ANALYTICAL CHEMISTRY(Seperation and Purification Techniques)

Dr.S.SURESHAssistant Professor

Email:avitsureshindia@gmail.com

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.

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.

Crystallization by Cooling a Hot Crystallization by Cooling a Hot Concentrated SolutionConcentrated Solution

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.

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

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.

Use of drying agents and their properties

S.No Drying agent Properties Uses

1 CaCl2 NeutralGood preliminary drying agent. It has large water absorption capacity giving the hexahydrate.

2 Sodium sulphate NeutralIt 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.,

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.

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.

SublimationSublimation

A mixture of two compounds can be separated by sublimation

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.

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.

• 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.

Fractional distillation

Distillation under reduced Pressure (or) Vaccum distillation

• Vacuum distillation- It is used for organic compounds which decompose at or below their boiling points.Example: GlycerolThe 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.

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.

Distillation under reduced pressure

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.

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".

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.

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.

Solvent ExtractionSolvent Extraction

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

Melting Point Apparatus

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.

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.