Science Buddies: "Ask an Expert"

I am helping my 2nd grader do the Make Your Own Markers Using Natural Pigments From Plants project. Part of the project is using chromatography paper to compare the dye from plants and a marker. We have done this experiment over and over and can't seem to get the colors pigments to separate out. Instead the dot of dye travels to the top of the strip of paper along with the water. The dot of plant dye looks the same as the marker dye. I've seen all the pictures of how the dyes should separate out and ours doesn't change color at all, it just travels up the strip of paper. We have followed the instructions exactly. I also tried it with 100% acetone and rubbing alcohol. When I did it with acetone and alcohol the color didn't separate out either. It just stayed in the same spot and snugged a little.We even tried it with food coloring and marker just to see if we could get any different result and we couldn't. Please help!!

Hello popstarholli!

Chromatography gives you results based on a property of dye molecules called polarity-- simply put, how unbalanced its electrons are. If the polarity of a molecule is close to the polarity of your solvent (like water, or rubbing alcohol), you will see a spot close to the top, and if their polarities are different, the material will stay closer to the bottom. Water is very polar-- acetone and rubbing alcohol are a little less so. So in this case my best guess is that the marker dye and your plant dye are also polar, and thus are traveling all the way up the strip of paper with the water.

As a first instinct, you could try mixing water and rubbing alcohol. Since the dyes moved all the way along the paper in water, and didn't move at all in rubbing alcohol or acetone, using a fifty-fifty mixture of the two might give you a good separation. The mixture can also be adjusted from here: adding more alcohol or water is definitely an option if you see that the chromatography isn't working.

I hope this makes sense; please don't hesitate to ask if I was unclear about something or you have additional questions!