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General Aspects of Separation Methods UNIT 1 GENERAL ASPECTS
OF
SEPARATION METHODS Structure 1.1 Introduction
Objectives 1.2 Separation Methods A Unified Science 1.3 Scope of
Separation Methods 1.4 Evolution of Chromatography 1.5
Classification of Separation Methods 1.6 Classification Based on
Property Resulting in Separation
Volatility Solubility Partition Ion Exchange Surface Activity
Molecular Geometry Electromigration
1.7 Classification Based on Equilibrium and Rate Processes
Classification Based on Equilibrium Processes Classification Based
on Rate Processes
1.8 Criteria for Selection of Separation Methods Selectivity
Detectability Reproducibility Yield, Speed and Convenience
Capability for Hyphenation Ease in Scaling up and Economics
1.9 Summary 1.10 Terminal Questions 1.11 Answers
1.1 INTRODUCTION The world around us consists of an innumerable
complex materials- organic, inorganic and those containing both
types. In order to know the impact of their existence on our life
and fruitfully recover useful materials from various natural
resources, we have to know their composition and develop chemistry
for their recovery. We may like to look at another scenario where
the human population of today has become very much demanding in
terms of purity of materials, better products and security,
particularly in terms of health. Simultaneously, science has
tremendously grown in its dimensions and the need for ultra pure
materials is fast increasing. You will, thus, realize that the
entire scenario requires developments in the methods of analysis
including separations. It is pertinent to point out here that
separations are not only important for analysis but assume great
significance in the synthesis and recovery of pure materials.
Separations touch every branch of science and technology and have
developed into a well established branch known as separation
science. If you just look at the developments, say in biological
sciences such as biochemistry, biotechnology, clinical
pharmacology, therapeutics and toxicology, the progress has taken
place prominently because of the advancements in the separation
methods.
After going through the preceding text, you would realize that
separation methods form an important component of chemistry and
particularly analytical chemistry. This course on Separation
Methods is designed to include theoretical aspects,
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Classical Methods instrumentation, applications, advantages and
limitations of some of the important methods of separations.
Before you study the individual separation methods, it may be
necessary to get an overview of the separation science. This
particular unit deals with the concept of separations as a unified
science, scope of separations highlighting their utility,
classification of separation methods and the criteria for the
selection of separation methods.
Objectives After studying this Unit, you should be able to
appreciate the separation methods as a unified science,
describe the scope and utility of separation methods,
discuss the evolution of chromatography
explain the classification of separation methods, and
list the criteria for the selection of separation methods.
1.2 SEPARATION METHODS A UNIFIED SCIENCE The separations play a
key role in the various branches of science and technology but they
themselves form a unified branch of science. The subject of
separation science essentially deals with the physical and chemical
phenomena involved in achieving the separations. The outcome of
separations is very much dependent upon the physicochemical
principles resulting into separations. It also involves the
development, application and reproducibility of various separation
processes. The separations have assumed such a paramount importance
that a common man has become familiar with the meaning of word
separation. However, with its requirement and usage, the definition
of separation has become a little complex. In the simplest terms,
separation is defined as an operation in which a mixture is divided
into at least two components having different compositions. But
this particular definition has a limitation as it does not cover
chiral separations in which molecules of same composition and
chemical structure are involved. The molecules differ only in their
stereochemistry. Therefore, a broader definition of separation will
be as under: Separation is a process by which a mixture is divided
in at least two components with different compositions or two types
of molecules with the same composition but different
stereochemistry.
After having learnt the definition of separation in true
chemical sense, you should be clear about the different objectives
for achieving separations:
i) Analysis of different constituents of a mixture. ii)
Procuring pure materials from complex mixtures. In analysis,
referred above in (i), there may be three aspects:
Removal of interfering constituents before the actual
quantitation of one or more known compounds.
Isolation of unknown constituents for subsequent
characterization.
Analysis of a complex unknown mixture by subjecting the entire
sample to separation into individual constituents.
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General Aspects of Separation Methods
Under (ii), for obtaining the pure materials from complex
mixtures, the constituent with the desired purity may be obtained
by applying a single process or using a number of separation
techniques. In some cases, in order to attain the required level of
purity, the separation steps of the same process may have to be
repeated.
The mixtures to be separated vary largely in terms of their
complexity. They may contain constituents which differ in their
molecular weights, solubility in a solvent, volatility or other
properties. The sizes of species may range from atomic dimensions
through organic molecules and macromolecules to molecular
aggregates.
A large number of separation methods are available that utilize
selected characteristics as means of separation. Each of these
methods can be further subdivided into different techniques using
unique characteristics. In certain cases, the properties of the
constituents may be so different that very simple techniques of
separation can be applied. A very simple example, in this regard,
is the recovery of common salt from sea water. However, in other
cases, the properties of constituents may be so similar that the
separation becomes a tedious job. A very well known example of a
difficult separation is the separation of Zn (IV) and Hf (IV). The
other example in this category is the separation of optical
isomers.
Another important parameter which is critical in choosing the
separation is the amount of mixture available. In some cases, the
amount may be a few molecules. However, in industrial processes it
may run in tonnes.
