Introduction to Chromatography

Introduction to Chromatography

Chromatography is a common technique used by most scientists to separate components of a mixture. The mixture is carefully applied to a medium (like paper, gel, or gas) and a solvent is then passed through it. Based upon solubility, the different components are dragged different distances, along with the solvent. This spreads them out and allows for identification. If two pigments travel the same distance, they have the same molecular structure and can be considered identical. It also leaves a distinct “fingerprint” for the mixture. Each component can be assigned a value based upon the distance that it travels. This is obtained by measuring the distance that the component travels, the distance that the solvent travels, and then dividing the first by the second value. This is called the “Rf” value, and it is a decimal less than or equal to 1. For example, a spot that moves halfway has a value of 0.5.

Purpose: The purpose of this lab is to become familiar with this common technique by doing paper chromatography on water-soluble markers.

Materials:

round filter paper

scissors

water, acetic acid, alcohol, or other solvents

water-soluble and permanent markers

cup or beaker

Procedure:

Part I

1.   Cut a piece of filter paper into 8 pie-shaped wedges.

2.   Roll one of these into a tight cone.

3.   Place a small hole in the middle of another piece of filter paper with the tip of a pair of scissors.

4.   Fold the paper into eighths and open it up.

5.   Apply tight spots of ink about 1 cm from the hole on each of the 8 lines. Samples should include:

1 permanent marker

2 pens that are the same color but are from a different manufacturer

5 of your own choice

6.   Number the spots 1-8 with a pencil along the outside edge of the paper.

7.   List the colors/ manufacturers in the table below:

Kind of Pen/Color

8.   Fill the beaker with water and wipe off the rim.

8. Insert the cone into the hole so that the point is sticking up fits tightly in the hole in the filter paper (called a chromatogram).

9. Carefully place the chromatogram on the beaker so that the cone is in the water.

10. Let the water run through the ink samples until it is about 1 cm from the edge.

11.   Remove the chromatogram, take the cone out and throw it away, and let the chromatogram dry.

Part II

1.   Choose a pen that showed two or more distinct colors on the chromatogram. Star the one that you choose in the table above.

2.   Cut a strip of filter paper about 3 cm wide and long enough to hang over the top of a beaker and into the solvent below.

3.   Place a spot from the pen that you chose around 1 cm from one edge and 1 cm from the bottom of the strip.

4.   Now place a spot from the permanent marker about 1 cm from the other edge and 1 cm from the bottom of the strip.

5.   Put a very small amount (1 cm) of a solvent other than water in your beaker. You may choose from several.

6.   Wipe the rim and place the strip so that the bottom edge barely touches the solvent and the top hooks over the top of the beaker or through a glass rod. Cover with plastic if the fumes are strong.

7.   Allow the solvent to run up the strip for at least as far as the solvent traveled on the round filter paper.

8.   Remove and dry, returning the solvent to the original container.

Analysis

1.   Calculate the Rf values for the two pens you ran on the strip.

Solvent Chosen:______

Questions:

1.   Was the ink in the marker that you chose for your strip an element, a compound or a mixture?

2.   Was it heterogeneous or homogeneous?

3.   Explain in your own words why the pigments in the markers separated.

4.   Look at the color pattern for the two pens that were the same color. Did the different manufacturers use any of the same pigments. Explain.

5.   Explain the behavior of the permanent marker in the water.

6.   Did the permanent marker behave differently in the second solvent? Describe.

7.   Were there any differences in the Rf values of the chosen pen in the two solvents? Was one better than the other, or just different?

8. Chromatography is often used at crime scenes. Name two mixtures that could be separated by chromatography which might help solve a crime.