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In school, we did a paper chromatography lab using different sharpie colors. I chose a pink sharpie color and while everyone else's pigments separated, my color just moved up and left a colorless space in between the line that was first drawn and where the solvent(Isopropyl alcohol) traveled. Why is this? Did I do something wrong?
Question Date: 2019-11-29
Answer 1:

I think you did everything correctly! Some inks are made of mixtures of pigments, while others are not. Dark inks like in black or brown sharpies contain the most different pigments, so they will show more bands of different colors when the inks separate. Other inks, usually lighter colors (like yellow, or pink as you have found) might only contain one type of pigment, so it won't separate further during the chromatography experiment.

Thank you!

Answer 2:

Paper chromatography separates mixtures into components by exploiting differences in the strength of attraction between the components of the mixture (the Sharpie ink), the mobile solvent (isopropyl alcohol), and the stationary phase (paper, but with a bit of water around the molecules). Components which are strongly attracted to the solvent are carried further by it, while those strongly attracted to the stationary phase (here, mostly the water around the cellulose molecules of the paper) do not move as much. Thus, the mixtures separate if they contain components which dissolve well in the mobile phase and others which are attracted to the stationary phase.

If your ink did not separate, there are a few possibilities. One is that the ink is not a mixture. If there is only a single component, then it cannot separate. Another is that the ink is a mixture, and that all of the components are essentially equally attracted to the solvent and the substrate, in which case they all traveled at the same rate. Running the same experiment with a different solvent may help to clarify which is the case.

Answer 3:

Your results sound fine. Your results say that there was only 1 pigment in the pink sharpie marker - or maybe more pigments that traveled together.

I'm impressed with the results you and the other students got. I have a paper chromatography on a paper towel, and I didn't get good separation for any of my markers. They all just traveled with the solvent line, with big smears below, and a couple left faint spots at the origin. I probably used 99% isopropyl alcohol for the solvent. I think I used were various kinds of colored markers.

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