SAQ 1 What are the main applications of separations?
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SAQ 2 Define separation in the real chemical sense.
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1.3 SCOPE OF SEPARATION METHODS We have learnt that the
separation science deals with a variety of problems at hand. Now,
it may be necessary to illustrate the utility of separations by
citing example from daily life to various branches of science and
technology. The number of even the important examples is so large
that they cannot be recounted here. However, by citing a few, you
will be able to appreciate the significance of separations.
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Classical Methods The art of separations is not new to the
present day community of scientists. Even our ancestors were
familiar with the various separation methods and were using them
for their daily needs. A very typical example is the distillation
of alcohol for drinking and other purposes. Isolation of various
dyes for coloring purposes from natural materials is a good
testimony of their skill for achieving separations. They were also
quite proficient in isolating metals for their use from ores by
applying different separation procedures. A number of drugs used to
be isolated from plants and herbs. The treatment of waters by solid
adsorbents is as old as the civilization. There are records
available that in the time of Aristotle, sand filters were used for
the purification of sea water and impure drinking water. Moses used
tree branches to make bitter water sweet.
Separation processes play a key role in our daily life. We
remove undesirable gases and particles from the air we breathe. The
municipal drinking water undergoes several purification steps. It
is well known that the identification and removal of contaminants
from food are largely possible due to separation processes. One of
best examples of use of separations in industry is the availability
of a variety of products from crude petroleum. The nuclear age did
really take off due to improvements in the methods of separation of
U238 and U235.
The requirement of high purity materials in industry,
particularly for semiconductor, is met due to advancements in
separation processes.
It has been possible to understand the different biochemical
processes taking place in our body due to advancements in
separation processes. The separation processes have given a unique
gift in the form of artificial kidney. The success in the studies
on human genome and proteomics owes a great deal to advancements in
separation sciences. The synthesis of different medicines is
possible due to efficiency of separation processes. The
identification of explosives has been possible due to a key role of
separations.
In a nutshell, there is hardly any walk of life where
separations do not play their dominant role.
It is well known that a large number of separation methods fall
under the category of chromatography and that is why the separation
methods have become synonymous with chromatography. Thus, before
proposing a classification of separation methods, it may be
important to give here a brief description of evolution of
chromatography.
SAQ 3 Give two examples of separations beneficial for our
environment. ..
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1.4 EVOLUTION OF CHROMATOGRAPHY There is a large number of
separation methods which vary in their utility in a particular
situation. Many of these important methods fall under the category
of chromatography. Therefore, before we discuss classification of
separation methods, it will be appropriate to give an idea of the
chromatographic science. In Unit 4 of this course, a discussion on
classification and general principles of chromatography has
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General Aspects of Separation Methods
been presented. Following this unit, some of the units discuss
important chromatographic methods in detail. In order to keep the
text of this particular unit properly jointed, a brief idea about
the evolution of chromatography is being presented here.
The subject of chromatography had a very modest beginning. The
principles and the applications were reported in 1906 by a Russian
botanist, Mikhail Tswett. He described the resolution of
chlorophylls and other plant pigments in a plant extract. A
petroleum ether solution of chlorophyll was passed through a
calcium carbonate column. By passing the solvent through the
column, the pigments were resolved into various zones. This
separation became practical if after the pigment solution, the pure
solvent was allowed to pass through the column. Such a preparation
was termed as the chromatogram and the corresponding method, the
chromatographic method. The term chromatography is derived from the
Greek words chroma and graphy meaning colour writing. The discovery
of chromatography was made with the separation of coloured
compounds but its potential for colourless compounds was realized.
Tswett himself anticipated the potential of this technique for a
wide variety of compounds. This technique is now termed as
liquid-solid adsorption chromatography.
Following the discovery of original form of chromatography,
several new advances were made in the form of ion exchange
chromatography, partition chromatography and gas chromatography.
The logic of naming Tswett method as chromatography does not hold
good today because most of the compounds separated by this
technique are not colored. The name is very well established and is
not likely to change. Not only this, many other techniques leading
to the redistribution of components of a mixture are included under
the head chromatography.
The most important advances in chromatography were introduced by
James and Martin. The impact of chromatography has been very great
on all the areas of analysis and on the general progress of
science. Recognition of this fact resulted in the award of Nobel
Prize in 1952 to Martin and Synge for their pioneering work in this
field.
As has already been mentioned, we see chromatography in its
different forms. A general definition of chromatography covering
its various forms is as given under:
Chromatography is a method of separation in which the components
to be separated are distributed between two phases, one of these is
called the stationary phase and the other the mobile phase which
moves on the stationary phase in a definite direction. The
stationary phase may be a solid or liquid and the moving phase may
be liquid, gas or supercritical fluid. The components of a mixture
redistribute themselves between two phases mainly by the process
which may be adsorption, partition, ion exchange or size exclusion.
For the purpose of simplification, in our discussion here, on the
classification of separation methods, we will not include
supercritical fluid as one of the mobile phases. However, this will
be taken up later.
