Chemistry, Third edition

19 · SEPARATING MIXTURES

column it separates into coloured bands of its components because the components

move through the column at different rates. For example, a compound that is not

strongly adsorbed but is very soluble in the eluting solvent, will move through the col-

umn rapidly. The bands are finally washed out at the bottom of the column and each

one is collected in a different flask (Fig. 19.19).

Column chromatography becomes a little more difficult if the separation involves

colourless compounds because the different bands cannot be detected by the eye. A

series of fractions of the same volume can be collected (say 25 cm^3 ) and the purity of

the fractions monitored by other chromatographic methods, such as TLC or GC.

Thin-layer chromatography (TLC)

Thin-layer chromatography (TLC) is a variation of column chromatography. A strip

of glass is coated on one side with a thin layer of adsorbent (e.g. alumina). The

substance to be tested is spotted near one end of the plate and the plate is put into a

developing jarwith a small amount of the solvent (to a level below the spot) and a cap

put on the jar (Fig. 19.20). The solvent rises up the plate, separating the sample into

its components. At the end of the experiment, a spot for each component is seen on

the plate. When the solvent is near the top of the plate, the position the solvent has

reached is marked with a penciland the plate removed and dried. If the components are

colourless, a number of techniques can be used to show them up:

1.Some compounds show up under ultraviolet light.

2.The dry plate can be placed in a stoppered jar with a few crystals of iodine – the

crystals sublime and most organic compounds show up as coloured spots when

exposed to iodine vapour.

3.The plate can be sprayed with a reagent that will react with a compound on the

plate to form a coloured product.

TLC needs only small amounts of sample and is generally used to check the purity of

the product of a chemical reaction. TLC can also be used to confirm the identity of

an unknown sample. The retention factor(Rfvalue) of a compound is

Rf=

the distance travelled by the compound

the distance travelled by the solvent

362

Fig. 19.19Column chromatography.

Fig. 19.20Running a TLC
plate.