i.
a
i.
FILTRATION
The process in which insoluble particles are separated from a liquid is called
filtration. For filtration we can use a Filtration paper or Filter crucible.
Filter
Paper:
The filtration by filter paper is a slow process. In this process solvent
passes through filter paper and insoluble particles are left behind on the
filter paper. Many types of filter papers are available. They have different
porosites (pore sizes). A suitable filter paper is chosen. Its size should be
large so that it should be
to full of precipitates at the end of
filtration.
to full of precipitates at the end of
filtration.|
The funnel should be large so that its rim should extend
1-2 cm above the filter paper. The stem of funnel should be long. It should
touch the side of beaker. Moreover stem of funnel should remain full of
liquid during the filtration. The filter paper should be fitted properly
otherwise it is time wastage.
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Folding
of Filter Paper:
i.
Filter paper should be
folded twice.
ii.
The first fold should
be along the diameter.
iii.
In second fold the
edges should not be quite matched.
iv.
The folded filter
paper should be opened on the larger section.
v.
A cone is formed by
three fold side and one fold side.
vi.
The apex angle should
be greater than 60o.
vii.
Then filter paper is
placed into the 60o funnel.
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viii. Finally wet the filter paper and press it firmly.
ix. The rate of filtration can be increased by gentle suction.
x. The rate of filtration can also be increased by fluted
filter paper.
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(Fluted
Filter Paper)
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Fluted
Filter Paper:
A Filter Paper Raving many elevations and depressions is called fluted filter
paper. A filter paper folded by fan like arrangement with alternate elevations
and depressions given fluted filter paper.
Filter
Crucibles:
We use filter crucible for an easy filtration. There are two types of filter
crucibles.
i.
Gooch Crucible
ii.
Sintered glass
Crucible
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i. Gooch
Crucible:
It is made of porcelain. Its bottom is perforated covered with paper pulp.
The Gooch crucible is placed on suction apparatus. It is shown in figure. It
is used to filter those precipitates which are to be heated at high
temperature. Some solutions like HCl, KMnO4 react
with paper. So their filtration is done by covering Gooch crucible with
asbestos [CaMg3 (SiO3)4].
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(Gooch
Crucible Fitted on Filtring Apparatus)
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ii. Sintered
Glass Crucible:
It is made of glass. Its bottom is sealed with sintered (porous) glass disc.
It needs no preparation. All solutions can be filtered by it.
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(Sintered
Glass Crucible)
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ii.
Crystallization
The process of crystals formation is called crystallization. When a hot
saturated solution of a substance is cooled, then crystals of solid substance
separate out. It is called crystallization. This process is used for
purification of solid compounds. E.g. Purification of naphthalene. The process
of crystallization involves the following steps.
i. Choice
of Solvent:
A suitable solvent is chosen by hit and trial method. An ideal solvent has
following properties
i.
It should dissolve
maximum amount of solute.
ii.
It should not react
with solute.
iii.
It should not dissolve
the impurities.
iv.
On cooling, it should
form pure crystals
v.
It should be cheap
(in-expensive)
vi.
It should be easily
removable.
vii.
Its use should be safe
viii.
An inflammable solvent
should be heated on water bath. The most common solvents are water, rectified
spirit(95% alcohol), absolute alcohol(99% alcohol), ether, chloroform, acetone,
acetic acid, carbon tetra chloride.
ii. Preparation
of Saturated Solution:
The substance is dissolved in minimum amount of solvent. To dissolve more and
more solute, the solution should be heated directly or on water bath. The
stirring of solution is necessary.
Precaution:There are certain solutions which are volatile and these should
be heated with the help of water bath.
Diagram……
iii. Filtration:
The hot saturated solution is filtered to remove insoluble impurities.
Sometimes premature crystals can form. It can be prevented by using hot water
funnel.
Diagram…..
iv. Cooling:
The hot filtered solution is cooled. The moderate cooling gives the medium
sized crystals and slow cooling gives bigger crystals. Therefore slow cooling
is avoided because it contains solvent as an impurity. Sometimes a few crystals
of pure solid are added in solution to start the crystallization process.
v. Collecting
the Crystals:
When the crystallization is complete, then mixture of crystals and mother
liquor is filtered through Gooch crucible using a vacuum pump. The mother
liquor is removed completely by full suction. Then the filter cake is pressed
to remove the rest mother liquor. Finally the crystals are washed with small
amount of cold solvent. This process is repeated many times till pure crystals
are formed. By evaporation of the mother liquor we can get a fresh crop of
crystals. The mixture through which crystals are formed called mother liquor.
Diagram………
vi. Drying
of Crystals:
There are three methods for drying the crystals.
a) Press the crystals between the folds of filter
paper. Repeat this process many times but by this method crystals are crushed
and fibers of filter paper can mix with them.
b) Dry the crystals at 100oC in an
oven. Here solid should not melt or decompose at 100oC. This method
is also not reliable because there are many solids which have low very low m.p
can not be dried.
c) Spread the crystals over watch glass and keep
in vacuum desiccators for many hours. The drying agents used in desiccators are
CaCl2, P2O5, Silica gel. This is the most reliable
method
Diagram…………….
vii. Decolourization
of Undesirable Colours:
Sometimes crystals become coloured due to impurities. To prevent it, the
substance is boiled with animal charcoal. Animal charcoal absorbs the coloured
impurity. The pure colourless crystals are formed by cooling the filtrate.
iii. Sublimation
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The process in which a solid, on heating directly changes into vapours
without passing through liquid state is called sublimation. The hot vapours
are then condensed to form the solid again. It is called sublimate. The
process of sublimation is used to purify the solids. E.g. Naphthalene, NH4Cl,
Iodine and benzoic acid etc. the solid substance is taken in a watch glass. A
funnel is inverted on it. The solid is heated slowly over a sand bath. The
funnel is cooled by wet cotton. The pure solid deposits on the inner sides of
funnel and impurities are left behind in watch glass.