SAQ 4 Define chromatography.
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Mikhail Tswelt (1872-1919)
R. L. M. Synge (1914-1994)
A. J. P. Martin (1910-2002)
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Classical Methods SAQ 5 Cite the main processes which are
responsible for separations by chromatography. ..
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1.5 CLASSIFICATION OF SEPARATION METHODS The subject of
separation science has grown very large in its dimensions. A
variety of methods have come up on the forefront to meet the
different needs. A number of well known techniques have undergone
modifications. The growth in the number of separation methods can
be attributed to the following factors:
Different separation goals,
Diversity of mixtures to be separated, and
Utilization of a variety of physicochemical phenomena for
separations.
The separation methods are generally named after the forces or
phenomena that give rise to separation. In this respect, we can
cite the examples of precipitation, crystallization, extraction,
adsorption and ion exchange. At times, the name is used to reflect
upon a distinct form of operation. Here, you can mention techniques
like filtration, distillation, and chromatography. Chromatography,
for example, can employ any number of forces such as adsorption,
partition, ion exchange and size exclusion.
The above discussion makes it clear that one of the
classifications of the separation methods may be based on the
property which results into separations. The other approach that
can be adopted for classification is based on the physicochemical
phenomena responsible for separation. To simplify, we can further
divide these phenomena in two major categories: equilibrium
processes and rate processes.
Thus, we can propose two types of classifications for separation
methods.
Based on the property resulting into separation,
Based on the equilibrium and rate processes. It may be pertinent
to point out that the details of the methods will not be discussed
here because either some of the methods are well known or a few of
them are being discussed in details in other units of this
course.
SAQ 6 What are the main reasons for the growth in the number of
separation mehods? ..
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General Aspects of Separation Methods
SAQ 7 What are the two main criteria employed for classifying
the different separation methods? ..
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1.6 CLASSIFICATION BASED ON PROPERTY RESULTING IN SEPARATION
The main properties which result into segregation of components
of the mixture are:
Volatility,
Solubility,
Partition,
Ion exchange,
Surface activity,
Molecular geometry, and
Electromigration.
1.6.1 Volatility The methods based on volatility mainly include
vaporization and distillation in its different forms. Sublimation
is a special case of distillation where a solid is directly
vaporized without passing through a liquid state. Vaporization is
simple where solvent is removed by using heat or air currents such
that the material concentrates to a solid. In separation by
distillation, all the components of interest in the mixture are
volatile. Sublimation is an exception to this because of the
physical state of the component. Distillation depends on the
distribution of constituents between the liquid mixture and the
vapor in equilibrium with the mixture. The more volatile component
is concentrated in the vapor while the less volatile components
remain in the liquid phase in greater concentration.
There are various forms of distillation such as fractional
distillation, flash distillation, vacuum distillation, steam
distillation and azeotropic distillation.
Fractional distillation involves the return of condensate to the
distillation unit under conditions such that this condensate is
continuously and counter currently in contact with the vapors. By
this method, you can achieve greater enrichment of vapors of more
volatile component than that obtained by simple distillation.
In flash distillation, there is an instantaneous and continuous
vaporization of a definite component of the mixture such that the
total vapor produced is in equilibrium with the remaining
liquid.
Vacuum distillation is actually distillation under reduced
pressure. It is used to separate high boiling mixtures or liquids
that decompose below their normal boiling points. Low pressure
reduces the boiling point.
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Classical Methods Steam distillation is employed to separate a
component from a nonvolatile impurity. The boiling temperature of
the mixture is reduced by vaporizing it in a stream of carrier
vapour say steam which upon condensation is immiscible with the
original mixture. And thus, a separation can be readily
achieved.
Azeotropic distillation is used for separating components of a
mixture which boil very close to each other. The relative
volatility of the components of the mixture is altered by adding
another substance. This description will give you an idea about the
different ways the distillation can be carried out for different
types of mixtures.
SAQ 8 What type of distillation is used to separate components
of a mixture if they decompose below their normal boiling point?
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1.6.2 Solubility When we refer solubility as a means of
separation it essentially implies precipitating the constituent of
interest from the solution. When the solubility limit of the solute
in the solvent exceeds, the material appears as a precipitate. If
this process is carried out in an appropriately selected solvent
and in a controlled manner, it can lead to crystallization.
Precipitation can be brought about in a number of ways out of which
some important ones are discussed below.
Solvent precipitation is achieved by adding another miscible
solvent to the solution such that the solubility of the component
of interest is reduced and the material appears as a precipitate in
the mixed solvent. The precipitation brought about by a chemical
reaction is well known. Right from the beginning in a chemistry
laboratory, you have been precipitating different ions by adding
appropriate reagents. A typical example is precipitating Ba2+ by
adding SO42-. There are many selective reagents known which can
precipitate one ion in the presence of other ions. A detailed
discussion about this is given in Units 14 and 15 of the Course on
Basic Analytical Chemistry. However, some organic compounds can be
precipitated by suitable adjustment of pH. It is possible to
precipitate weakly basic organic compound from an aqueous solution
by making the solution more basic. Similarly, organic acids can be
precipitated by making the solution more acidic by strong
acids.
