Chromatography: Separating Mixtures

Chromatography: Separating Mixtures

Introduction:

Magic marker inks are often mixtures of several compounds. Paper chromatography is a common method of separating various components of a mixture. After separation, you can observe the different colors that make up a particular color of magic marker ink.

You can also calculate a ratio Rf, which compares how far each compound traveled to how far each solvent (substance that dissolves another substance) traveled during the experiment.

Rf = Ds/Df

Ds = Distance traveled by the compound

Df = Distance traveled by the solvent

Materials:

coffee filter, tape, rubbing alcohol, water, 3 different color magic markers (not permanent); 3 identical tall, narrow drinking glasses; metric ruler, 3 pencils

Pre-lab Questions:

1. List one extensive and one intensive property of marker ink.

2. Define mixture.

3. Is tap water a heterogeneous or homogenous mixture? Explain your answer.

4. Is rubbing alcohol a heterogeneous or homogenous mixture? Explain your answer.

Procedure:

1. Cut 3 strips of coffee filter. Be sure that the strips are much narrower and slightly taller than the drinking glasses.

2. In the center the strips, about 3 cm from one end, place a dot of the marker to be tested. The dots should be about 0.2 cm in diameter and dark enough to be clearly visible.

3. Place about 2 cm of water in each glass.

4. Tape the unmarked end of each strip to the center of a pencil so that the strip hangs down when the pencil is held horizontally.

5. Carefully insert the strips into the glasses, dotted end down. The dot must be above the water, and the sides of the coffee filters cannot touch the sides of the glasses.

6. Let the setup stand for 20 minutes. Follow procedure 8 as your setup stands.

7. Record your observations after 20 minutes in the table below. Measure Df (in cm) for the solvent. Measure the Ds (in cm) for each color that you see on the coffee filter (there may not be three). Calculate the Rf for each color using the equation in the Introduction section.

8. Repeat steps 1–7, except place about 2 cm of rubbing alcohol in each glass instead of water.

9. Clean up your work area.

Data:

Marker Color
Solvent: water / Df / Colors / Ds / Rf / Df / Colors / Ds / Rf / Df / Colors / Ds / Rf
Solvent: rubbing alcohol / Df / Colors / Ds / Rf / Df / Colors / Ds / Rf / Df / Colors / Ds / Rf

Post-Lab Questions:

5. What evidence is there that marker ink is a mixture?

6. Each compound in the marker ink is represented by a color. Did one compound travel farther than the others? Explain why you think that this is the case.

7. Did the compounds travel farther in the water or the rubbing alcohol? Explain why you think that this is the case.

8. How could you improve upon the accuracy of your Rf measurements?