Methods of Purification of Solids
1. Cystallisation In this process, a saturated solution of impure substance is prepared in
hot solvent and heated with animal charcoal which adsorbs the impurities. The solution
is filtered and filtrate on cooling deposits crystals of pure compound. Success of the
process depends upon the selection of the solvent. The impurities must be least soluble.
2. Fractional crystallisation It is based on the different solubilities of different
compounds in a solvent. The compound having less solubility crystallises out first on
cooling leaving behind others in solution. Sometimes mixture of two solvents,
e.g., alcohol and water. chloroform and petroleum ether, give better results
3. Sublimation Some solids directly convert into vapours when heated without converting
into liquid. These are known as sublimate and this process is called sublimation.
Camphor, naphthalene and anthracene are purified by sublimation
Methods of Purification of Liquids
1. Simple distillation The vaporisation of a liquid by heating and subsequent condensation
of vapours by cooling is known as distillation.
2. Fractional distillation It is employed for separating mixture of two or more volatile
liquids having boiling points close to each other e.g , acetone (boiling point 60°C) and
methanol (boiling point 65°C). Components of petrolium are separated by this method.
3. Distillation under reduced pressure or vacuum distillation Some liquids decompose
when heated to their boiling points e.g., glycerol. Such liquids can be purified
4. Steam distillation The liquids insoluble in water, steam volatile in nature, having high
molecular weight and high vapour pressure are purified by steam distillation provided
the impurities present are not steam volatile.
Chromatographic Method
1. Adsorption Chromatography
Stationary phase- Solid or ion exchange resin. Mobile phase-Liquid or gas.
It includes liquid-solid chromatography, gas-solid chromatography or ion exchange
chromatography.
2. Partition Chromatography
Fixed phase-liquid supported on inert solid. Mobile phase-liquid or gas.
3. Paper Chromatography
The principle of paper chromatography is based on the fact that solutes have the capacity to
migrate through filter paper at different rates as a solution is drawn into strip of paper by
capillary action.
(The ratio of the distance travelled by a component to the distance travelled by the solvent front
is characteristic of each component and is known as the R
f
value
Rf
= (distance in em from starting line to the centre of zone/distance in cm from starting line to
the solvent front)
4. Column Chromatography
It is an example of adsorption chromatography. Adsorbents used are alumina, silica gel,
cellulose powder, animal charcoal, keiselguhr etc.
Liquid solvents used are benzene. petroleum ether, alcohol etc
Chemical Methods of Purification
1. Mixture of amines (1°, 2° and 3°) is separated by Hinsberg’s method.
2. Acetic acid from pyroligneous acid is separated by forming calcium salt.
3. Acids are separated by froming sodium derivatives with NaHCO3
·
Qualitative Analysis of Organic Compounds
This is done by heating the given organic compound with dry cupric oxide in a hard glass test
tube when carbon present is oxidised to carbon dioxide and hydrogen is oxidised to water
1. Detection of Carbon and Hydrogen
This is done by heating the given organic compound with dry cupric oxide in a hard glass test
tube when carbon present is oxidised to carbon dioxide and hydrogen is oxidised to water
Carbon dioxide turns lime water milky.
Water condenses on the cooler parts of the test tube and turns anbydrous copper sulpbate blue
Lassaigne’s Test
The organic compound is fused with a small piece of Na metal. When
element (N, 8, X) of the organic compound combine to give NaCN,Na
2
S or NaX; the red hot
tube is plunged in distilled water, boiled and filtered. The filtrate is called Lassaigne’s extract
or sodium extract. The Lassaigne’s extract is usually alkaline. If not, it is made alkaline by
adding a few drops of a dilute solution of sodium hydroxide.
The purpose of fusing the organic compounds with sodium metal is to convert halogens, N, S,
P etc., present in the organic compound to their corresponding soluble sodium salts (ionic
compounds).
Na + C + N → NaCN
2Na + S → N
2
S (where, X = Cl, Br, I)
Na + X → NaX
1. Detecton of Nitrogen
To a part of this alkaline solution is added a few drops of a freshly prepared solution of ferrous
sulphate, because a dilute solution of FeSO
4
after a long time oxidise to basic ferric sulphate
which is useless for analysis. The contents are warmed a little, cooled and then acidified with
dil. H
2SO
4
. Appearance of a green or Prussian blue colouration indicates the presence of
nitrogen.
If S is also present alongwith N, a red colour in place of Prussian blue in the test of nitrogen
appears, due to the formation of Fe(CNS)
3
.
Hydrazine does not give Lassaigne’s test for nitrogen since it does not contain carbon
2. Detection of Sulphur
(i) Sodium fusion extract is acidified with acetic acid and acetate is added to presence of
sulphur.
(ii) On treating sodium fusion extract with sodium nitroprusside, apperance o f a violet colour
further indicates the presence of sulphur.
3. Detection of Halogens
The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate.
X
-+ Ag
+
→ AgX
X represents a halogen -Cl Br, or I.
AgCl white ppt, AgBr-dull yellow ppt, AgI-bright yellow ppt.
Detection of Phosphorus
The compound is heated with an oxidising agent (sodium peroxide). By this the phosphorus
present in the compound is oxidised to phosphate.
Detection of Oxygen
There is no direct method to detect oxygen in compounds. It is present in the form of functional
groups such as -OH, -COOH. -NO2
etc.