1.6.3 Partition The methods based on partition require two
phases and there has to be a redistribution of components between
these two phases. Some important types of methods fall under this
category and they are as follows:
Liquid-liquid extraction
Liquid-liquid partition chromatography
Gas-liquid chromatography
Liquid-liquid extraction is popularly known as solvent
extraction. When a solute is brought in contact with two immiscible
solvents one of which is invariably water and the other organic,
then it distributes itself in them in a fixed ratio. In certain
favorable conditions, the solute of interest can be more or less
completely transferred from one
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General Aspects of Separation Methods
phase to another. This purification technique finds applications
in separation, purification and enrichment. This separation method
has been discussed in detail in Units 2 and 3 of this course.
In liquid-liquid partition chromatography, there is a support
which holds the stationary liquid phase and a mobile liquid phase
runs over it. Because of the requirement that the two liquids be
immiscible, it follows that they differ markedly in polarity.
Either the more polar or less polar may be immobilized. Most
commonly, a polar solvent is held on the support. If a non polar
solvent is held after making the support hydrophobic, this is
called reversed phase chromatography. The components of the mixture
redistribute themselves in the two liquid phases resulting into
separation. It is in some ways similar to liquid-liquid extraction
discussed above. If the support material is packed in a column, the
method is known as column liquid-liquid partition chromatography.
The details of this method are discussed in Unit 5 of this course.
A simpler version of this technique is available in the form of
planar or two dimensional chromatography. The support is either a
thin layer of inert material coated over a glass plate or a
chromatographic paper. And hence, we have two types of two
dimensional chromatography: thin layer chromatography and paper
chromatography. In these two cases, the mobile phase can be made to
move either from top to bottom or bottom to top and, thus, there
are two modes of operations descending and ascending
chromatography. The details of planar chromatography are discussed
in Unit 6 of this course.
An improved version of liquid chromatography is in the form of
high performance/ high pressure liquid chromatography (HPLC). It is
one of the most widely used separation techniques. The particle
size of the packing material is much smaller and a high pressure
around 6000 psi is applied at the top of the column. A high
pressure version of thin layer (HPTLC) is also available. Unit 8 of
this course incorporates the details of this technique.
In gas-liquid chromatography, the stationary phase is a liquid
coated in a column or on a support packed in the column. The mobile
phase is a carrier gas which carries with it the sample in form of
a gas. The sample is volatilized to be carried through. The
partitioning takes place between the carrier gas and the coated
liquid. The different aspects of this separation technique are
elaborated in Unit 7.
SAQ 9 Name the two types of liquid-liquid partition
chromatography. ..
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1.6.4 Ion Exchange In ion exchange, there is an exchange of
ions, cations or anions, between an insoluble solid material and
the solution in contact with it. The solid material called ion
exchanger carries exchangeable cations and anions. When the
exchanger is in contact with an electrolyte, these ions can be
exchanged for a stoichiometrically equivalent amount of other ions
of same charge. Carriers of exchangeable cations are known as
cation exchangers and carriers of exchangeable anions as anion
exchangers. Certain materials are capable of both cation and anion
exchange and are known as amphoteric
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Classical Methods exchangers. Generally, ion exchange is
performed in columns. However, in some cases, batch operations are
carried out. A very special case of ion exchangers is chelating
resins. These resins contain functional groups which are chelating
ligands. They form multiple bonds with complex forming metal ions.
These resins have much higher affinities for transition metal ions
than for alkali metal ions.
SAQ 10 Name the type of ion exchanger which shows both cation
and anion exchange properties.
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1.6.5 Surface Activity In surface activity, it is the property
of adsorption which is mainly used for separation. The methods can
be chromatographic and nonchromatographic. The chromatographic
methods are as follows:
Liquid-solid adsorption chromatography
Gas-solid adsorption chromatography
In liquid-solid adsorption chromatography, an active adsorbent
acts as a stationary phase and the liquid as a mobile phase. The
stationary phase can be in the form of a column or a thin layer on
a glass plate. Paper chromatography cannot be exclusively
classified as liquid-liquid partition chromatography. If the
cellulose of the paper is playing an active role as an adsorbent,
it will be justified to put it under the head of adsorption
chromatography. The high pressure/ high performance versions of
adsorption chromatography both on column and thin layer are, in
use.
In gas-solid adsorption chromatography, the carrier gas with the
volatile components of the mixture flows over an active adsorbent
packed inside the column. Because of differences in adsorption
affinity of the components, the segregation of the components takes
place. As compared to gas-liquid chromatography in which a liquid
is coated over an inert material packed inside the column, here in
gas solid chromatography, an adsorbent is packed.
The details of the two types of adsorption chromatography and
the high performance version of liquid-solid chromatography are
discussed in the respective Units 5 and 6 of Block 2 and Units 7
and 8 of Block 3 of this Course.
