Download Slide 1

Experiment 15
A: Isolation of Pure Aspirin
From Aspirin Tablets
B: TLC of a Dye Mixture
Objectives



Isolation of a pure component from a
commercial tablet
To become familiar with the technique
of thin-layer chromatography
To separate a dye mixture into its
constituent components
Introduction – 15A






Commercial pharmaceutical tablets
contain a variety of components
Drug molecule
Binders
Flavours
Colours
Co-administered drug molecules
Experimental






Weigh one tablet of aspirin
Grind and transfer to a 50 ml flask
Add 20 ml of acetone and heat
Filter off the insoluble binding material
Evaporate to dryness
Record the weight of crude aspirin
Experimental





Crude aspirin is recrystallized from
water
Dissolve in 3-4 ml of boiling water
Allow to cool and crystallize
Isolate the crystals
Allow to dry, get weight and mp
Introduction – 15B





Thin Layer Chromatography
Common separation technique
Separation based on polarity
Qualitative analysis
Used to test purity, reaction completion,
decomposition
Introduction




TLC plate – sheet of metal covered in a
thin layer of silica
Silica is polar – polar compounds have a
greater affinity for silica
Solvent allowed to travel up the plate
Compounds with a higher affinity for
the solvent will travel further up the
plate
Introduction





Choice of solvent is important
Affects distance travelled by spots
Affects separation of spots
Common solvent systems include
mixtures of Ethyl Acetate/Hexane and
Dichloromethane/Methanol
Large scale – purification by column
chromatography
Rf values


Rf value of a spot is
the distance
travelled by the spot
over the distance
travelled by the
solvent front
Rf depends on
solvent system
dX
Rf 
ds
Thin Layer Chromatography
Rf = 0.74
Rf = 0.74
Rf = 0.53
Rf = 0.52
Rf = 0.16
Rf = 0.16
Viewing TLC plates




Spots not always visible – can be colourless
Can be viewed under UV light
Developed using iodine to stain spots
Other dyes/stains – vanillin, ninhydrin
O
HO
O
O
O
vanillin
O
ninhydrin
Viewing plates
Experimental




Use a capillary spotter to spot the dye
mixture on a TLC plate
Insert slide into a jar for development
Solvent level must be below the spot
level – otherwise spots dissolve
Allow solvent front to rise close to top,
remove plate, mark the solvent front
Experimental




Using pencil, circle all the spots
Calculate the Rf values for each colour
spot in the dye mixture
Develop the TLC plate in both toluene
and ether – solvent affects the distance
travelled and separation
Keep toluene and ether in the
fumehood at all times
Report



Record the distance travelled and Rf
value for each colour component in
both the solvents used
Comment on the relative solubilities of
the colour components in the two
solvents used
Brief discussion on TLC