Science with Dr Bob - Chromatography

Science with Dr Bob segment, WABI TV5, 24 August 2017

Dr Bob explains an important but complex scientific technique – chromatography – to WABI TV’s Joy Hallowell and Wayne Harvey in an engaging and visually interesting way. 

How do we separate molecules when they are too small to see?

     Although molecules are too small to see or touch individually, we can see groups of them when the molecules are colorful. Food coloring is made of just such molecules. Dye molecules and food coloring molecules can be separated using a technique scientists use to purify other compounds like drugs. This separation technique, called chromatography, is easy to do at home and produces some brilliant results. 

     In today’s segment, we use chromatography to see the recipe of food coloring molecules that distinguishes regular colors from their neon counterparts.  

How to do the experiment:

  The kind of chromatography we did today is called "paper chromatography" because we used paper as the solid material that does the separating. For this experiment, you will need a bit of absorbent thick paper, a plate, some water, and water soluble (that is, washable) felt tipped pens. Although I used laboratory filter paper circles today, I’ve done the experiment with thick paper towels and they work great too.   

     Set out a shallow plate with a small puddle of water on it. Dot some dots of water-soluble ink from washable felt tip markers in a line on the paper towel about a half inch from the bottom edge of the paper. Dip the end of the paper towel into the water and – here’s the trick – hold the paper vertical vertical so only its edge near the dots sits in the water. The water must only get to the dots by running up through paper. If the water is deep enough so the dots touch the puddle, the ink will run down into the puddle rather than up into the paper.  

     When the water reaches the drops of ink, it will pick up the ink and carry it up into the paper. The distance that the molecules go depends on the amount of time the paper towel soaks up water. The speed with which the individual colors move in the paper is the reason for the separation. Molecules that are small will tend to run faster and move ahead of bigger molecules, and molecules that are more sticky for the paper will tend to run more slowly and fall behind less adherent molecules. Try different colors (browns work particularly well) and try different solvents (rubbing alcohol from the drug store will even dissolve and separate permanent Sharpie marker a bit!) If you use food coloring, see if the number of colors on the ingredient list on the box matches the number of colored spots that seem to separate from each other. Have fun experimenting!