Food dye chromatography

[fmk]

My daughter is doing the candy chromatography experiment. We are running into a problem with measuring the Rf factor of the commercial food color dyes. The level of the dye is always the same as the solvent, resulting in Rf value of 1 or near 1. We know this can't be right We are getting good results with the dyes from the candies themselves. But using the commercial food dyes- and we have tried red, blue, green dyes, in a salt solution or just using a plain water- the dye always moves up the strip at an equal rate to the solvent and always reaches the same distance. We've even tried purchasing our own food coloring instead of using the dyes that came with the Science Buddies kit, to no avail. Any ideas of what we can do to change our results would be much appreciated.

[kgudger]

Hello and welcome to the forums

I haven't done this experiment before, but in reading it I had a question. For each chromatography strip, it says:

Let the solvent rise up the strip (by capillary action) until it is about 0.5 cm from the top then remove the strip from the solvent. Keep a close eye on your chromatography strip and the solvent front— if you let it run too long the dye may run off the paper and become distorted.

Is it possible that the food coloring strips moved so fast that you missed when the solvent got to the top? Each strip will have a different time for the solvent, so you can't just run multiple strips at once and assume that they all end at the same time.

Other than that, the experiment also says:

Note: It is possible that other components in the candies may affect how well the food coloring dyes travel through the paper. Why do you think this might be? (Hint: Think about solubility and re-read this part of the Introduction.) If you have unexpected results, do you think this might help explain them?

Please let us know what you find out.
Best, Keith

[fmk]

Hi, Keith,

Thanks for your quick response, and on a holiday, too! Regarding your suggestions, I'm pretty sure It's not the case that the dyes moved too fast to get to the top. We watched them pretty closely. From the time the strips were placed in the solvent, the dyes were always at the same level as the solvent on the strip, and they moved up together. Also, the problem wasn't in the candy dyes, only in the food color dyes, so I don't think it's additional ingredients in the candies. The candy dyes seem to get good results. It's the food color dyes that consistently give Rf values of 1. Any other ideas?

[deleted-71487]

Hmmm... I'm stumped on that one. It should work. I assume you followed the procedure that the strips barely be touching the salt solution? If the strips were, as you say, "placed in the solvent", it might be that too much water is soaking into the strips too fast. The solvent has to move pretty slowly through the strip for this to work very well (at least that's what little I remember from doing chromatography when I was a kid... some 40 years ago .