ACKNOWLEDGEMENT:
Figure
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Source of Figure
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Figure 13
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www.nuffieldfoundation.org
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Figure 14
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resources.schoolscience.co.uk
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Technique 9: Paper Chromatography
Paper Chromatography
is a technique based on relative solubilities.
Figure 13:
Figure 13 shows the
apparatus used for Paper chromatography. Paper chromatography works on the
basis of relative solubilities. In this, a base line is drawn using a pencil on
a cellulose paper. The dye or dyes are dropped on this base line. The paper is
hanged or adjusted in the solvent dish in such a way that the base line is
above the solvent. This prevents the dye from dissolving in the solvent
directly.
As the reaction
progresses, the solvent rises up the cellulose paper, by capillary action, and
in doing so, it dissolves the many pigments of the dye based on their relative
solubilities. Some pigments dissolve more than the others and travel up the
paper along with the solvent, others cause spoting on the way up the paper.
This difference in solubilities, enable paper chromatography to separate the
dye into its constituent pigments.
Figure 14: A typical
Chromatogram.
Figure 14 shows a
typical chromatogram. A, B, C and D are
reference dyes, broken down to their constituent pigments. Dye X is compared
with these references to deduce that X contains Dye D and Dye A, as their
constituent pigments spot the paper on the same distance from the starting
line.
Calculating Rf Value:
Rf Value of
a certain pigment in dye is given by:
It is a measure of how
much a pigment dissolves in a solvent.
Paper chromatography
is also used for colourless substances. Therefore, a locating agent such as
Ninhydrin is sprayed on the dried paper chromatogram to locate/mark the spots,
after chromatography.
Amino acids produce
purple spots when ninhydrin is sprayed on it.
Solved example for Paper Chromatography:
An analytic chemist
was investigating the presence of various pigments in a particular dye X. His
results are shown in the chromatogram below:
- Why is it necessary to keep the solvent level below the starting
line?
- The solvent level must be below the starting line
to ensure that the dye placed on the starting line does not dissolve in
the solvent reservoir. The dissolved dye in the reservoir would not be
able travel up the paper as required and the results of chromatography
would be incorrect.
- Which pigments are present is
Dye X?
- Pigment A and C are present
in dye X.
- Which pigments are absent is Dye X?
- Pigment B and D are absent in
dye X.
- Calculate the Rf value of Pigment D.
