Paper Chromatography

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Paper Chromatography

Chromatography is a method for analyzing complex mixtures (such as ink) by separating them into the chemicals from which they are made. Chromatography is used to separate and identify all sorts of substances in police work. Drugs from narcotics to aspirin can be identified in urine and blood samples, often with the aid of chromatography.

For a printable version of this project, click here.

Materials

• Paper coffee filters
• One black permanent pen
• Black water soluble pens
• Container full of water
• Several sheets of paper
• Small glasses or plastic containers
• Isopropyl rubbing alcohol*
• Pencils
• Tape
• Scissors
• Stapler

*Read and obey warnings on rubbing alcohol label.

Instructions

Part I - Separating Black Ink
1. Cut several coffee filters into long strips, one strip per pen.
2. Fold the end of each strip over then staple it to form a loop.
3. Place a dot of ink near the bottom of each strip. Use a pencil to identify which strip belongs to which pen.

Paper Chromatography Instructions 2

4. Poke a pencil through one of the loops you just made. Use the pencil to suspend the strip in a small glass or container.
5. Carefully add water to the glass until it reaches the bottom of the paper strip just below the ink dot. Be sure the ink stays above the water and the paper stays in the water.
6. Allow the water to soak up the strip and watch what happens to the ink drop.
7. If the ink you are testing does not spread out, re-test it using rubbing alcohol.
8. Repeat this process for each strip and compare your results.
9. Let the strips dry and tape them on a sheet of paper as a record of the different pen types.

Paper Chromatography Instructions 3

Part 2- Secret Note Challenge
1. Turn your back while someone uses one of the pens you just tested to write a secret note on a piece of coffee filter.
2. Cut out several individual letters from the note.
3. Staple each letter to the bottom of a strip of coffee filter.
4. Conduct the chromatography experiment above to determine which pen was used to write the secret note.

(Watch how the ink spreads up the paper. Compare it to your known samples of ink.)

What’s Happening

Because molecules in ink and other mixtures have different characteristics (such as size and solubility), they travel at different speeds when pulled along a piece of paper by a solvent (in this case, water). For example, black ink contains several colours. When the water flows through a word written in black, the molecules of each one of the colours behave differently, resulting in a sort of “rainbow” effect.
Many common inks are water soluble and spread apart into the component dyes using water as a solvent. If the ink you are testing does not spread out using water, it may be “permanent” ink. In such cases, you will have to use a different solvent such as rubbing alcohol.