An ultimate extension of adsorption chromatography is affinity
chromatography. It is relatively a new advancement and is an
important tool for biomedical research. The technique of affinity
chromatography exploits the unique biological specificity of in a
ligand-macromolecule interaction. The concept of affinity
chromatography is realized by covalently attaching the ligand to an
insoluble support through an spacer arm and packing the material
into a chromatographic bed. If a mixture of several proteins is
passed through the column, only that protein that displays
appreciable affinity for the ligand will be retained or retarded.
The others which show no recognition of the ligand will pass
unretarded. The adsorbed protein can be subsequently eluted by
changing the composition of the solvent to permit dissociation from
the ligand. The
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General Aspects of Separation Methods
potential of this technique for the purification and isolation
of biological molecules is unlimited. Specific adsorbents can be
designed for the purification of enzymes, antibiotics, nucleic
acids and proteins.
1.6.6 Molecular Geometry There are a number of methods based on
molecular geometry. They require a permeable barrier to provide
separation. The methods are based on the following:
Molecular sieves
Gel filtration The permeable barrier separation process of
practical importance utilizes semipermeable membranes as the
restrictive surface. The membranes permit the passage of certain
chemical species completely while preventing or strongly retarding
the permeation of others. It may be mentioned here that the
transport rate of permeation has to be high enough to achieve
reasonably rapid separation. A good mechanical and chemical
stability of these membranes is important. There are different
types of membranes available which are as follows:
Microporous membranes,
Homogenous membranes,
Charged membranes, and
Thin membranes. There are numerous applications of these
membranes. Some of these are listed below;
Ultrafiltration,
Reverse osmosis,
Dialysis, and
Electrodialysis. Unit 11 of Block 5 of this course is assigned
for the detailed discussion on membrane separations.
Gel filtration has taken the form of a regular chromatography.
This is also known as gel permeation or size exclusion
chromatography. The column packing materials are polymer beads and
silica based particles containing a network of uniform pores into
which the solute and solvent molecules can diffuse. In a column
operation, the solute molecules which are small enough to enter the
pores of gel, are retarded. While the molecules large enough not to
enter the pores, will spend all their time in the mobile phase and
move rapidly through the column. This is a useful technique for the
separation of high molecular weight natural products from low
molecular weight species and salts. It is also used for the rapid
determination of molecular weight. The separation by gel permeation
can be carried out at high pressure with an HPLC instrument. The
details of this branch of chromatography are given in Unit 10.
SAQ 11 What are the main requisites of a semi-permeable membrane
to be used for various applications?
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Classical Methods 1.6.7 Electromigration The simplest example of
separation by migration under electric field is controlled
potential electrodeposition. This method is quite effective for the
separation of certain metal ions in the presence of other metal
ions. But in the normal classification of separation methods, it is
not given its due recognition. However, the techniques like
electrophoresis and electrochromatography in which an electric
field is utilized to produce or affect the relative motion of
charged species in solution, have gained more prominence. The two
frontline techniques included under this head are electrophoresis
and electrochromatography.
In electrophoresis, only the electric field causes the motion
while in electrochromatography, the movement is caused by the
resultant of an electric and a gravitational (or other
non-electrical) forces. Electrophoretic separations are performed
in two different ways. One is called slab electrophoresis and the
other capillary electrophoresis.
In slab electrophoresis, a filter paper, porous glass or a gel
cast in the form of a bed can be used. Samples are introduced as
spots or band and a dc potential is applied across the slab for a
fixed period. The separated species are visualized by staining in a
way similar to TLC.
In capillary electrophoresis, a buffer filled capillary tube is
used. The tube extends between two buffer reservoirs that also
holds the platinum electrodes. The sample is introduced at one end
and a dc potential is applied. The separated analytes are observed
by a detector at the other end.
The simplest form of electrochromatography is by fusion of
electrophoresis and paper chromatography. The sheet of paper is
usually suspended vertically with the solvent (buffer) descending
from the top. The electric field is applied horizontally along the
sides of paper. The capillary electrochromatography is a more
sophisticated version of the paper electrochromatography. It is a
hybrid of capillary electrophoresis and HPLC and offers some of the
advantages of the two techniques. The different aspects of
separation by electromigration are discussed in Unit 12.
1.7 CLASSIFICATION BASED ON EQUILIBRIUM AND RATE PROCESSES
Now we have seen that the different types of separation methods
have been classified under various categories based on property
resulting into separation. Practically, all these methods can be
divided into various classes using equilibrium and rate processes
as criteria. The equilibrium processes are based on differences in
the properties of individual components. These processes are
generally based on phase equilibria and involve the distribution of
substances between two phases. Rate processes are based on the
kinetic properties of the components. It is already clear that a
majority of important separation processes are chromatography
based; therefore, it may be reasonable to introduce a
sub-classification of the method listed under this head as
chromatographic and non-chromatographic methods.
1.7.1 Classification Based on Equilibrium Processes It has been
pointed out in the preceding section that in equilibrium processes
there are two competing phases and they could be as follows:
Gas-liquid,
Gas-solid,
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General Aspects of Separation Methods
Liquid-liquid,
Liquid-solid. a) Gas-liquid
Some of the important methods in which gas and liquid are the
two phases involved are as follows:
1. Non-chromatographic methods
i) Distillation ii) Foam fractionation When we say distillation,
it means distillation in all its forms. The well-known example of
foam fractionation is concentration of ores by froth floatation
process.
