Introducing paper chromatography

Celebrating the 90s - 50th Celebration of Chem 13 News

Reprint 1995

For several years, I have been using the following story to explain the ideas of chromatography to my grade 9 science classes. They seem to understand the concepts so well that it becomes a useful reference when we talk about solubility and intermolecular forces again in senior chemistry classes. I present it as a fable and embellish it with lots of spurious details. [Any gender inequities are presented as an unfortunate situation produced by the primitive social situation "back then".]

“Once upon a time in a kingdom far far away”

There was a king who, for military reasons, decided to marry off his daughter to the strongest knight in the kingdom. So he announced to his daughter that he would hold a tournament so the suitors could challenge one another in combat. The strongest knight was sure to win.

The princess would have none of it, saying that she didn't want to end up married to someone with some parts broken or missing! After much discussion, the princess agreed to the marriage, and the king allowed his daughter to design the test of strength that would produce the proper winner.

The day of the great contest, the princess ordered all of the suitors to gather in full armour, on the bridge over the river running through the town. Then, she ordered the serfs to cut down a few trees so that the trunks fell across the river from bank to bank.

[DRAW this on a chalkboard with the current flowing from the bottom to the top of the board.]

At her signal, all of the knights jumped into the river and were swept kicking and spluttering down the stream. Thankfully, they came to the first log and they all hung on. After a while, one by one, they began to lose their grip and drop off into the river again. Some were swept down to the second log and some to the third log, and so on...

[SHOW this using various colours of chalk to map the drift of each knight Everyone can root for their favorite.]

When there was only one last knight left on the first (or second) log, the contest was declared over and that knight won the hand of the princess.

"Just like the knights, coloured molecules cling to the paper with different forces of attraction. The molecules with the least attraction are swept along by solvent flowing up the paper. The strongest move the least distance in the time the chromatogram is running. Since every compound is a unique molecule, the forces will be different for each compound and the compounds will gradually separate on the paper."

The analogy is close enough to the theory of the situation that it gives a good basis for discussion, one that they tend to remember.

In grade 9, we follow it up that day by doing paper chromatograms of water-soluble markers, and of colouring from black jellybeans. Homework is to use coffee filter paper to analyze the ink in their highlighter pens and to create chromatog-art. The next day, I present a "robbery note" and three black markers. They use their chromatographing skills to find out which marker wrote the note.

As with most of the ideas we share, it's hard to remember which parts are borrowed and which wrinkles are original. These are gleaned from several sources and I hope they are useful to others.