Recrystallisation, Simple distillation, Fractional distillation, Extraction, Solvent Extraction

1. ANALYTICAL CHEMISTRY
(Seperation and Purification Techniques)
Dr.S.SURESH
Assistant Professor
Email:avitsureshindia@gmail.com

2. Purification of solid organic compounds
• Purification of organic compounds is
important because, organic compound
made in the laboratory contains
impurities.
• The common methods for purification are
crystallisation, sublimation, distillation,
chromatography, etc.

3. CrystallizationCrystallization
• Crystallization is the process of forming crystals
Crystallization by Cooling a Hot Concentrated SolutionCrystallization by Cooling a Hot Concentrated Solution
• In this method the impure substance is dissolved in water or
some other suitable organic solvent such as alcohol, petrol, etc.,
• The solution is filtered to remove any suspended impurities. The
filtrate is heated over a water bath so that the vapors of the
solvent may not catch fire.
• Then the hot solution of the organic compound is cooled, so that
the crystals begin to separate out.
• The crystals are removed by filtration and the impurities are
passed on to the filtrate.

4. Crystallization by Cooling a HotCrystallization by Cooling a Hot
Concentrated SolutionConcentrated Solution

5. Recrystallisation
• Recrystallisation: It is a method used to
purify an organic solid.
• A small amount of the solvent is added to a
flask containing an impure solid. The
contents of the flask are heated until the
solid dissolves. Then the solution is cooled. A
more pure solid separates out, leaving
impurities dissolved in the solvent.

6. Example for recrystallisation:
1.0 g of crude benzoic acid is taken in a 50 ml flask.
Add a little water and boil
Some contaminants will not dissolve
Transfer to a conical flask and cool
Filter the crystals and allow to dry for 5-10 mins
Impure benzoic acid Benzoic acid after
recrystallisation

7. Drying agents
• A drying agent is an inorganic salt which readily
takes up water to become hydrated.
• Example: LiAlH4, CaCl2, CaSO4, K2CO3, Na2SO4
LiAlH4 : It reacts violently with water liberating
hydrogen and is a powerful drying agent for
organic compounds. It is widely used for drying
ethers. It finds extensive application in purifying
organic chemical substances by the removal of
water and carbonyl containing impurities.

8. Use of drying agents and their properties
S.No Drying agent Properties Uses
1 CaCl2 Neutral
Good preliminary drying agent. It has
large water absorption capacity giving
the hexahydrate.
2 Sodium sulphate Neutral
It has large water absorption capacity
for the absorption of water forming the
decahydrate.
3 Barium oxide Basic Suitable for drying organic bases
4 Sulphuric acid Acidic
It is suitable for drying bromine,
saturated hydrocarbons, etc.,

9. SublimationSublimation
Sublimation is the direct change of
• a solid to vapour on heating
• without going through the liquid state
Used to separate volatile organic compounds
from non volatile impurities e.g. Naphthalene,
benzoic acid, anthracene, camphor.

10. Sublimation
• The substances that sublime can be purified by
this method, provided the impurities present
does not sublime.
• Many organic compounds directly form
vapours, when solid compounds are heated,
without becoming a liquid at any stage. On
cooling the vapours the solid is directly
obtained.

11. SublimationSublimation
A mixture of two compounds can be
separated by sublimation

12. Purification of Liquid organic compounds
• Organic compounds in the liquid state are purified by
distillation.
• Distillation involves the heating of a liquid to boiling
and then collecting their vapours to condense them in
liquid state.

13. Simple Distillation
Simple distillation is designed to evaporate a volatile
liquid from a solution of non-volatile substances; the
vapor is then condensed in the water condenser and
collected in the receiver.

14. • 2-Fractional Distillation
If the boiling point of the liquids in the mixture are very
close to each other then such mixtures can be purified
by fractional distillation. The difference in boiling points
of the mixture is usually less than 40ºC.
e.g. – acetone(b.p. 56ºC)and methyl alcohol(b.p. 65ºC).
For fractional distillation, a suitable fractionating column
is placed between the flask and the condenser.

15. Fractional distillation

16. Distillation under reduced Pressure
(or) Vaccum distillation
• Vacuum distillation- It is used for organic
compounds which decompose at or below
their boiling points.
Example: Glycerol
The boiling point of a liquid is the
temperature at which the total vapour
pressure is equal to the external pressure.
This means that by lowering the pressure the
boiling point of the liquid can be lowered.

17. Vaccum distillation
• If we have an organic substance which
decompose at its boiling point, we can make
it to boil at a temperature lower than its
boiling point. All that we have to do is to
create a partial vaccum. Under reduced
pressure, the substance boils at a much
lower temperature and distils over
undecomposed.

18. Distillation under reduced pressure

19. Extraction
• Extraction is a very common laboratory procedure
used when isolating or purifying a product
• In organic laboratory, liquid-liquid extraction is most
commonly used. Liquid-liquid extraction requires two
immiscible liquids known as the organic phase and the
aqueous phase. The aqueous phase is water-based
and the organic phase is an organic solvent.

21. General extraction procedure
• Place the solution to be extracted in the separatory
funnel. As the organic solvent and water are not
miscible with each other, you should be able to see the
two layers (organic and aqueous layers) clearly.
• Now, shake the separatory funnel to increase the
contact between these substances and the water.
When finished, the funnel can be returned to the
stand and the layers are allowed to separate.
• You should also have two beakers ready, one labelled
"organic layer" and the other labelled "aqueous layer".

22. Solvent ExtractionSolvent Extraction
• Extraction with solvents is used as a method of
separation of dissolved substances from
solutions.
• The common solvent which is used for the
extraction are diethyl ether, benzene, etc.,
• A good solvent for extraction should satisfy two
important conditions.
• (a) The substance to be extracted should be highly
soluble in the solvent.
• (b) After the extraction the solvent should be easily
separable from the solute.

23. Solvent ExtractionSolvent Extraction
• The mixture of urea and benzoic acid can be separated
using solvent extraction process.
• The mixtures are taken in a separating funnel, to the
substances(Urea and benzoic acid) which is solid,
diethyl ether is added.
• The two mixtures are shaken well to get a solution.
• Only benzoic acid is soluble in ether, while urea is not.
• Urea is at the bottom of the separating funnel, and it is
collected.
• Now the benzoic acid in ether is left, which on heating,
we get benzoic acid.

24. Solvent ExtractionSolvent Extraction

25. Criteria and test for purity
• The following physical properties are used
for checking the purity of substances
(a) Melting point
(b) Boiling point
(d) Density
These properties have long been utilized in
identification and characterisation of organic
compounds

26. Melting Point Apparatus

27. Criteria and test for purity
• Melting point: The melting point of a pure
substance is a constant. The presence of impurities
in a substance lowers the melting point and hence if
the melting point of a substance is correct then the
substance is taken as pure
• Boiling Point: The boiling point of a pure substance
under a particular pressure is a constant and
therefore this property can be used to check the
purity of a liquid.

28. Criteria for purity
• The purity of an organic compound can be
checked by the following criteria:
(a) Sharp melting point which does not change
on further purification.
(b) In case of solids, mix the solid with known
sample of pure compound and its melting point
is noted. The m.pt should remain the same.
(c) Concordant boiling point
(d) Definite crystalline shape.