Chemistry Lab
Paper Chromatography
Pre-Lab Discussion
It is often desirable to separate mixtures into their components. Sometimes this
separation is easy, as in boiling saltwater to separate the salt from the water. Some
mixtures are more difficult to separate, so chemists have devised a variety of techniques
to accomplish their goal. One important separation technique is called chromatography.
There are many types of chromatography, but the easiest and most common type is paper
chromatography.
Chromatography takes advantage of the fact that each substance has a unique solubility in
a particular solvent. Consider, for example, if I put a spot of black ink at the bottom of a
strip of paper. The ink is held to the paper by adhesion forces that are determined by the
type of paper used and by the nature of the ink molecules. A different type of ink would
be held to the paper by different adhesion forces. Now let’s dip the bottom edge of the
paper into a solvent. The solvent will rise up the strip of paper by capillary action. As
the solvent moves past the ink dot, the ink molecules will partially dissolve in the solvent.
There is a tug-of-war between the adhesion forces trying to hold the ink in place and the
solvent trying to drag the ink up the paper with it. Here’s the important part – the relative
strengths of the forces determines how rapidly the ink moves up the paper with the
solvent. Some inks will dissolve readily and move up rapidly with the solvent; others
will be more strongly attracted to the paper and lag far behind. Thus, if my spot is made
of a mixture of inks, chromatography can separate the mixture into its components. We
will get colored dots of different inks spread out along the strip of paper.
How far the ink moves depends on the type of paper, the specific ink used, the solvent
used, and how far the solvent traveled up the paper. The relative locations of the ink dots
are often measured by a Rf value. Rf for a dot can be found by
Rf = ddot / dsolvent
where ddot is the distance the dot moved from its starting point and dsolvent is the distance
the solvent moved. By finding Rf values of known inks, we can then identify unknown
inks by their experimental Rf values.
Objective
The objective of this lab is to separate the components of inks and find the Rf value for
each component.
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Chemistry Lab
Paper Chromatography
Methodology
1. Cut a few strips of chromatography paper. Wrap one end around a pencil and allow it
to hang from the pencil into a beaker. The paper should hang so that it just barely
misses the bottom of the beaker.
2. Select a solvent and pour a small amount into the beaker. There should be just
enough solvent to cover the bottom of the beaker.
3. Using a dropper, place a small dot of ink onto the bottom of the chromatography
paper strip. If the dot is not dark enough, wait until it dries and apply a second coat
on top of the first. Make sure the dot is far enough from the bottom that it will not
touch the solvent when placed in the beaker.
4. Mark the original location of the dot by drawing a line across the paper with a pencil.
Also, label the top of the paper with the name of the solvent and the type of ink used
to make the dot.
5. Place the paper gently into the beaker and cover carefully with a piece of parafilm.
Wait a few minutes for the solvent to rise up along the surface of the paper. When it
has reached a point near the top, remove the paper from the beaker and immediately
draw a line in pencil showing the edge of the solvent front.
6. Repeat with a different ink sample. Feel free to try things like Sharpies, your pens,
and so on. Try each ink with a variety of solvents and determine which solvent
would work best for separating a mixture.
7. Obtain a mixture of inks and use your chosen solvent to separate it.
Analysis
Calculate Rf values for each solvent-ink combination. Use these values to discuss your
choice of solvent for separating the mixture. Identify the components in the mixture and
calculate the Rf value for each component. Calculate a relative error for each Rf value,
assuming that the original values for the unmixed dyes are the theoretical values.
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