Objectives
1)
Determine the Rf value.
2)
To identify the components in the analgesic tablet
by a TLC comparison 1with standard compounds.
Introduction
Chromatographic are used extensively in organic chemistry
laboratories for routines analysis. Thin layer chromotography (TLC) can be used
to determine the purity of compound , to analyze the composition of a mixture
are differentiated by exposing to two competing phases , the stationary phase
is a polar adsorbent such as silica gel or alumina , which has been coated on
plastic plate. The mobile phase is an organic solvent .The solvent moves up the
plate capillary action.
As the solvent moves past the spot that was aaplied an
equilibrium was achieved for each component of the mixture between the
molecules of that component which are adsorbed on the solid and the molecules
which is are in solution . In principle , the components will differ in
solubility and in the strength of their adsorption to the adsorbent and some
components will be carried further up the plate than the others.When the
solvent has reached on the top of the plate, the plate will be removed from the
solvent , it will dried and the developing components will be visualised . UV
lamp and iodine are used to visualized the components .
In this experiment the TLC was used to examine the
composition of known analgesic whic is the pain relieving drug such as
paracetamol. Several common analgesics are aspirin and acetomeniphen . Caffein
is sometimes added to these formulations to overcome drowsiness.
The distance travelled by each component is measured and
this value is called the retardation factor which is designated as Rf
value.Rf value for a component is calculated using the following
expression :
Rf = Distance
traveled by the component
Distance traveled by the solvent
There is some chemicals that we are using in this
experiment are shown below.
Acetaminophen
Aspirin
Caffeine
Result and observation
|
Label
|
A
(aspirin)
|
B (upthamol)
|
C
(caffeine)
|
D
(acetaminophen)
|
|
Reading
1 - Distance travelled from start line to center spot(cm)
|
2.6
cm
|
2.3
cm
|
1.1
cm
|
2.5
cm
|
|
Reading
2 - Distance travelled from start line to center spot(cm)
|
2.8
cm
|
2.4
cm
|
1.1
cm
|
2.4
cm
|
(TLC reading 2) (TLC reading
1)
The distance from starting point to the end point of
reading 1 is 5 cm while for reading 2 is
5.1 cm
Rf = Distance
traveled by the component
Distance traveled by the solvent
Calculation for reading 1
Rf of
A (aspirin) = 2.6 / 5 = 0.52
Rf
of B (upthamol) = 2.3 / 5 = 0.46
Rf
of C (caffeine) = 1.1 / 5 = 0.22
Rf
of D (acetaminophen) = 2.5 / 5 = 0.5
Calculation for reading 2
Rf of
A (aspirin) = 2.8 / 5.1 =0.55
Rf
of B (upthamol) = 2.4 / 5.1 = 0.47
Rf
of C (caffeine) = 1.1 / 5.1 = 0.22
Rf
of D (acetaminophen) = 2.4 / 5.1 = 0.47
The TLC
reports that the drug containing is caffeine
Discussion
Thin layer chromotography is a
special of adsorption in which thin layer of adsorbent supported on a flat surfaces
is utilized instead of a column of adsorbent , in a simple words thin layer
chromatography techniques is used to separate mixtures.Thin layer
chromotography techniques was performed on a sheet of glass or aluminium foil
which is coated with thin layer adsorbent material usually silica gel.This
layer was known as stationary phase.Silica gel is a form of silicon dioxide ,
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
hydroxyl group . The surface of the silica gel is very polar , this is because
of the hydroxyl igroup it can formed hydrogen bonds with suitable compounds
around it as well as van der waals dispersion forces and dipole-dipole attraction.
Differrent compounds in the sample
mixture travel at different rates due to the differances in their attraction to
the stationary phase and because of differences in solubility in the solvent .
Fom the above calculation we can see that different compound has different Rf
value.
For reading 1 , Rf for
aspirin is 0.52 , upthamol is 0.46 , caffein is 0.22 and acetaminophen is 0.5 . From reading 2 Rf value for
aspirin is 0.55 , upthamol is 0.47 , caffein is 0.22 and acetaminophen is 0.47
. Reading 1 and 2 has same reading Rf value for caffeine so the drug
is caffeine .
There is some precaution that we
should be aware by doing this experiment which is we cannot allow UV light to
shine to anyone eyes , it can cause permanent eyes damage .While we are doing
line on the TLC sheet we should use pencil instead of pen because it can
contaminated our TLC paper.The solution point maybe moves with the pen ink.
Conclusion
The Rf value was
determined which is for reading 1 ,Rf for aspirin is 0.52 , upthamol is 0.46 ,
caffein is 0.22 and acetaminophen is 0.5
while for reading 2 , Rf
value for aspirin is 0.55 , upthamol is 0.47 , caffein is 0.22 and
acetaminophen is 0.47 .From the calculation the compound in the TLC tablet is caffeine.
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