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, 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.
Procedure:
1. Using a coffee filter, make 9 horizontal lines, numbered 1 – 9, approximately 1cm from the bottom of the filter.
2. Place a small spot of ink on each line according to the data table.
3. Pour rubbing alcohol in a pie pan until the bottom is completely covered. Be sure the solvent doesn’t cover the 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 substances with their 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.
Conclusions:
1. Calculate the Rf values for each substance.
2. The solvent, rubbing alcohol, is very polar. Based on your evidence, which color is the most polar? Why?
3. Would the same substances that dissolve in alcohol dissolve in water? Why?
4. What does this lab tell you about the polarity of graphite? How do you know?
5. What could you do to improve the separation of the colors?
6. 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?
7. Is all of permanent marker “permanent”? Why?
8. What does a large Rf value tell you? What does a small Rf value tell you?
9. Which dyes would dissolve in hexane, a nonpolar solvent?