SCY1611 – Analytical Chemistry
Unit - I
1
, UNIT - I
SEPARATION AND PURIFICATION TECHNIQUES
Principles involved in the separation of precipitates, solvent extraction and
electrophoresis. Purification of solid organic compounds, extraction - use of immiscible
solvents, soxhlet extraction, crystallization, use of miscible solvents, fractional
crystallization, sublimation. Purification of liquids, experimental techniques of
distillation, fractional distillation, vacuum distillation, steam distillation, tests for
purity.
1. INTRODUCTION
The separation of mixtures of compounds to give the pure components is of great practical
importance in chemistry. Separation techniques constitute an important aspect of experimental
chemistry. Almost all compounds of biochemical interest occur naturally as components of very
complex mixtures from which they can be separated only with considerable difficulty. Separations
can be achieved by differences in physical properties, such as differences in boiling point, or by
chemical means, wherein differences in physical properties are enhanced by chemical reactions. In
this chapter we will consider some separations of compounds based on differences in physical
properties. The chemical and physical aspects of purification of matter are interesting. The
purification techniques have undergone vast improvements with the development of modern
technology. Though expensive instruments are sometimes used for purifying a compound, the
well-known principle of adsorption-desorption is used in chromatography. In spite of the
availability of modern gadgets for separation and purification of chemicals, the simple and
inexpensive techniques like distillation, filtration, sublimation, etc., continue to remain
indispensable in all laboratories.
1.1 Separation Techniques:
Definition:
• It consists of important aspects of experimental chemistry.
• The quest for isolating a chemical in a 100% pure form has resulted in the invention and
perfection of innumerable separatory methods.
• The low cost technique is filtration and the computer aided technique are HPLC etc., which are
expensive.
Types:
1.2 Precipitation:
It is an important method of separation in various analytical techniques.
In qualitative analysis, the principle of separation of cations is based on by adding suitable
reagents.
Eg:
HCl precipitates Hg, Pb & Ag.
Thus the separation of these ions from Cu2+, Al3+, Zn2+ etc., is possible.
Mn ions can be separated from Zn ions by adding NaOH.
2
, In a mixture of Cu & Ni ions, Cu ions are precipitated as sulphide and removed before Ni is
estimated gravimetrically using DMG(dimethyl glyoxime).
A carbonyl compound can be precipitated by adding Borsche’s reagent).
1.1.1 Solvent Extraction:
• Extraction with solvents is used as a method for separation of dissolved substances from solutions.
• It can also be used for the separation of one constituent from a solid mixture as well as for the
removal of undesired soluble impurities from mixtures.
• Extraction with a second solvent is an application of the Nernst distribution law which states that
“at constant temperature a solute distributes itself between two immiscible solvents only in a
particular ratio”.
• When a substance distributes itself between two solvents without the complications of dissociation
or association, it is possible to calculate the weight of the substance that can be removed by a
series of extractions.
• If v1 ml of a solution contains Wg of a substance and if the substance is
repeatedly extracted with v2 ml of another solvent, the weight of the substance Wn remaining in
the first solvent after n extractions is given by,
Wn = W [K v1 / K v1 + v2 ]n
Where K = distribution coefficient for the substance between the two solvents.
Larger the value of K, more efficient is the extraction.
1.1.2 Solvent Extraction Using Separating Funnel:
• The extraction with a second solvent can be carried out in the laboratory with a separating funnel.
• It is provided with a ground glass stopper and a stop-cock.
• It is mounted in a ring on a stand.
• When an aqueous solution is shaken with ether in a separating funnel and allowed to settle, two
sharply defined layers are formed.
• The two layers can be separated by opening the stop-cock and allowing the lower aqueous layer to
drain slowly into a beaker.
Fig. 1.1. Separating Funnel
3
, 1.1.3 Continuous Extraction:
• When one has to separate a component which is slightly soluble in the extracting solvent from a
mixture whose other components are essentially insoluble, large quantities of solvent would have
to be used.
• The method of continuous extraction can be used in such cases with a smaller quantity of the
solvent.
• The apparatus used for continuous extraction when the extracting solvent is lighter than the
original solution containing the solute.
• The extracting solvent is kept in a flask which is heated.
• The vapours condense and the liquid solvent drains into the vessel containing the solution to be
extracted.
• After extracting a little of the solute, the extracting solvent being lighter rises to the top.
• When the level exceeds a certain limit, it drains once again into the original flask from where it is
vaporised and then utilised for extraction once again.
No back pressure
cooling water inlet
Condenser
cooling water outlet
spill over tube
Extraction flask
Collection flask
Fig. 1.2 Continuous Extraction
1.1.4 Soxhlet Extraction:
• For separation of the components of a solid mixture by continuous extraction, a Soxhlet apparatus
is used.
• The solid is kept in the porous thimble.
• The extracting solvent is taken in the boiling flask.
