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Unit 1 General Aspects of Separation Methods

Nov 20, 2015




General Aspects of Separation Methods

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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?






    SAQ 2 Define separation in the real chemical sense.






    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 n