AP Chemistry Lab
Name ________________
Paper Chromatography
Introduction: Chromatography is a means of separating mixtures of substances that are in the same phase.
There are many types of chromatography including gas, high pressure liquid, thin film and paper. The
basic principle involved is the fact that when forced to travel some distance through for example a matrix
of paper fibers, the larger, heavier molecules travel more slowly than the lighter ones and are thus separated
from each other. It is a very common technique for chemical analysis.
Purpose: In this lab, you will perform paper chromatography on a number of food colorings. These dyes
are used in a great many consumer food products. You are to find out which dyes contain a single color,
which are mixtures, and how many different individual dyes are present in the samples. You are also to
identify the individual dye components of any mixtures, using color and Rf values to support your
determinations.
Materials:
2 pieces of chromatography paper
2 plastic cups
3 Toothpicks
6 FD&C food dyes
flimsy
Ruler
Method:
1. Wash your hands with soap and water and dry thoroughly. Handle the papers by the edges as
much as possible as skin oils and moisture can adversely affect the experiment.
2. Obtain a piece of filter paper.
3. Obtain a flimsy and place 1 drop of each dye on it.
4. Using one end of a toothpick, dip the wood in to the dye and use it to put a spot of dye on the
intersection of the two pencil lines.
5. Allow the spot to dry and repeat the procedure once more. The spot should be fairly small and
concentrated when you finish.
6. Repeat for all of the dyes and record the colors corresponding to the numbers 1-6.
7. Use a second piece of paper and test 6 different vis-à-vis colors.
8. Roll the paper into a cylinder and staple it together, without overlapping the edges of the paper.
9. Barely cover the bottom of the plastic cup with water.
10. Place the cylinder of paper in the cup and make sure the dye spot remains above the water level.
11. Allow the strips to develop for 10 minutes or until the solvent front nears the top of the paper.
12. When the solvent front has nearly reached the top, remove the strips from the chamber and mark the
solvent front with a pencil.
13. Place the strips on a paper towel and allow them to dry.
14. Using a ruler, measure the distance that the water solvent traveled from the line at the bottom of the
strip to the highest point reached by the solvent front, also marked in pencil near the top of the strip.
This distance will be labeled as Dsol.
15. Measure how far each dye in the sample moved from the pencil line at the bottom of the strip. If
the dye color is a smear, use your judgment as to which part of the colored area you will measure.
BE CONSISTENT. This distance will be labeled as Ddye.
16. Clean the flimsy and paper cup. The dyes are sink safe and may be washed down the drain. Save
the strips to include in your report.
Calculations:
An important value calculated in chromatography experiments is Rf. It is a ratio of how far the substance
in the mixture traveled versus how far the solvent traveled. It is easily calculated as:
Rf = Ddye/Dsol
Rf value is inversely proportional to the molecular mass. Larger molecules have smaller Rf values and vice
versa.
Show at least one sample calculation of Rf for one of the dyes.
Report the Rf values for each dye measured in the data/calculations table.
Data/Calculated Data
FD&C
Food Dye
1
Rf Value
2
3
4
5
6
Vis a Vis
Color
1
Dye 1
Color
Dye 1 Rf
Dye 2
Color
Dye 2 Rf
Dye 2
Color
Dye 3 Rf
2
3
4
5
6
Questions for Analysis
1. Show a sample calculation to calculate the Rf value for 1 FD&C dye molecule. Rank the different
FD&C dye molecules (colors) from lightest to heaviest.
2. Which Vis-à-vis marker colorings are mixtures?
3. Compare the dyes used in each Vis-à-vis color to the FD&C food dyes. Which dye molecules can
you identify as an FD&C food dye.
4. Did the chromatography process separate the dye mixtures adequately? Explain why or why not.