2. Chromatographic method
i) Gas-liquid chromatography b) Gas-solid
When the two phases are gas and solid, a few examples of the
methods are as given below:
1. Non-chromatographic methods
i) Sublimation ii) Adsorption
2. Chromatographic methods
i) Gas-solid chromatography ii) Exclusion chromatography
In the case of sublimation, the solid is directly vaporised and
the equilibrium exists between the gas and the solid. Adsorption of
gases by solids is a well known method of separation and this forms
the basis of gas-solid chromatography. Exclusion of molecules based
on shape and size of both gas and liquid can take place.
c) Liquid-liquid In this case, two immiscible liquids come in
contact with each other. Here, the separation takes place because
of favorable partition of one or more components in one of the
phases.
1. Non-chromatographic method
i) Liquid-liquid extraction
2. Chromatographic methods
i) Liquid-liquid column chromatography ii) Liquid-liquid planar
chromatography iii) High pressure liquid chromatography
d) Liquid-solid 1. Non-chromatographic methods
i) Precipitation
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Classical Methods ii) Fractional crystallization 2.
Chromatographic methods
i) Adsorption chromatography (column and planar) ii) Ion
exchange chromatography iii) Affinity chromatography iv) Exclusion
(gel permeation) chromatography
1.7.2 Classification Based on Rate Processes The separations
based on rate processes take place due to differences in the
kinetic properties of the components of the mixture. The
separations are mainly achieved due to the following two
reasons.
Different diffusion rates through permeable barriers such as
membranes,
Different migration velocities under various fields like
electrical, gravitational and thermal
a) Different diffusion rate through permeable barriers The most
commonly used permeable barrier is semi = permeable membrane. The
methods based on its use are
i) Ultrafiltration, ii) Reverse Osmosis, iii) Dialysis, and iv)
Electrodyalysis.
b) Field separations Electrical field is commonly used to affect
separations and the different methods which are based on the
application of this type of field are
i) Electrodeposition, ii) Electrophoresis, iii) Capillary
electrophoresis and iv) Electrochromatography.
The examples for separations resulting due to application of
gravitational and thermal fields are ultracentrifugation and
thermal diffusion, respectively.
After going through both types of classification, one can see
that most of the known techniques are included in one or the other
category. But it may be mentioned that the list of separation
methods considered for classification is not exhaustive. Some of
the lesser known methods have not been taken into consideration.
The other point which has to be kept in mind is that in some of the
methods, particularly chromatographic, more than one mechanism is
operative and the method may be classified under different
categories.
SAQ 12 Give one example each of a non-chromatographic separation
process when the following two phases are in equilibrium.
i) Gas-liquid
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General Aspects of Separation Methods
ii) Liquid-liquid iii) Liquid-solid
..
..
..
SAQ 13 Name the separation methods which are performed under the
influence of electrical field. ..
..
..
..
1.8 CRITERIA FOR SELECTION OF SEPARATION METHODS
Now we have some idea about a majority of separation methods.
The advantages, applications and limitations of some of the
important methods will be discussed in the different units of this
course. It may be a little premature to make a comparison of the
methods, particularly, when we do not have sufficient background of
the different separation methods. Moreover, the assessment of
utility of a method is really a situation based problem. To
elaborate this point, one can cite that the selection of a method
will be determined by the physical state of mixture, its
complexity, amount of sample available, the speed required and the
level of purity desired. Over and above this, one important point
that has to be kept in mind is that whether the method has to be
used for analysis or synthesis/recovery/purification. If a method
has to be used on an industrial scale, some different
considerations also get prominence. In a situation like this, it is
a little difficult task to impart special importance to one or the
other criteria. However, having known about the utility of the
separation methods, we can talk about some of the general criteria
for the selection of the separation methods. They are
Selectivity,
Detectability,
Yield, speed and convenience,
Capability for hyphenation, and
Ease in scaling up and economics. We will now briefly explain
each one of them.
1.8.1 Selectivity The foremost and most important is the
selectivity. The selectivity relates to the capability of the
technique to separate the desired component effectively from a
complex mixture particularly containing closely similar components.
The term effective separation is expressed in terms of resolution,
separation factor, decontamination factor and percentage purity.
All these terms, in one form or the other, will appear in different
units of this course.
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Classical Methods 1.8.2 Detectability The term detectability
refers to the minimum amount that can be detected. It is also
termed as sensitivity. In the case of detectors, it can be
calculated based on certain assumptions such as detection at two or
three times the noise level observed at the baseline. Sensitivity
can also be determined practically. Sensitivity can be improved by
properly optimizing the separation conditions using the same
detector. Sensitivity assumes paramount importance when the
quantity of mixture to be analyzed is very small or trace
impurities for obtaining an ultrapure material are to be
assayed.
