INTRODUCTION:
Chromatography is a technique used widely in chemistry and in the biological sciences for: 1) separating mixtures of compounds, and 2) to identify unknown substances. All types of chromatography employ two different immiscible phases in contact with each other. One of the phases is moving, the mobile phase, and the other is not, the stationary phase. In paper chromatography, for example, a solvent moves from one end of a piece of paper to the other end, as the paper absorbs it. The solvent is the mobile phase because it is moving, and the paper is the stationary phase.
A mixture of chemicals can be separated using paper chromatography. A small amount of mixture to be separated is placed near the edge of an absorbant paper. That edge of the paper is wetted with solvent. The solvent travels up the paper by capillary action, carrying the mixture with it. Separation occurs because different chemicals in the mixture travel different distances, depending upon whether they have a stronger attraction for the mobile phase, the solvent, or for the stationary phase, the paper. When the solvent has moved the entire length of the paper, the paper is removed from the solvent and dried. Once developed, the paper, called a chromatogram, will contain different chemicals located at different positions on the paper. The color and location of each compound can be used as a basis for identification because it can be matched with the color and location of known compounds subjected to the same conditions.
In this lab you will use the technique of paper chromatography to separate the colored dyes in felt-tipped pens, and state why you get the results you obtain.
MATERIALS:
PROCEDURE:
1. Cut a piece of filter paper so that the bottom edge is straight and the paper will fit into a battery jar without the sides of the paper contacting the jar.
2. Make the filter paper just tall enough so that the paper can be suspended from a pencil or ruler placed atop the jar, and the bottom of the paper stops just short of touching the bottom of the jar.
3. Make a horizontal pencil line about 2 cm from the bottom of the filter paper. Place light pin points of ink from the wet erase markers about one to two centimeters apart on the pencil line of the filter paper.
4. Repeat this procedure for a second piece of filter paper on which dots of permanent marker ink are placed.
5. Add distilled water to the battery jar to a depth of about 1 cm.
6. Carefully place the two filter papers with the marker ink dots, secured with tape to a pencil or ruler, in place into the jar with lower edges just barely submerged in the water. Be careful that the dots are never in direct contact with the water.
7. Allow the solvent to move up the paper (elute). This will take about 15 minutes. Be sure to remove your chromatography paper from the solvent once it has reached one to one half cm from the top. Do not allow the solvent to reach all the way to the top of the filter paper.
8. Allow the chromatograms to dry. When dry, you will staple them to the back of your group's lab report to turn in.
9. Rinse and dry the battery jars and return them to the front table. Clean up your lab stations thoroughly.
CALCULATIONS:
In chromatograpy, the symbol Rf is used to denote the position of a component on a chromatogram relative to the distance the solvent moved. This Rf value is a quantitative reflection of the physical interaction of each component with the mobile phase (the solvent) and the stationary phase (the paper).
Measure the distance from the origin of the marker dots (the pencil line) to the center of the eluted marker ink. Divide that number by the distance the solvent moved, ie. from the origin to the solvent front. The best way to estimate the center of a non-circular spot is to draw the best elllipse around the spot and estimate its center. Create a data table for Rf values on each of your inks tested. Clearly indicate whether the ink was from a wet erase pen or a permantent marker.
CONCLUSIONS AND QUESTIONS FOR ANALYSIS
1. What happens to the ink spots as the chromatogram develops? Why?
2. If one dye moves more rapidly than another, which one dissolves more readily in the moving solvent?
3. Which of the ink dyes that you used are probably made up of only one compound? How can you tell? Which are made up of more than one compound? Why?
4. How can you tell if two different brands of pens use the same combination of dyes to make black, blue, or another color?
LABORATORY ERROR: Discuss any possible sources of error in this laboratory investigation.
FURTHER INVESTIGATIONS:
Design further experiments whereby you would try to obtain a more informative chromatogram of the permanent marker pens. What procedural or material changes could you make?
Design a procedure whereby you possibly could identify the dye types used in the marker inks tested.