They are numerous techniques of identification and separation of mixtures. The most valuable clues in the identification
of a compound are the physical properties such as melting points, specific
gravity and even the index of refraction of a liquid sample. It is also necessary
to carry out qualitative analysis so as to establish which elements are present.
The method of separation depends of
various properties of the existing compounds in the mixture, whether they are
soluble in a polar or non-polar solvent, whether they have low or high boiling
points etc. The chemist will then decide what method to use based of these
factors. We will look at one method of identification and separation that are
famous for their accuracy and ease of use relative to other methods.
Identification of chemical compounds
Nuclear Magnetic Resonance Spectroscopy
This is a method of identification
A sample of a compound, dissolved in a suitable solvent, is placed in a
precision tube between the pole faces of a massive, highly homogenous magnet. The
effective homogeneity of the field is enhanced by spinning the sample at high
speed about a vertical axis. It is then ‘swept’ in the absorption region, maintaining
the frequency of an oscillator at a constant value whilst varying the magnetic
The environment of each nucleus in a molecule is different, and the absorption
of energy by the nucleus is depends on its environment. The hydrogen atoms in a
CH3 group can thus be
distinguished from those in a CH2 group which in turn can be
distinguished from those in an OH group, and so on.
Applications of Nuclear Magnetic Resonance Spectroscopy.
This technology is used very frequently in the food industry to identify
organic and bio-organic substances and to elucidate their chemical structures.
2. Separation of Mixtures
Chromatography is a laboratory technique for the separation of a mixture.
The sample is spotted on to the paper, which is suspended
vertically in a glass tank containing a small quantity of a suitable
partitioning solvent. The bottom edge of the paper should dip into the solvent
and the spot should be located just above the surface of the solvent. The
components become partitioned between the water in the cellulose fibres of the
paper and the mobile organic solvent. When the solvent has nearly reached then
end of the paper, the paper is removed from the tank and the position of the
solvent front is marked. The paper is then dried and developed with a suitable
reagent e.g. butanol. Each component can then be identified from its RF value
(ratio of the distance moved by the sample to the distance moved by the solvent
front) which is a constant for each compound for a particular combination of
eluting and adsorbing phases.
Chromatography provides a sensitive method for separating
very small samples of mixtures. It is particularly useful when the physical
properties of the components are similar. Separation is brought about by the differential
movement of the components of a mixture in relation to two immiscible liquid
phases or to a solid and a liquid phase. In an adsorption column
chromatography, the sample, in a suitable solvent, is introduced into a column
containing a solid adsorbent stationary phase. A continuous process of
separation is established by the subsequent addition of further quantities of solvent.
The different adsorption properties of the components of the sample cause them
to pass through the column at different rates and form distinctive bands in the
In liquid-liquid chromatography, the components of the sample become
partitioned between the water, which serves as a stationary second solvent
supported by an inert porous medium, and a mobile organic solvent.