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BACKGROUND

Professor Y. N. Howe arrived at work today and found a note taped to his door.  It read "Class is cancelled".  Professor Howe was confused - he had spent several hours the night before preparing a special exam for his class, and he couldn't think of any reason that his class would be cancelled - it wasn't snowing outside and there didn't appear to be any emergencies in the building.  "AHA!" he thought  "Somebody must be playing a trick on me, but I am going to find out Who did it! I have a feeling that one of my students didn't want to take the test today, but we can do an experiment to find out which pen was used to write this note.  I will make a Chromatograph with this note and then I will have the students do Chromatography with their pens during class.  By the end of class I may have a good idea of who wrote this note!"

In the center of your table are laminated samples of the chromatograph which Professor Y. N. Howe got from the note he found on his door this morning.  Using an assortment of pens on the table, please help him find out which pen was used to write the note!
 

GETTING READY

"Samples" of  chromatographs from Dr. Y. N. Howe's note need to be prepared ahead of time.   Using a black washable marker (NOTE:  we find the black Vis-à-Vis® overhead projection pens have an excellent separation), draw a large dot or write a short word (e.g., "is") 2 cm from the bottom of a 5-cm x 12-cm strip of filter paper.  Follow Steps 4-5 below.  After about 5 minutes, when the colors have separated well on the filter paper, remove the paper from the cup and allow it to dry.  Label it "Dr. Y. N. Howe's Chromatograph, and if possible, laminate it.
 

PERFORMING THE EXPERIMENT

1. Obtain a 5-cm x 12-cm (2-inch x 4 3/4-inch) strip of filter paper.

 
2. Obtain three different colors of washable markers.  Choose colors which you think may be composed of  the colors Dr. Y. N. Howe obtained on his chromatograph..  (Hint:  Try at least one dark color.) 

 
3. Make three dots (one dot of each color) in a line, all 1.5 cm (1/2 inch) from one end of the filter paper and from each other as shown in the sketch.  This is the bottom end of the filter paper.  Make three dots using the same colors and same order about 1.5cm (1/2  inch) from the other end as shown in the sketch.  This is the top end of the filter paper. 

 
4. Push a short wooden skewer through the center of the top end of the filter paper about 1 cm (less than 1/2 inch) from the top.

 
5. Squeeze enough water from a squeeze bottle into the plastic cup to fill it just to the line less than 1 cm from the bottom of the cup. 

 
6. Suspend the wooden skewer across the mouth of the cup to hang the filter paper in the cup so that it just touches the surface of the water in the cup.  The colored dots should be initially at least 1 cm above the surface of the water.

 
7. Let the experiment stand 5 minutes, and observe any changes.  Compare your chromatograph with the sample of Dr. Howe's chromatograph which is on the table. 

 
8. If none of the inks on your chromatograph match Dr. Y. N. Howe's, repeat steps 1-7 with different pens. 

 
9. When you think you have discovered the color and type of pen which the prankster used, check your answer with the presenter.

 
10. Remove the filter paper, and let it dry.  Leave the skewer and plastic cup for the next person.  You may take the filter paper home with you. 

 

1.  How might a crime laboratory use chromatography to identify ink used to write a ransom note?

2.  How could you show that washable black ink is a mixture of a number of colors?

3.  Yellow food coloring #5 causes allergic reactions in some people.  How might you determine whether yellow, green, orange, or brown M & M®´s contained Yellow food coloring #5?
 

EXPLANATION

Washable pens are used in this activity because the pigments in the inks are soluble in water.  Inks may appear to be a single color but are often composed of differently colored pigments.  Water can dissolve these pigments to effect a separation and produce a decorative pattern.  The filter paper functions as a stationary phase on which the separation takes place whereas the water functions as a mobile phase to carry the pigments through the filter paper.  Pigments which are more soluble in the water and attracted more strongly to the water and less strongly to the filter paper move through the filter paper at a faster rate than those which are less soluble.  Although colors of different manufacturers may appear to be the same, often different pigments and different ratios of pigments are used to produce the same color.  Thus, it is possible to tell what type of pen was used.  However, without further testing, it would not be possible to tell which particular pen was used.
 

EQUIPMENT NEEDED

12-oz plastic tumblers [cups about 11 cm (4 1/2 inches) tall]; 5-cm x 12-cm (2-inch x 4 3/4-inch) strips of white filter paper; short wooden skewers (for hors d´oeuvres); centimeter rulers; black Vis-à-Vis® overhead projection marker; an assortment of washable colored pens; squeeze bottles of water; paper towels.
 

REFERENCE

Modified from Mickey Sarquis and Jerry Sarquis, Fun with Chemistry: A Guidebook of K-12 Activities, Volume 2, Institute of Chemical Education, University of Wisconsin - Madison, Madison, WI, 1993, pages 3-5.
 

Kids Involved Doing Science is at http://www.kids.union.edu          Modified 04/25/02  by C. & P. Scaife