1.8.3 Reproducibility The method should be such that it gives
reproducible results of separation. It is usually expressed in
terms of standard deviation or coefficient of variation based on
replicate measurements. Now at this point, it may be important to
point out that the separation conditions should not be such that a
slight variation in the conditions may alter the result
significantly. A flexibility in the conditions of separation is
always preferred. This is particularly important if the process is
to be scaled up on a commercial scale.
1.8.4 Yield, Speed and Convenience In all those methods in which
a simultaneous quantification of the separated product is done, a
quantitative recovery of the separated product is a must.
Naturally, a method giving a higher yield is preferred. In cases
where the yield is not high, the remaining portion may have to be
recycled.
Speed of the separation is another critical parameter. Any
technique which is awefully slow in achieving the separation goes
in the background. By properly adjusting the experimental
parameters, it is sometimes possible to improve the speed of the
separation process. Speed, as a criteria, assumes great
significance if the constituents of the mixture degenerate or
decay. A typical example is the separation of radionuclides with
very short half-lives. In such a situation, speed becomes more
important than the yield.
Literally, the convenience of a separation method is a very
broad term. The method of separation should be such that there may
be little preparative chemistry before subjecting the sample to
separation. The conditions of separation should not be very
stringent to the extent that it may be difficult to keep a control
of the conditions. The product should be obtained in such a form
that it can be easily put to use without much chemistry. Moreover,
the method can be an asset if it is easily put on-line operation or
easily automated.
1.8.5 Capability for Hyphenation In a majority of
chromatographic methods, there is a detector which quantifies the
effluent coming out of the column. These detectors are not very
sensitive. Also they have a limited capability of identifying the
unknowns. It is known that a mass spectrometer is a very powerful
universal detector. Therefore, it is being interfaced with some of
the chromatographic techniques. A very important example in this
regard is interfacing of a gas chromatograph with a mass
spectrometer (GC-MS).
Gas chromatography is an ideal separator, whereas mass
spectrometry is excellent for identification. The aim of an
interfacing arrangement is to operate both a gas chromatograph and
a mass spectrometer without affecting the performance of either
instrument. The problem is compatibility. The interface provides
the link between the two instruments. On a similar line, gas
chromatography is coupled with infrared
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General Aspects of Separation Methods
spectrometry. The techniques like high performance liquid
chromatography and simple liquid chromatography are coupled with a
mass spectrometer. The interfacing makes the technique as a
powerful analytical tool. Thus, in nutshell, a separation technique
is conveniently interfaced with an effective detecting device like
mass spectrometer or spectrophotometer, this can be an additional
advantage of the technique. There have been serious efforts in this
direction and even the simple techniques like thin layer
chromatography has been interfaced with a mass spectrometer.
1.8.6 Ease in Scaling up and Economics It has been repeatedly
pointed out that one of the main roles of the separations in
technology is in the synthesis, recovery and purification of
products. Therefore, the separation methods do not remain
necessarily confined to the bench scale. The basic data has to be
used to scale up the process to plant scale. The method should be
such that it can be conveniently scaled up on a commercial scale
without much alteration in basic experimental parameters. Once we
talk of the use of the method on a commercial scale, the economics
of the process figures in. Not only the economics, the
environmental considerations assume significant importance. The
commercial method of separation should also be environmental
friendly.
SAQ 14 What is the usual way of expressing the reproducibility
of a separation method? ..
..
SAQ 15 In what particular type of separations, the speed of the
separation becomes very important?
..
..
SAQ 16 What different parameters gain prominence when a
separation method is to be scaled up for plant production? ..
..
..
..
..
1.9 SUMMARY The present unit provides an introduction to the
course on separation methods. It highlights the utility of
separations in the analysis and recovery of pure products from
complex mixtures. The scope of separations extends from the needs
of everyday life to complex technological processes. There is
hardly any branch of science or technology which has not been a
beneficiary of the developments in separation science.
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Classical Methods Separations encompass different
physicochemical principles and the subject has itself matured to a
unified science. Most of the well known methods of separations fall
under the category of chromatographic methods.
It is a little difficult task to categorize the variety of
separation methods available today. A simpler classification can be
made on the basis of property which results into separation. The
other approach that can be adopted for classification is based on
the physicochemical phenomena responsible for separation. In other
words, these phenomena can be divided under two categories, the
equilibrium and the rate processes. In equilibrium processes, there
have to be two competing phases. However, in rate processes, the
constituents move differently through a permeable barrier or show
different migration velocities under various fields, mainly
electrical.
In both the classifications, it has been possible to classify
most of the well known separation methods in one or the other
category. But some of the methods are such that they can be put in
more than one category.
As regards the criteria to be used for the assessment of utility
of a separation method, the whole problem is situation based. This
point can be clarified by the fact that the selection or the
utility of a method will be determined by the physical state of the
mixture, its complexity, amount of sample available, the speed
required and the level of purity desired. It has to be kept in mind
that whether the method is to be used for analysis or
synthesis/recovery. But this only does not necessarily determine
the importance of the different criteria cited in the text.
1.10 TERMINAL QUESTIONS 1. In what ways the separations help in
chemical analysis?