Limitation: Those substances can be sublimed which have
high v.p than their m.p
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iv. Solvent
Extraction
We can separate a solute from a solution by shaking it with an immiscible
solvent in which solute is more soluble. It is called solvent extraction. It is
mainly used to separate organic compounds from water. It is done by an
apparatus called.
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i. Separating
Funnel:
It is shown in figure. The example of solvent extraction is ether extraction.
In this method an aqueous solution containing an organic compound is taken in
separating funnel. The ether is also put into the funnel. It is stoppered,
shaken well and allowed to stand. After sometime two layers are formed. The
organic compound goes to ether layer and
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Inorganic impurities
remain in water layer. The ether layer is separated. It is evaporated on water
bath to get organic compound. The repeated extraction using small portions of
solvent (ether) are more efficient than a single extraction using large volume
of solvent. The solvent extraction is an equilibrium process. So it is based
upon Distribution law or Partition law.
This law states that a
solute distributes itself between two immiscible liquids in a constant ratio of
concentration irrespective of its added amount. The ratio of concentration of a
solute dissolved in two immiscible solvents is a constant. It is called
distribution coefficient. It is given as
For example distribution of I2 in water and Carbon
tetrachloride. The I2 is not very soluble in water but readily
soluble in aqueous KI. In water I2 is soluble in the form of
tri Iodide ion (
). When ccl4 is added to
aqueous solution of ions, then 
equilibrium takes place.
). When ccl4 is added to
aqueous solution of ions, then equilibrium takes place.
When two solvents are gently shaken, then their area of contact increases and
transfer of I2molecules improves. Thus a good separation is
achieved. The result is that brown colour of in aqueous layer fades and purple
colour of free I2 in CCl4 layer appears. When
equilibrium is established, then rate at which iodine passes from CCl4 to
water equals the rate at which it passes from water to CCl4 .
The distribution coefficient is given as k = [I2(CCL4)]
in aqueous layer fades and purple
colour of free I2 in CCl4 layer appears. When
equilibrium is established, then rate at which iodine passes from CCl4 to
water equals the rate at which it passes from water to CCl4 .
The distribution coefficient is given as k = [I2(CCL4)]
[I3(water)]
v.
Chromatography
The word chromatography is derived from Greek word “Khromatos” which means
colour writing. A technique by which we can separate components of a mixture
due to their different distribution between stationary and mobile phase is
called chromatography. The phase over which mobile phase flows is called
stationary phase. It may be a solid or liquid. The phase which flows over the
stationary phase is called mobile phase. It may be a liquid or gas.
Principle:
The principle of chromatography depends upon two points.
i.
Relative affinity
(solubility) of components in two phases.
ii.
Distribution
coefficient of components in two phases. The distribution coefficient K is
given as 
A component with
smaller value of K remains in stationary phase.
A component with grater
value of K remains in mobile phase.
Types of Chromatography
The chromatography has two types
(i) Adsorption Chromatography
(ii) Partition Chromatography
i. Adsorption
Chromatography:
The Chromatography in which solid substance is used as stationary phase is
called adsorption chromatography. e.g. Column Chromatography.
ii. Partition
Chromatography:
The Chromatography in which a liquid substance is used as stationary phase is
called partition chromatography. e.g. paper chromatography.
Paper Chromatography
It is a kind of partition chromatography. Here stationary phase is liquid (H2O)
adsorbed on paper. The mobile phase is an organic liquid which passes over the
paper. The solutes are distributed between two liquids. There are three kinds
of paper chromatography.
i.
Ascending paper
chromatography
ii.
Descending paper
chromatography
iii.
Radial / Circular
paper chromatography
Here will explain only ascending type.
Ascending Paper Chromatography
The chromatography in which solvent front or mobile phase travels upwards by
capillary action is called ascending paper chromatography.
Method:
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A solvent mixture is taken in a chromatographic tank. A 20 cm long strip of
whatmann’s paper no.1 is taken and a pencil line is drawn on it at 2.5 cm
from one end. A spot point of mixture solution is put on the line. Two other
spots of pure components are also put on the line as standard. Now spots are
dried and paper is suspended into the tank such that its lower end dips 5-6
mm into the solvent. The glass cover is put on the tank. The solvent passes
the spots. The solutes begin to move upward according to their distribution
coefficients.
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When the solvent has covered 
length of paper, then strip is removed,
position of solvent is marked and paper is dried. The pattern on the is called
chromatogram.
length of paper, then strip is removed,
position of solvent is marked and paper is dried. The pattern on the is called
chromatogram.
If spots are coloured, then they can be easily identified. If spots are
colourless, the chromatogram is developed by physical or chemical methods. The
retardation factor (Rf) of each component is calculated as 
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Each component has its specific Rfvalue. For example the shown
chromatogram tells that mixture solution (A) has two components (B, C). Their
Rf values are given as
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Rf(B)= x/y
(x is distance covered by component B, Z is distance covered by
component C and y is distance covered by solvent)
Rf(C)= z/y
Think! Why Rf value can never be greater
than 1
Uses
of Chromatography:
i.
The chromatography is
useful in separation of organic compounds.
ii.
It is used for
identification of compounds.
iii.
It is useful in
qualitative and quantitative analysis.
iv.
It is used in checking
purity of a compound.
Important note:A pure compound gives only one spot.
Think! Why Paper Chromatography is time
consuming process?
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