Sedimentation or Dorn Effect

Sedimentation or Dorn Effect

1) We know that the dispersed phase remains suspended in Dispersion medium.

2) It is a two phase system and very stable.

3) But even though these colloidal dispersions are fairly stable, the particles slowly settle down under the influence of gravity when kept undisturbed for a longer period of time.

4) It is a very slow process and is just like falling of a spherical body in highly viscous medium.

5) While settling the heavy particles settle first at the bottom and the light particles at the top.

6) Such settling of the colloidal particles under the influence of an gravity is called as Sedimentation or Dorn Effect.

7) In 1880, DORN observed this phenomenon first ,hence it is known as DORN effect.

8) The Potential developed due to this is called as Sedimentation Potential.

9) Sedimentation Potential can be measured by placing the electrodes at different levels in the falling stream of the colloidal particles.

10) It is just reverse of Electrophoresis i.e in Electrophoresis there is movement of colloidal particles due to an applied potential while in Dorn effect the potential is developed due to movement of colloidal particles.

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Electro-Osmosis - An Electro Kinetic Phenomenon

Electro-Osmosis

Electroosmosis was first observed by a Russian scientist F.F. Reuss in 1809.

1) We know that a COLLOID is a substance which is microscopically dispersed (widely spread) evenly throughout another substance.

2) The Colloidal particles form Dispersed phase and the medium used for this is called as Dispersion medium.

3) Also we all know that Electrophoresis is a phenomenon in which the colloidal particles move under the influence of an electric field.

4) But,Suppose if it happens that, the colloidal particles are not allowed to move under the influence of an electric field, then What Will Happen...?

Nothing but.....

The dispersion medium itself will begin to move under the influence of the electric field.

5) This movement of the dispersion medium under the influence of an electric field is called as "Electroosmosis" .

6) The pressure required to counter the Electro osmotic flow is called as Electroosmotic pressure.

7) To study the Phenomenon of Electroosmosis consider the experimental setup as shown in the figure below:-

8) The colloidal solution(sol) is taken in X-compartment whereas the Y and Z compartments are filled with Dispersed medium,here water is taken as dispersion medium

9) The X compartment is seperated from Y and Z compartments by a semipermeable membrane or a porous partition.

10) A positive and a negative  electrode is immersed in the Y and Z compartments as shown in the figure.

11) When a potential is applied at the electrodes the dispersion medium will move from one side to other which can be observed on the basis of the movement of the boundries in the side tubes.

12) The direction of the movement of dispersion medium will be opposite to the direction of flow of the colloidal particles ,i.e., if colloidal particles are positively charged then the dispersion medium moves towards the negative electrode and vice versa.

13) The Zeta Potential developed due to electroosmosis is given by :-

                          

                                     Pπr² /2DE

Where,

P = Electroosmotic pressure in N/m²

r = radius of side tube in cm

E = Potential applied

D = Dielectric constant of medium

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Electrophoresis - An Electro Kinetic Phenomenon

Electrophoresis

Electrophoresis is one of the Electro Kinetic Phenomenon (a family of several different effects that occur in heterogeneous fluids i.e the fluids containing particles which can be solid, liquid or gaseous). It was first observed by Ferdinand Frederic Reuss in 1807.

1) The Colloidal particles carry an Electric charge on it either Positive(+) or Negative(-).

2) Due to this charge present on colloidal particles, they move towards one of the electrodes when an electric Potential is applied to the Sol(Colloidal solutions).

3) This migration of colloidal particles under the influence of an electric field is called as "Electrophoresis".

4) In the other words, on the basis of Electrophoresis phenomenon it is possible to detect the charge present on the colloidal particles.

5) This can be studied with the help of an simple experimental set up.

6) Consider that in a U-shaped glass tube Colloidal solution is taken. A small amount of electrolyte having lower density than sol is also introduced in the U-tube.

7) Due to it's lower density the electrolyte is displaced upward and a sharp boundary is formed between the sol and the electrolyte.

8) Now a Positive electrode is immersed at one end of the U-tube whereas the Negative electrode is immersed at the another end and the electricity is passed.

9) The colloidal particles will move towards one of the electrodes.

10) If they move towards Positive(+) electrode then they are Negatively (-) charged whereas if they move towards Negative(-) electrode then they are Positively(+) charged.

11) Electrophoresis shown by positively charged particles i.e cations is called as Cataphoresis , while Electrophoresis shown by negatively charged particles i.e anions is called as Anaphoresis .

12) Amino acids,Nucleotides,Polypeptides,Proteins and many other compounds in their colloidal states can be separated by Electrophoresis.

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Origin of Charges on Colloidal Particles (COLLOIDS)...!

Origin Of Charges On Colloidal Particles (Colloids)

• As we know that in Colloids the Dispersed phase remains suspended in Dispersion medium.
• The Colloidal system is a two phase system and very stable.
• It does not grow in size or lead to precipitation.
• This remarkable stability of Colloids is due to the presence of a small quantity of electrolyte in the medium and presence of charge on the surface of colloids.
• The colloidal particles carry an electric charge on it either positive or negative.
• Due to the presence of similar type of charges, the colloidal particles repel one another and do not combine together to form larger particles.
• This keeps them dispersed in the medium and the colloids remains stable.
• The type of charge present on the Colloidal particles can be detected by Electrophoresis experiment.

The various factors responsible for the origin of charges on the

colloidal particles are as follows :-

1) Dissociation of Molecules :-

The charge on the colloidal particles can be generated by the dissociation of molecules.

For eg :-

Soaps are sodium salts of fatty acids. In solutions they dissociate to give rise to ions as shown in the reaction below :-

R-COONa -----> RCOO- + Na+

The RCOO ions consists of two parts,long hydrocarbon chain ' R ' which is hydrophobic i.e water repelling and the polar group i.e ' COO ' which is hydrophilic i.e water loving.

As they form ions their hydrocarbon chain points at centre and COO remain outward.

This gives the colloids extra stability.

2) Medium :-

The charge present on the particles might be due to the medium in which they are present.

For eg:- Consider Proteins,

a) Amino acids are the building blocks of Proteins.

b) Amino acids have two functional groups i.e., Primary Amine & Carboxylic Acid.

c) In Acidic medium ---->The Primary Amine i.e the NH2 group accepts a proton from an acid and becomes positively(+) charged.

H2N - RCOOH --(H+)--> NH3 -RCOOH

Thus In Acidic Medium the proteins will move towards Negative(-) electrode.

d) In Alkaline medium ----> The Carboxylic Acid i.e the R-COOH group loses a proton from COOH group and becomes Negatively(-) charged.

RCOO + H2H2N - RCOOH --(OH-)--> H2N - O

Thus In Alkaline Medium the proteins will move towards Positive(+) electrode.

3) Surface Adsorption :-

The Colloidal particles have large surface area and are capable of adsorbing. Sometimes it happens that Certain kinds of ions from Dispersion medium get adsorbed on the colloidal particles and this can be the reason for the charge present on colloidal particles.

For eg:- Consider that,

AgI ' Sol ' is prepared by the reaction between Silver Nitrate

(AgNO3) and Potassium Iodide (KI).

Now, If excess of KI is added to AgNO3 , AgI sol absorbs I- ions preferably,forming a Negative(-) colloid, Whereas, If excess of AgNO3 is added to KI, then AgI sol absorb Ag+ ions preferably,forming a Positive(+) colloid.

4) Frictional Electrification :-

Sometimes it happens that the dispersed phase particles i.e the Colloidal particles gets rubbed with the particles of the dispersion medium giving rise to friction and this could be the reason for the charge present on the colloidal particles.

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