2. Name the properties which are generally used for achieving
separations.
3. What are the different types of distillation processes?
4. What are the different ways of affecting separations by
precipitation?
5. What is paper electrochromatography?
6. Name the different criteria which are commonly used for the
selection of separation methods.
1.11 ANSWERS
Self Assessment Questions 1. Separations have the following two
main types of applications:
i) In the analysis of materials ii) In the synthesis and
recovery of pure materials.
2. Separation is a process by which a mixture is divided in at
least two components with different compositions or two types of
molecules with the same composition but different
stereochemistry.
3. The two important examples beneficial for our environment are
as follows:
i) Purification of municipal drinking water.
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General Aspects of Separation Methods
ii) Removal of undesirable gases and particulate matter from the
factory emissions.
4. Chromatography is a method of separation in which the
components to be separated are distributed between two phases, one
of these is called the stationary phase and the other mobile phase
which moves on the stationary phase in a definite direction. The
stationary phase can be a solid or liquid and the moving phase may
be a liquid, gas or supercritical fluid.
5. The main processes responsible for separations by
chromatography are as follows:
i) Adsorption ii) Partition iii) Ion exchange iv) Size
exclusion.
6. The different reasons for the growth in separation methods
are as under.
i) Different separation goals. ii) Diversity of mixtures to be
separated. iii) Utilization of variety of physicochemical phenomena
for
separations.
7. The two main criteria employed for classifying different
separation methods are as given below.
i) One criteria is based on the property which results in
separation. ii) The other criteria is based on the physicochemical
phenomena
responsible for separation. These phenomena can be further
divided in two categories, the equilibrium processes and the rate
processes.
8. Vacuum distillation or actually distillation under reduced
pressure is used to separate components of a mixture which
decompose below their normal boiling point.
9. The two types of liquidliquid partition chromatography are as
follows:
i) Column chromatography ii) Planar chromatography
10. The type of ion exchanger which shows both cation and anion
exchange properties is known as amphoteric exchanger.
11. The main requisites of a semipermeable membrane to be used
for separation are as follows:
i) It has to permit the passage of certain chemical species
completely by restricting or retarding permeation of others,
ii) The transport rate of permeation has to be high enough and
iii) It should have good chemical and mechanical stability.
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Classical Methods 12. Phases in Equilibrium Examples of
non-chromatographic Separation Process
i) Gas-liquid distillation ii) Liquid-liquid solvent extraction.
iii) Liquid-solid precipitation
13. The methods are
i) Electrodeposition, ii) Electrophoresis, iii) Capillary
electrophoresis, and iv) Electrochromatography.
14. Standard deviation () or coefficient of variation.
15. If the species degenerates or decays fast with time.
16. The different parameters which gain prominence are
i) Convenience in scaling up. ii) Economics of the process. iii)
Potential for automation. iv) Environmental friendliness.
1.12 TERMINAL QUESTIONS 1. The different ways in which the
separations help in chemical analysis are stated
below:
i) Removal of interfering constituents before the actual
analysis of one or more constituents of the mixture.
ii) Isolation of an unknown constituent for subsequent
characterization. iii) Analysis of complex unknown mixture by
subjecting the entire sample to
separation into individual constituents.
2. The properties which are generally used for achieving
separations are
i) Volatility, ii) Solubility, iii) Partition, iv) Ion exchange,
v) Surface activity, vi) Molecular geometry, and vii)
Electromigration.
3. i) Fractional distillation, ii) Flash distillation, iii)
Vacuum distillation,
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General Aspects of Separation Methods
iv) Steam distillation, and v) Azeotropic distillation.
4. Separation by precipitation can be affected in the following
ways:
i) Solvent precipitation is achieved by the addition of another
miscible solvent such that the solubility of the component of
interest is reduced and it appears as a precipitate.
ii) By chemical reaction between a selective precipitating agent
which reacts with the desired constituent to give a precipitate,
and
iii) Some weakly basic and acidic compounds are precipitated by
altering the pH of the solution.
5. Paper electrochromatography is a fusion of electrophoresis
and paper chromatography. The sheet of paper is usually suspended
vertically and a solvent (buffer) is made to travel from top to
bottom. The electric field is applied horizontally.
6. The criteria for the selection of separation method(s)
are
i) Selectivity, ii) Detectability, iii) Reproducibility, iv)
Yield, speed and convenience, v) Capability for hyphenation and vi)
Ease in scaling up and economics.
Further Reading 1. Chromatography and Separation Science, By
Satinder Ahuja, Academic Press. 2. Quantitative Analysis, By R.A.
Day Jr. and A.L. Underwood, Prentice and Hall
of India.
3. Principles of Instrumental Analysis, By D.A. Skoog, F.J.
Holler and T.A. Nieman, Thomson India.
4. Instrumental Methods of Chemical Analysis, By G.W. Ewing,
McGraw Book Company.
5. Basic Concepts of Analytical Chemistry, By S.M. Khopkar,
Wiley Eastern Limited.
6. Analytical Chemistry, By G.D. Christian, John Wiley &
Sons.