Q1.
How you will account for the presence of electrical charge on colloidal
particles?
Ans:In a particular colloidal solution, all the colloidal
particles carry the same type of charge, while the dispersion medium has an
equal but opposite charge. Thus, the charge on colloidal particles is balanced
by that of the dispersion medium and the colloidal solution as a whole is
electrically neutral.
For example:
In a ferric hydroxide {Fe(OH)3} sol, the colloidal ferric hydroxide
particles are positively charged, while the dispersion medium carries an equal
and opposite negative charge.
Q2. How is the presence of charge on
colloidal particles responsible for the stability of colloidal sols?
Ans: The stability of a colloidal
solution is mainly due to the presence a particular type
of charge on all the
colloidal present in it. Due to the presence of similar and equal charges, the
colloidal particles repel one another and are thus unable to combine together
to form larger particles or settle down . This keeps them dispersed in the
dispersion medium and the colloidal remains stable. This is why sol particles do
not settle down even on standing for a long time.
Q3. Explain the classification of
colloidal solution depending upon the nature of charge on the particles of the
dispersed phase?
Ans:
the colloidal solutions can be classified into positively charged and
negatively charged colloids. Some of the typical examples of it are as follows:
# THEORIES OF ORIGIN OF CHARGE ON COLLOIDAL PARTICLES
Q4: How will you account for the
origin of Charge on Colloidal Particles?
Ans: There are several views regarding
the origin of charge on colloidal particles. According to these views,
colloidal particles acquire charge due to the following reasons.
1.Due to dissociation of the
adsorbed molecular electrolytes: (i) (i) Colloidal particles have a strong tendency to adsorb reactant
or product molecules. The molecules thus adsorbed on the surface of colloidal
particles may undergo dissociation/ionization and may impart charge to them.
For example: During the preparation of sulphide
sols (e.g., As2S3 sol), H2S molecules get adsorbed on colloidal particles. H2S
molecules thus adsorbed undergo ionization and release H+ ions into the medium.
Consequently, colloidal particles are left with negative charge.
(ii)
Due to the dissociation of molecules forming colloidal aggregates: The
molecules responsible for the formation of aggregates of colloidal dimensions
may themselves undergo dissociation/ionisation resulting in the development of
charge on the colloidal particles formed by their aggregation.
For example: The soap
molecules (RCOONa) dissociate to give RCOO- and Na+ ions. RCOO- ions aggregate
due to weak attractive forces present in the hydrocarbon chains together to
form micelles which carry negative charge as explained earlier.
(2) Due to
frictional electrification
(i) It is supposed that the
frictional electrification due to the rubbing of the dispersed phase particles
with that of dispersion medium results in some charge on the colloidal
particles.
(ii) The dispersion medium must also
get some charge, due to the friction. As it does not carry any charge, theory
does not appear to be correct.
(3) Due to selective adsorption
of ions / Due to preferential adsorption of
ions from solutions: The colloidal particles have a tendency to preferentially
adsorb a particular type of ions from the solution. A colloidal particle
usually adsorbs those ions which are in excess
and are common to its own lattice.
For
example: When a ferric hydroxide sol is prepared by the hydrolysis of
ferric chloride in warm water, the colloidal particles of Fe(OH)3 formed have a
tendency to adsorb preferentially the Fe3+ ions present in the solution. This
is because Fe3+ ions are common to the lattice of Fe(OH)3 particle. The Fe3+
ions thus adsorbed impart positive charge to the colloidal particles present in
the sol.
(ii) For example: When a small quantity of silver
nitrate (AgNO3) solution is added to a large quantity of potassium
iodide KI solution, colloidal particles of the silver iodide adsorb I-
from the solution to become negatively charged, (at this stage
KI is in excess, and I-being common to AgI)
But, when a small quantity of
potassium iodide (KI) solution is added to a large quantity of silver nitrate
solution (AgNO3); the colloidal silver iodide particles adsorb Ag+
from the solution to become positively charged, (at this stage
AgNO3 is in excess and Ag+ is common to AgI),
1. Electrophoresis: Due
to the presence of a particular type of electrical charge, the colloidal
particles present in a colloidal dispersion move towards a particular electrode
under the influence of an electric field.
The
direction of movement of the colloidal particles is decided by the nature of
charge present on them. If
the colloidal particles carry positive charge, they move towards cathode when
subjected to an electric field and vice versa. The phenomenon is called
electrophoresis.
It may be defined as “ the movement of colloidal
particles towards a particular electrode under the influence of an electric
field”.
Electrophoresis
is an important phenomenon and finds several applications in industry.
2. Electro-Osmosis:
It
may be defined as the movement of
dispersion medium under the influence of an electric field in the situation
when the movement of colloidal particles is prevented with the help of a
suitable membrane.
The
colloidal solution is placed between two partitions made by semi permeable
membranes.The outer compartments consisting of platinum electrodes and side
tubes are filled with water. On passing electric current, water level begins to
rise in one of the side tubs and falls in the other.
The phenomenon can be explained as follows
:
We have already seen that the colloidal
particles and dispersion medium carry charges which are equal but opposite in
nature. Under the influence of an electric field, both have a tendency to move
towards the oppositely charged electrodes. Semi permeable membranes do not
allow the passage of colloidal particles. However, dispersion medium can pass
through them. Therefore during electro-osmosis, colloidal particles are checked
and it is the dispersion medium that moves towards the oppositely charged electrode.
3. Electrokinectic Potential OR Zeta Potential
Having
acquired a positive or a negative charge by selective adsorption on the surface
of a colloidal particle as stated above, this layer attracts counter ions from
the medium forming a second layer, as shown below. AgI/I- K+ AgI/Ag+ I – .
The
combination of the two layers of opposite charges around the colloidal particle
is called Helmholtz electrical double layer. According to modern views,
the first layer of ions is firmly held and is termed fixed layer while the
second layer is mobile which is termed diffused layer.
Since
separation of charge is a seat of potential, the charges of opposite signs on
the fixed and diffused parts of the double layer results in a difference in
potential between these layers. This potential difference between the fixed
layer and the diffused layer of opposite charges is called the electrokinetic
potential or zeta potential.
This potential difference between the fixed layer and the diffused layer
of opposite charges is called the electrokinetic potential or zeta potential.
Noted Credit: Mr Pradeep Sharma
