Study Chemistry notes on Thin Layer Chromatography for Secondary. Useful for revision, homework, and exam preparation in Zimbabwe.
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This is the Chemistry notes from 2018. It is a free notes for revision.
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S CHIMANIKIRE MACALLISTER MABHUREKI ; CHINHOYI 2 HIGH SCHOOL 2018 CHEM -NOTE
Thin Layer Chromatography
Thin layer chromatography (TLC) is a chromatographic technique used to separate the components of a mixture using a thin stationary phase supported by an inert backing. It may be performed on the analytical scale as a means of monitoring the progress of a reaction, or on the preparative scale to purify small amounts of a compound. TLC is an analytical tool widely used because of its simplicity, relative low cost, high sensitivity, and speed of separation.TLC functions on the same principle as all chromatography: a compound will have different affinities for the mobile and stationary phases, and this affects the speed at which it migrates. The goal of TLC is to obtain well defined, well separated spots.
Retention Factor
After a separation is complete, individual compounds appear as spots separated vertically. Each spot has a retention factor (Rf) which is equal to the distance migrated over the total distance covered by the solvent. The RfRf formula is
$\mathrm{Rf}=$ distance traveled by sampledistance traveled by solvent $\mathrm{Rf}=$ distance traveled by sampledistance traveled by solvent
The RfRf value can be used to identify compounds due to their uniqueness to each compound. When comparing two different compounds under the same conditions, the compound with the larger RfRf value is less polar because it does not stick to the stationary phase as long as the polar compound, which would have a lower RfRf value.
RfRf values and reproducibility can be affected by a number of different factors such as layer thickness, moisture on the TLC plate, vessel saturation, temperature, depth of mobile phase, nature of the TLC plate, sample size, and solvent parameters. These effects normally cause an increase in $\mathrm{RfRfvalues}. \mathrm{However}$, the case of layer thickness, the RfRf value would decrease because the mobile phase moves slower up the plate.
If it is desired to express positions relative to the position of another substance, x , the RxRx (relative retention value) can be calculated:
$\mathrm{Rx}=$ distance of compound from origindistance of compound x from origin $\mathrm{Rx}=$ distance of compound from origindistance of compound x from origin RxRx can be greater than 1 .
Apparatus
Plates
As stated earlier, TLC plates (also known as chromatoplates) can be prepared in the lab, but are most commonly purchased. Silica gel and alumina are …