(TLC) and

A layer of sand is placed over the top of the adsorbent, and the whole column is wetted with the
solvent to be used. A solution of the substance to be purified in this solvent is then applied
evenly to the top of the column, and this solution is allowed to pass down into the column so that
the dissolved solid is adsorbed at the top of the column. The column is then eluted by passing
down a number of solvents of increasing polarity. In this way, weakly adsorbed substances will
pass rapidly through the column while the more strongly adsorbed substances will pass through
at a slower rate. By eluting with a series of solvents of increasing polarity it is therefore possible
to separate the components of a mixture and to elute them, one after the other, from the solid
adsorbent.
Choice of Adsorbent
The order in which the compounds are eluted will depend on how strongly they are adsorbed on
the surface of the stationary phase. A few adsorbents used in column chromatography, with
different binding abilities are Cellulose, Calcium oxide, Silica gel (oxides of silicon), and
alumina (aluminum oxide). Alumina unless specially pretreated is slightly basic and hence
strongly adsorbs acidic substances or materials capable of forming hydrogen bonds to the basic
oxygen atoms of the alumina. Compounds without the ability to form hydrogen bonds but with
substantial dipole moments will be somewhat less strongly adsorbed due to electronic
interactions between their dipoles and those of the alumina. Compounds with neither acidic
hydrogens nor dipole moments are only very weakly adsorbed due to dipole induced dipole
interactions. The complete order for the strength of all these bonding interactions is generally the
following: Salt Formation > Coordination Complexes > Hydrogen Bonding > Dipole-Dipole
Interactions > Van der Waals. Some examples of these bonding interactions with alumina are
shown:
Choice of Solvent
The choice of solvents used to elute the various components of the mixture from the column will
depend upon the components in the mixture. For a very weakly adsorbed component a very nonpolar
solvent such as petroleum ether or benzene would be used. For more strongly adsorbed
components, a more polar solvent such as ether might be used. For very strongly adsorbed
components, a very polar solvent such as ethanol or even acetic acid might be required to
displace the material from the column. A list of common solvents in order of increasing eluting
power follows:
Petroleum Ether
Carbon Tetrachloride
Cyclohexane
Carbon Disulfide
(non-polar)
increasing Benzene
Toluene increasing
eluting Methylene Chloride
Chloroform polarity