Thin Layer Chromatography

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Experiment 5
Title : Thin Layer Chromatography
i. To distinguish polar and non-polar solvents.
ii. To familiar with the analysis technique by using the thin layer chromatography. iii. To differentiate the retention factor, Rf for different compounds. [pic]
|Compound |Distance traveled by the compound | |o-nitroanaline |2.45 | |p-nitroaniline |1.70 | |Unknown sample |2.45 |


The Rf value for each dye is then worked out using the formula: [pic]


Rf= 2.45


Rf= 1.75

Unknown sample:

Rf= 2.45


Silica gel is a form of silicon dioxide (silica). The silicon atoms are joined via oxygen atoms in a giant covalent structure. However, at the surface of the silica gel, the silicon atoms are attached to -OH groups. So, at the surface of the silica gel you have Si-O-H bonds instead of Si-O-Si bonds. The diagram shows a small part of the silica surface. [pic]

The surface of the silica gel is very polar and, because of the -OH groups, can form hydrogen bonds with suitable compounds around it as well as van der Waals dispersion forces and dipole-dipole attractions. As the solvent begins to soak up the plate, it first dissolves the compounds in the spot that you have put on the base line. The compounds present will then tend to get carried up the chromatography plate as the solvent continues to move upwards. How fast the compounds get carried up the plate depends on two things: • How soluble the compound is in the solvent. This will depend on how much attraction there is between the molecules of the compound and those of the solvent. • How much the compound sticks to the stationary phase - the silica get, for example. This will depend on how much attraction there is between the molecules of the compound and the silica gel. Suppose the original spot contained two compounds - one of which can form hydrogen bonds, and one of which can only take part in weaker van der Waals interactions. The one which can hydrogen bond will stick to the surface of the silica gel more firmly than the other one. We say that one is adsorbed more strongly than the other. Adsorption is the name given to one substance forming some sort of bonds to the surface of another one. Adsorption isn't permanent - there is a constant movement of a molecule between being adsorbed onto the silica gel surface and going back into solution in the solvent. Obviously the compound can only travel up the plate during the time that it is dissolved in the solvent. While it is adsorbed on the silica gel, it is temporarily stopped - the solvent is moving on without it. That means that the more strongly a compound is adsorbed, the less distance it can travel up the plate. In the example we started with, the compound which can hydrogen bond will adsorb more strongly than the one dependent on van der Waals interactions, and so won't travel so far up the plate. What if both components of the mixture can hydrogen bond?

It is very unlikely that both will hydrogen bond to exactly the same extent, and be soluble in the solvent to exactly the same extent. It isn't just the attraction of the compound for the silica gel which matters. Attractions between the compound and the solvent are also important - they will affect how easily the compound is pulled back into solution away from the surface of the silica. However, it may be that the compounds don't separate out very well when you make the chromatogram. In...
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