvent sheath,removal of surface charge only cause their complete precipitation.Thus,precipitation of any colloids can be done by the following three methods:

i)Electrophoresis

ii)By mixing of two oppositely charged sol

iii)By addition of electrolyte

Electrophoresis

The migration of disperesed phase(solute particles) under the influence of electric field in colloidal solution is known as electrophoresis or cataphoresis.

Electrophoresis of positively charged particles (cations) is called cataphoresis, while electrophoresis of negatively charged particles (anions) is called anaphoresis. Electrophoresis is a technique used in laboratories in order to separate macromolecules based on size. The technique applies a negative charge so proteins move towards a positive charge. This is used for both DNA and RNA analysis.

By mixing of two oppositely charged sol

When oppositely charged sols are mixed in almost equal proportions, their charges are neutralised. Both sols may be partially or completely precipitated as the mixing of ferric hydroxide (+ve sol) and arsenious sulphide (–ve sol) bring them in precipitated form. This type of coagulation is called mutual coagulation or meteral coagulation.

By addition of electrolyte

The particles of the dispersed phase i.e., colloids bear some charge. When an electrolyte is added to 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 solution is added to sol the ions are attracted by the negatively charged sol particles and their charge gets neutralised. This lead to coagulation.

Hardy Schulze Rule

Hardy schulze Rule states that the precipitating power of an ion to the sol of opposite charge depends on the valency of ion.According to this rule,higher is the valency of ion,higher will be its power to coagulate the colloidal solution.For example:

i)To cause the precipitation of positively charged ferric hydroxide sol,trivalent anions are more effective than bivalent anions and bivalent anions are more effective than monovalent anions.

ii)To cause the precipitation of negatively charged arsenious sulphite sol,trivalent cations are more effective than bivalent cations and bivalent cations are more effective than monovalent cations.

So,Hardy Schulze Rule states that:

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

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

Question:Why are alums added in the town water supply?

Water consists of susppended impurities of colloidal dimension.When alums are added in such water,the ions provided by alums remove the surface charge present in the colloidal impurities so that their precipitation occurs.Such precipitation can be easily removed by filtration and water will be obtained in pure form.

Question:Why FeCl₃ solution is applied in a cut to prevent bleeding?

Blood is a colloidal solution and impurities of colloidal dimension.When FeCl₃ solution is applied in a cut,the ions provided by FeCl₃ molecule neutralise the charge present in colloidal particles and precipitation or clotting occurs at the surface.Due to this phenomenon,excess of bleedind is prevented.

Origin of charge on colloidal particles

Generally,charge is originated in colloidal particles due to adsorption of common ion present in the solution.Ferric hydroxide sol formed by adding FeCl₃ in hot water absorb Fe³⁺ ion from the solution so that sol particles become positively charged.Similarly As₂O₃ sol absorb sulphite ion from the solution and its particles become negatively charged.

Some sol absorb ion from the solution that are present in excess.For example:AgCl sol formed by precipitation reaction of NaCl and AgNO₃ adsorb Ag⁺ ion if it is in excess and becomes positively charged.Similarly it adsorbs Cl‾ is excess and becomes negatively charged.

The charge on the surface active agent originate due to the ionization of surface active group.

Cleansing action of soap and detergent

When water comes in contact with the dirt particles,it cannot be removed by washing due to the formation of interfacial tension between two phase.When soap or detergent is added to the awter,they minimize the interfacial tension because of their adsorption on water surface.Actually interfacial tension is minimized due to the decreasement of surface tension of water.When interfacial tension is minimized,two phases start to mix with each other and the dirt particle can be removed by washing.

Alternatively,the cleansing action of soap and detergent can be explained by considering their behaviour when added in water.

Let us consider sodium sterate(C₁₇H₃₅COONa) is an example of typical soap.When sodium sterate is added in water,it ionises as:

Sodium sterate(C₁₇H₃₅COONa)→C₁₇H₃₅COO^‾ + Na⁺

At or above a particular concentration,C₁₇H₃₅COO⁻ groups of soap start to aggregate and give a globular structure called miscelle.In a miscelle,the long chain of non-polar hydrocarbon tail face inward and polar groups face outward.

When such miscelle comes in contact with the dirt particle,then the non-polar hydrocarbon tail of miscelle attack the dirt particle so,the emulsification of dirt particle takes place.Such an emulsified dirt particles can be easily removed by washing.

Associated Colloids

The colloids that are formed by aggregation or association of large number of particles are called as associative colloids.Miscelle is a typical example of associative colloid.

Miscelle formation occurs at or above a particular concentration called Critical Miscellation Temperature(CMC).

Below CMC,soap particles exist as an individual entities but when CMC is reached they start to aggregae and give a miscelle.The conversion of individual entities to miscelle is a reversible process and miscelle can be broken by decreasing concentration or by increasing dilution.

Flocculation andd Flocculation value

Flocculation is a special case of precipitation in which the precipitated particles are float on the surface of liquid.

The number of millimoles of an electrolyte that is to be added in one litre of colloidal solution so as to cause the complete precipitation is known as flocculation value.

Emulsion

Emulsions are colloida system in which both dispersed phase and dispersion medium are liquid.There are two types of emulsions.They are:

i)Oil in water type emulsions:In this emulsion,oil act as dispersed phase and water cat as dispersion medium.Example:milk

ii)Water in oil type emulsions:In this type of emulsion,water act as dispersed phase and oil act as dispersion medium.Example:curd.

Preparation of colloids

Emulsion can be prepared by agitating two liquid mixture or by passing the two liquid mixture through a colloidal milk or homogenizer.Emulsionprepared by mixing of two liquids are generally nstable due to the formation of interfacial tensionbetween two liquids.To increase the stability of emulsion,certain substances are added to it.These substances minimize the interfacial tension because of their adsorption.These substances are called as emulsifying agent.

Demulsification

Demulsification is the phenomenon of breaking of emulsion into its constituent liquids.Demulsification can be done by the following methods:

i)heating

ii)centrifuging

iii)by addition of electrolyte

Gels

Gels are a type of colloidal system in which liquids are disperesed in interconnecting network of solid.There are two types of gel.They are:

i)Elastic gel:

A gel is said to bof gel,the fibrils are elastic in nature if its completely dehydrated form can be regenerated on addition of water.In this type of gel,the fibrils are(outermost layer) are elastic in nature so that it can easily absorb water.Example:Agar-Agar gel.

ii)Non-elastic gel:

A gel is said to be non-elastic if its completed dehydrated form cannot be regenerated on addition of water.In this type

of gel,the fibrils are of rigid type.Example:silica gel.

Properties of gel

i)Swelling:

Partially dehydrated gel absorb water and increase in size.This process is known as swelling or expansion.

ii)Syneresis:

Certain gel undergo shrinkage on long standing due to the loss of water.This property of gel is known as syneresis.

iii)Thioxotropy:

Certain gel convert to sol form on shaking and revert to gel form on standing.This sol-gel conversion is known as thioxotropy.

References

Paula, P.Atkins & J.de. Elements of Physical Chemistry. New Delhi: Oxford University Press, 2009.