Paper Chromatography Experiment
Process Objectives
· To observe the separation of a mixture by the method of paper chromatography.
· To infer the identity of the unknown inks by comparing their chromatograms to those of the known inks.
Learning Objectives
· To gain experience with chromatography-one of the most widely used laboratory techniques of the analytical chemist.
· To separate the colors of different inks.
Introduction
Paper chromatography is a method of separating mixtures by using a piece of absorbent paper. In this process, the solution to be separated is placed on a piece of dry filter paper. This is the stationary phase. A solvent (the moving phase) is allowed to travel across the paper by capillary action. As the solvent front moves, the components of the mixture separate. The components of the mixture that are most soluble in the solvent and least attracted to the paper travel the farthest.
In this experiment you will place a small spot of black, water-soluble ink near the center of a piece of filter paper. By means of a wick, water will be drawn to the center of the filter paper and will then spread outward toward its edge. The colored molecules that make up the black ink mixture will be distributed by the solvent along a path to the edge of the paper. Because each different brand of black ink is a unique mixture of colored molecules, each pattern on the paper, or chromatogram, is characteristic of the brand of pen used.
Chromatography is an important tool of the forensic chemist in solving crimes. This method can help identify the brand of pen used to write a ransom note. Using chromatography, ink manufacturers can quickly determine if a competitor has stolen their "secret" formula.
Safety
Take the necessary precautions before beginning this experiment. Wear safety goggles, apron, and gloves. Read all safety cautions in your procedures and discuss them with your teacher. It is important to use good safety techniques while conducting experiments. See pages 8 through II.
Apparatus
filter paper, 12 cm
six numbered pens, each with a different water-soluble black ink
filter paper wick, 2-cm equilateral triangle
petri dish
beaker, 250 mL
filter paper with four spots of unknown black ink.
Recording Your Observations
Where instructed in the procedures, record your observations in the Data Table.
Procedures Known
1. Use a pencil to sketch a circle about the size of a quarter in the center of the piece of filter paper. Write the numbers one through six around the inside of this circle. See Figure 4-1.
2. On the circle beside the number 1, use black pen number 1 to make a large dot. Use black pen number 2 to make a dot beside number 2, and repeat this procedure of placing dots from each pen beside its corresponding number on the filter paper until all six dots are placed.
3. Use the pencil to poke a small hole in the center of the piece of filter paper. Insert a rolled-up piece of wick through the hole.
4. Fill the petri dish to the halfway point with water. Set the wick of the filterpaper into this water and wait for the chromatogram to develop. See Figure 4-2.
5. After 15 minutes, or when the water is about I cm from the outside edge of the paper, remove the paper from the petri dish and allow the chromatogram to dry. A sample
chromatogram is shown in Figure 4-3. Record the colors that have separated from each of the six different black inks in the Data table. You may want to use colored pencils to 
record this information.
6. Ask your instructor for a piece of filter paper with four black dots of unknown ink. See Figure 4-4. Repeat steps 3 through 5.
7. Record the colors that separated from each of the four different black inks in the Data 'Mble. You may want to use colored pencils to record these patterns. Then, use this information to identify the pens that made the four
Questions
1. Some components of ink are minimally attracted to the stationary phase and very soluble in the solvent. Where are these components located on the filter paper during chromatography?
2. What can be said about the properties of component ink that travels only half of the distance to the final solvent front?
3. Predict the results of forgetting to remove the chromatogram from the water in the petri dish until the next day.
Data Table
Record of Chromatogram on Filter
|
Pen No. |
Dot No. |
Center |
Middle |
Edge |
|
1 |
1 |
|
|
|
|
2 |
2 |
|
|
|
|
3 |
3 |
|
|
|
|
4 |
4 |
|
|
|
|
5 |
5 |
|
|
|
|
6 |
6 |
|
|
|
|
No. of Unknown Pen |
Dot No. |
Center |
Middle |
Edge |
|
|
7 |
|
|
|
|
|
8 |
|
|
|
|
|
9 |
|
|
|
|
|
10 |
|
|
|
General Conclusions
1. Investigators at the scene of a crime strongly suspect that a hand-written promissory note for $10,000.00 has been altered to read $40,000.00. How might this be proven?