Undergraduate · 20 min

Column chromatography on silica

Pack a silica column and elute a mixture of ferrocene, acetylferrocene and a polar impurity: the bands separate because each compound partitions differently between silica and the mobile phase.

Goal

Rank the three compounds by polarity from their retention behaviour and choose a fraction-collection plan for clean separation.

Apparatus and reagents

Glass column, silica gel 60 slurry in hexane, sand layer, mixture of ferrocene/acetylferrocene, hexane–ethyl acetate gradient, collection tubes.

Procedure

Load the mixture as a narrow band on top of the packed silica bed.
Start with a non-polar eluent and raise the flow slowly; watch the first band migrate down.
Collect the yellow fraction (ferrocene), then switch to a more polar eluent to move the orange acetylferrocene band.
Explain which band corresponds to the most polar compound and why it elutes last.

What to observe

Three bands move at different speeds: the least polar compound travels fastest, the polar impurity barely leaves the top of the column.
Faster flow shortens analysis time but widens and overlaps the bands — resolution is traded against speed.

Explanation

Silica is a polar stationary phase: polar compounds adsorb strongly and move slowly, non-polar compounds travel with the eluent. The retention factor reflects the partition equilibrium between the two phases; raising eluent polarity (e.g. adding ethyl acetate to hexane) competes for the sites and speeds up the retained bands — the same principle as reversed-phase HPLC, only with the polarities inverted.

History of the experiment

Mikhail Tsvet coined “chromatography” in 1906 separating leaf pigments on a chalk column; Archer Martin and Richard Synge (Nobel 1952) put it on the theory of partition, paving the way for GC and HPLC.

Chemists behind it

Archer J. P. Martin Richard L. M. Synge