Paper Chromatography Lab
Purpose:
-
To determine the polarity of various dyes bases on Rf values
To determine the effect of solvent composition on retention of dyes by varying the solvent used for the
separation
Background Information:
Molecules with similar arrangements of their atoms or molecular structures are attracted to each other. Water
(H2O) molecules have the structure shown below in which the two hydrogen atoms form a 104° angle with the
oxygen at the vertex. Because of this structure the oxygen end of the molecule has a small negative electrical
charge and the hydrogen end has a small positive charge. Liquid water is held together by the attraction between
the charges on adjacent water molecules.
A molecule with these charged regions is called a polar molecule. Methanol (CH3OH) has a similar structure
(see below), and the methanol molecules are very soluble in water because of the mutual attraction between the
two polar molecules.
A more complex, yet still similar molecule is cellulose, a molecule which is the basic component of paper. It is
a very long molecule (a polymer) in which thousands of rings of six atoms each are linked together like beads.
A portion of a cellulose molecule is shown below.
The polar -OH regions of these molecules are attracted to the -OH groups on adjacent cellulose chains helping
to hold the fibers together in paper. Not surprisingly, water molecules, being polar, are also attracted to these
regions and when paper is wet it loses strength because the water molecules get between the cellulose chains
and weaken the attraction between them.
When the end of a piece of paper is dipped into water, the water molecules keep finding new places (polar
regions) to stick to and so the water molecules climb up the paper being replaced by new water molecules
below. Other molecules which might be dissolved in the water will also be carried along up the paper. This is
applied to the separation of dyes in a technique known as paper chromatography. If a spot of dye is placed on
the paper above the level of the water and the water moves up, it will carry with it the dye molecules if they are
more strongly attracted to the water molecules than to the paper molecules. If they are more strongly attracted to
the paper than to the water, they will move more slowly than the water or even not at all. What if the dye is a
mixture? If two or more dyes have been mixed to form an ink, then they may move at different rates as the
water moves up the paper. If this happens, they will separate and we can identify them by their colors. This is
shown in the drawings below. In this example, a small spot of green ink was chromatographed and separated
into the yellow and cyan dyes which were mixed to make the ink.
A very good way to compare dyes on different chromatograms is to measure the distance each dye moves
relative to the solvent. This is called the Rf value of the dye and it provides a way to judge whether two different
dyes are the same. If the Rf values are close and the dyes have the same color, than they are probably the same.
The Rf value is calculated by dividing the distance that the leading edge of the dye spot moved by the distance
that the solvent has moved. (see figure below) Since the dye cannot move any farther than the solvent moves,
the maximum value for the Rf is 1.0. This would be found if the dye did not stick to the cellulose molecules at
all. If it was strongly attracted to the cellulose then the Rf would be very small because the dye would move
very little compared to the solvent.
In the example above the yellow dye moved 5.8 cm and the solvent moved 8.5 cm. The Rf value for the yellow
dye is 0.72. Note that there are no units and there are 2 digits (significant figures) after the decimal point. We
can use these values to help answer several questions about the inks.
Procedure:
The inks to be used in this experiment are of two kinds: "washable" and "permanent.”
1. Using a coffee filter, make ten horizontal lines, numbered 1 – 10, approximately one centimeter from the
bottom of the filter.
2. Place a small spot of ink on each line according to the data table.
3. Pour approximately 100ml of rubbing alcohol in a pie pan, or until the bottom is completely covered.
Be sure the solvent does not cover the ink spot.
4. Allow the solvent to rise on the filter until it reaches the top crease. Remove and use a pencil to mark
the location of the solvent front or where the solvent stops on the filter. Suspend in air to dry.
5. Calculate the Rf value for each dye by measuring the distances traveled by the dye and the solvent using
the equation given in the Background for this experiment.
Data and Calculations:
Create a data table that illustrates all of the dyes or pigments, and their Rf value – you might have more than one
dye or pigment for each substance.
Calculations:
Show all of your calculations required for the Rf values and your conclusions.
Conclusions:
1.
2.
3.
4.
Calculate the Rf values for each substance.
What type of physical bonds are in rubbing alcohol?
Does rubbing alcohol dissolve in water? Why is this?
The solvent, rubbing alcohol, is very polar. Based on your evidence, which color is the most polar?
Why?
5. Would the same substances that dissolve in alcohol dissolve in water? Why?
6. Is this process chemical or physical? Why?
7. Suggest a way to change the Rf value without changing the solute, or the substance getting dissolved.
8. What does this lab tell you about the polarity of graphite? How do you know?
9. What could you do to improve the separation of the colors?
10. Are all of the colors the same? For example, is blue coloring in green food coloring the same as blue in
a marker? Why or why not?
11. Is all of permanent marker “permanent”? Why?
12. What does a large Rf value tell you? What does a small Rf value tell you?
13. Which dyes would dissolve in hexane, a nonpolar solvent?