4
Unit - I
1
, UNIT - I
SEPARATION AND PURIFICATION TECHNIQUES
Principles involved in the separation of precipitates, solvent extraction and
electrophoresis. Purification of solid organic compounds, extraction - use of immiscible
solvents, soxhlet extraction, crystallization, use of miscible solvents, fractional
crystallization, sublimation. Purification of liquids, experimental techniques of
distillation, fractional distillation, vacuum distillation, steam distillation, tests for
purity.
1. INTRODUCTION
The separation of mixtures of compounds to give the pure components is of great practical
importance in chemistry. Separation techniques constitute an important aspect of experimental
chemistry. Almost all compounds of biochemical interest occur naturally as components of very
complex mixtures from which they can be separated only with considerable difficulty. Separations
can be achieved by differences in physical properties, such as differences in boiling point, or by
chemical means, wherein differences in physical properties are enhanced by chemical reactions. In
this chapter we will consider some separations of compounds based on differences in physical
properties. The chemical and physical aspects of purification of matter are interesting. The
purification techniques have undergone vast improvements with the development of modern
technology. Though expensive instruments are sometimes used for purifying a compound, the
well-known principle of adsorption-desorption is used in chromatography. In spite of the
availability of modern gadgets for separation and purification of chemicals, the simple and
inexpensive techniques like distillation, filtration, sublimation, etc., continue to remain
indispensable in all laboratories.
1.1 Separation Techniques:
Definition:
• It consists of important aspects of experimental chemistry.
• The quest for isolating a chemical in a 100% pure form has resulted in the invention and
perfection of innumerable separatory methods.
• The low cost technique is filtration and the computer aided technique are HPLC etc., which are
expensive.
Types:
1.2 Precipitation:
It is an important method of separation in various analytical techniques.
In qualitative analysis, the principle of separation of cations is based on by adding suitable
reagents.
Eg:
HCl precipitates Hg, Pb & Ag.
Thus the separation of these ions from Cu2+, Al3+, Zn2+ etc., is possible.
Mn ions can be separated from Zn ions by adding NaOH.
2
, In a mixture of Cu & Ni ions, Cu ions are precipitated as sulphide and removed before Ni is
estimated gravimetrically using DMG(dimethyl glyoxime).
A carbonyl compound can be precipitated by adding Borsche’s reagent).
1.1.1 Solvent Extraction:
• Extraction with solvents is used as a method for separation of dissolved substances from solutions.
• It can also be used for the separation of one constituent from a solid mixture as well as for the
removal of undesired soluble impurities from mixtures.
• Extraction with a second solvent is an application of the Nernst distribution law which states that
“at constant temperature a solute distributes itself between two immiscible solvents only in a
particular ratio”.
• When a substance distributes itself between two solvents without the complications of dissociation
or association, it is possible to calculate the weight of the substance that can be removed by a
series of extractions.
• If v1 ml of a solution contains Wg of a substance and if the substance is
repeatedly extracted with v2 ml of another solvent, the weight of the substance Wn remaining in
the first solvent after n extractions is given by,
Wn = W [K v1 / K v1 + v2 ]n
Where K = distribution coefficient for the substance between the two solvents.
Larger the value of K, more efficient is the extraction.
1.1.2 Solvent Extraction Using Separating Funnel:
• The extraction with a second solvent can be carried out in the laboratory with a separating funnel.
• It is provided with a ground glass stopper and a stop-cock.
• It is mounted in a ring on a stand.
• When an aqueous solution is shaken with ether in a separating funnel and allowed to settle, two
sharply defined layers are formed.
• The two layers can be separated by opening the stop-cock and allowing the lower aqueous layer to
drain slowly into a beaker.
Fig. 1.1. Separating Funnel
3
, 1.1.3 Continuous Extraction:
• When one has to separate a component which is slightly soluble in the extracting solvent from a
mixture whose other components are essentially insoluble, large quantities of solvent would have
to be used.
• The method of continuous extraction can be used in such cases with a smaller quantity of the
solvent.
• The apparatus used for continuous extraction when the extracting solvent is lighter than the
original solution containing the solute.
• The extracting solvent is kept in a flask which is heated.
• The vapours condense and the liquid solvent drains into the vessel containing the solution to be
extracted.
• After extracting a little of the solute, the extracting solvent being lighter rises to the top.
• When the level exceeds a certain limit, it drains once again into the original flask from where it is
vaporised and then utilised for extraction once again.
No back pressure
cooling water inlet
Condenser
cooling water outlet
spill over tube
Extraction flask
Collection flask
Fig. 1.2 Continuous Extraction
1.1.4 Soxhlet Extraction:
• For separation of the components of a solid mixture by continuous extraction, a Soxhlet apparatus
is used.
• The solid is kept in the porous thimble.
• The extracting solvent is taken in the boiling flask.
4
