3.16.3Organic Chemistry
Column Chromatography
5 min read 4 parts Lesson 54 of 55
Section 1 of 4Setting Up the Column
1
A glass tube is packed with a slurry of solid silica or alumina, which forms the stationary phase.
2
A small amount of the mixture dissolved in a little solvent is added carefully to the top of the column.
3
Solvent, the mobile phase or eluent, is added continuously at the top and flows down through the column under gravity.
4
Components move down at different rates, so they separate into bands in the column.
5
Fractions of the liquid leaving the bottom are collected in separate containers as each component emerges.
Section 2 of 4Why Components Separate
Adsorption
- Polar components are held more strongly on a polar stationary phase and so move more slowly.
Solubility
- Components that dissolve well in the eluent are carried along faster.
Retention time
- The time a component takes to leave the column; strongly held components have long retention times.
Changing the eluent
- Using a more polar solvent later speeds up strongly held components, a technique that improves separation.
Identification
- Fractions can be analysed by TLC, or evaporated so that the pure product is recovered.
Section 3 of 4Comparison with TLC
Comparison with TLC
Scale
- TLC is used to analyse tiny amounts, while column chromatography can separate larger amounts for purification.
Collection
- In a column, each separated compound is collected in fractions; in TLC the spots stay on the plate.
Same principles
- Both use the same stationary phase (silica or alumina) and both rely on adsorption and solubility.
Link
- TLC is often used first to choose the solvent for a column.
| Feature | TLC | Column |
|---|---|---|
| Purpose | analysis | separation and purification |
| Stationary phase | silica on plate | silica in column |
| Mobile phase | solvent rising by capillary action | solvent flowing down by gravity |
Section 4 of 4Exam Checklist & Tips
3.16.3 – Key Points for the Exam
- Eluent The solvent that carries the mixture through the column.
- Retention time The time between injection or loading and the component leaving the column.
- Fraction A portion of liquid collected from the bottom of the column.
- Adsorption Attraction of a component to the surface of the stationary phase.
Common Mistakes
- Letting the column run dry, which introduces air bubbles and ruins the separation.
- Saying the least polar compound is retained longest on silica; polar compounds are held most strongly.
- Confusing the mobile phase (moves) with the stationary phase (fixed in the column).
- Using column chromatography only for analysis; its main use is purification.