1. Cystallisation In this process, a saturated solution of impure substance is prepared in
hot solvent and heated with animal charcoal which adsorbs the impurities. The solution
is filtered and filtrate on cooling deposits crystals of pure compound. Success of the
process depends upon the selection of the solvent. The impurities must be least soluble.
2. Fractional crystallisation It is based on the different solubilities of different
compounds in a solvent. The compound having less solubility crystallises out first on
cooling leaving behind others in solution. Sometimes mixture of two solvents,
e.g., alcohol and water. chloroform and petroleum ether, give better results
3. Sublimation Some solids directly convert into vapours when heated without converting
into liquid. These are known as sublimate and this process is called sublimation.
Camphor, naphthalene and anthracene are purified by sublimation
Methods of Purification of Liquids
1. Simple distillation The vaporisation of a liquid by heating and subsequent condensation
of vapours by cooling is known as distillation.
2. Fractional distillation It is employed for separating mixture of two or more volatile
liquids having boiling points close to each other e.g , acetone (boiling point 60°C) and
methanol (boiling point 65°C). Components of petrolium are separated by this method.
3. Distillation under reduced pressure or vacuum distillation Some liquids decompose
when heated to their boiling points e.g., glycerol. Such liquids can be purified
4. Steam distillation The liquids insoluble in water, steam volatile in nature, having high
molecular weight and high vapour pressure are purified by steam distillation provided
the impurities present are not steam volatile.
Chromatographic Method
1. Adsorption Chromatography
Stationary phase- Solid or ion exchange resin. Mobile phase-Liquid or gas.
It includes liquid-solid chromatography, gas-solid chromatography or ion exchange
chromatography.
2. Partition Chromatography
Fixed phase-liquid supported on inert solid. Mobile phase-liquid or gas.
3. Paper Chromatography
The principle of paper chromatography is based on the fact that solutes have the capacity to
migrate through filter paper at different rates as a solution is drawn into strip of paper by
capillary action.
(The ratio of the distance travelled by a component to the distance travelled by the solvent front
is characteristic of each component and is known as the R
f
value
Rf
= (distance in em from starting line to the centre of zone/distance in cm from starting line to
the solvent front)
4. Column Chromatography
It is an example of adsorption chromatography. Adsorbents used are alumina, silica gel,
cellulose powder, animal charcoal, keiselguhr etc.
Liquid solvents used are benzene. petroleum ether, alcohol etc
Chemical Methods of Purification
1. Mixture of amines (1°, 2° and 3°) is separated by Hinsberg’s method.
2. Acetic acid from pyroligneous acid is separated by forming calcium salt.
3. Acids are separated by froming sodium derivatives with NaHCO3
·
Qualitative Analysis of Organic Compounds
This is done by heating the given organic compound with dry cupric oxide in a hard glass test
tube when carbon present is oxidised to carbon dioxide and hydrogen is oxidised to water
1. Detection of Carbon and Hydrogen
This is done by heating the given organic compound with dry cupric oxide in a hard glass test
tube when carbon present is oxidised to carbon dioxide and hydrogen is oxidised to water
Carbon dioxide turns lime water milky.
Water condenses on the cooler parts of the test tube and turns anbydrous copper sulpbate blue
Lassaigne’s Test
The organic compound is fused with a small piece of Na metal. When
element (N, 8, X) of the organic compound combine to give NaCN,Na
2
S or NaX; the red hot
tube is plunged in distilled water, boiled and filtered. The filtrate is called Lassaigne’s extract
or sodium extract. The Lassaigne’s extract is usually alkaline. If not, it is made alkaline by
adding a few drops of a dilute solution of sodium hydroxide.
The purpose of fusing the organic compounds with sodium metal is to convert halogens, N, S,
P etc., present in the organic compound to their corresponding soluble sodium salts (ionic
compounds).
Na + C + N → NaCN
2Na + S → N
2
S (where, X = Cl, Br, I)
Na + X → NaX
1. Detecton of Nitrogen
To a part of this alkaline solution is added a few drops of a freshly prepared solution of ferrous
sulphate, because a dilute solution of FeSO
4
after a long time oxidise to basic ferric sulphate
which is useless for analysis. The contents are warmed a little, cooled and then acidified with
dil. H
2SO
4
. Appearance of a green or Prussian blue colouration indicates the presence of
nitrogen.
If S is also present alongwith N, a red colour in place of Prussian blue in the test of nitrogen
appears, due to the formation of Fe(CNS)
3
.
Hydrazine does not give Lassaigne’s test for nitrogen since it does not contain carbon
2. Detection of Sulphur
(i) Sodium fusion extract is acidified with acetic acid and acetate is added to presence of
sulphur.
(ii) On treating sodium fusion extract with sodium nitroprusside, apperance o f a violet colour
further indicates the presence of sulphur.
3. Detection of Halogens
The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate.
X
-+ Ag
+
→ AgX
X represents a halogen -Cl Br, or I.
AgCl white ppt, AgBr-dull yellow ppt, AgI-bright yellow ppt.
Detection of Phosphorus
The compound is heated with an oxidising agent (sodium peroxide). By this the phosphorus
present in the compound is oxidised to phosphate.
Detection of Oxygen
There is no direct method to detect oxygen in compounds. It is present in the form of functional
groups such as -OH, -COOH. -NO2
etc.
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