Engineering Chemistry S Datta

CHROMATOGRAPHY 507
• The paper is folded perpendicular to the line drawn.
• The edges of the paper are held with clips.
• The tank is prepared by placing the mobile phase (about 1 cm depth of the tank) and
the lid is placed.
• Some time is allowed to saturate the tank with solvent vapour.
• The folded paper is placed in the tank followed by placing the lid.
• The development is allowed to take place i.e., eluted until the solvent reaches a suitable
height (15-20 cm).
• The paper is removed, dried and now comes the question of location of the component.
Detection of components
If the components or substances in a mixture are colourless they are invisible on the
paper. If so, the analyst has to ‘develop’ the paper or plate in case of tlc with a suitable ‘locating
agent’. Common developing agents are (i) iodine vapour (ii) KMnO 4
(1%) (iii) bromocresol
green (0.05%) (iv) ninhydrin for amino acid etc. Amino acids give purple spot.
Advantages
• Rapid and reliable separation and identification.
• The technique may also be used for quantitative purpose.
Disadvantages
• Large scale separation of components is not possible.
• Time consuming, it takes much time to ‘elute’ compared to tlc.
Thin Layer Chromatography (tlc)
Here the stationary phase is a thin layer of a solid, generally silica gel or alumina, on a
glass or plastic plate while the mobile phase is a solvent. The rate at which a component moves
up a tlc plate depends on the equilibrium between adsorption on the solid (stationary phase)
and solution in the solvent (mobile phase). The position of equilibrium varies from one substance
to another as the components of a mixture separate.
Advantages of tlc
• tlc is quick and cheap.
• Only a very small sample is required.
The technique is widely used in laboratories as well as in industries. It can be used
quickly to check whether a chemical reaction is taking place as expected. During purification
of a product tlc can indicate whether or not all the impurities have been removed from the
reaction product.
Detection of the components
• Coloured compounds are easy to detect on a tlc plate.
• A quick way of detecting the position of a colourless organic compound, to allow to
stand the plate in a covered chamber with iodine crystals. The iodine is absorbed by
the compound giving coloured spots.
• Other methods such as spraying with certain reagents are also employed.
• Alternatively a tlc plate may be impregnated with a fluorescent chemical. When the
plate is placed under a UV lamp the whole plate glows except in the areas where organic
compounds absorb radiation, so that they are located as dark spots.
R f
values are used to record the distances moved by component chemicals of a mixture
relative to the distance moved by the solvent.