Science Buddies: "Ask an Expert"

I need help on this experiment. I no about the retention factor what does it mean in respect to colors? Do washable pens just travel with the solvent because they are heterogeneous or is it because of the solvent being water? I know my experiment was to prove heterogenous verus homogenous based on whether they traveled with the water on the paper because of their attraction to the paper versus the solvent but I am confused somehow. You see I did permanent markers of several colors and they all just stayed at he base line and did not travel at all so their Rf value was 0 but the water soluble traveled all the way the distance of the solvent. Some of the water soluble did seem to have on a main color with a halo around it of another color and I didn't know if that meant it was really hetero or not and then I did several regular markers all in the same colors as the perm. and water soluble and they all separated out into several colors. I didn;t know where to measure that value at the top of the color, in the middle or do a range and take the average.
Thank you for your help

Hi Morgan,

Your experiment may not have worked because the ink in the permanent markers you used was not soluble in water. Please try your experiment again using an organic solvent, such as isopropanol or acetone. Or, switch to a set of washable marking pens. If there is no migration of the ink from the origin, you can't make any conclusions about your results.

Please let us know if these suggestions don't solve the problem.

Donna Hardy

I did as you suggested and I ran the experiment with acetone. The perm. black marker separated into like a gray then black and then pink at the top. The colors only magrated half way up the paper but I guess that is ok.

Anyway I had some other questions that were not answered the last time I wrote. On some of the regular markers that I did with water the colors separated out but in a very long stretch of the paper. Do I measure the Rf value at the first sign of a change in color or do I measure at the top or do I measure the top and bottom and make a range or take the average of the two numbers? Does it really matter? I was really only looking to see if the markers were homo or heterogenous and I can tell that just by seeing if they separated out into different colors, can't I?

Oh one more thing on the water solvent markers the markers traveled all the way to the top of the paper and then looked like one color with the halo of another color around it. Does qualify for two colors I thought yes but I want to be sure.

You see I really wanted to see if there was a pattern or anything I could tell from doing muliple experiemnts so I used red, blue green and black markers all in perm., regular, and water soluble types to compare the difference. It was fun but a little overboard I guess.

Thanks for all your help.
I will be waiting for an answer., We didn't have school today so I am trying to finish up.

Morgan

Hi Morgan,

Congratulations! It sounds like your results are much better now. It's nice to have some results to write about. Here are answers to your questions:

1. For the Rf value, you should measure to the top of the color line. It's important to do this the same way every time so you can compare results from run-to-run. But, yes, if you see more than one color band, you know the ink is heterogeneous.

2. On the water makers with two colors at the top, you can see the two colors, but you didn't separate them. So it's probably best to say that it looks like there are two colors, but results are not conclusive. You would need to additional experiments to separate them and this would require finding a different solvent or running the separation with a longer piece of paper. There's nothing wrong with having inconclusive results, especially if you explain what you might do to solve the problem.

3. How are you going to present your results? Be sure to use your Rf values to compare the different inks.

Do you have any other questions on your write-up?

Donna Hardy

How do I use the rf values to compare inks? Is there a value for colors?

It is esy for me to say that the markers are heterogenous because most of them separated out different colors except red didn't separate well for any of the markers. It traveled up the paper but didn't really separate nicely. Why is that?
So if the question in my problem is about the markers being hetero vs. homo why do I need the rf values?

morgan marin wrote:Some of the water soluble did seem to have on a main color with a halo around it of another color

For this scenario, I think it's fair to say that you have two colors in the original ink, though, if you want to explore the issue in depth, you could offer alternative explanations in your write-up. For example, some dyes react under light (or in the presence of other chemicals) to form different compounds. It is conceivable that the outer halo of color is a reaction product that was formed after the ink was added added to the paper and dissolved in the solvent.

morgan marin wrote:How do I use the rf values to compare inks? Is there a value for colors?

The Rf value will vary depending on the solvent and the paper, so there is no set-in-stone value for different dyes.

morgan marin wrote:. . .red didn't separate well for any of the markers. It traveled up the paper but didn't really separate nicely. Why is that?

Maybe those reds are homogeneous dyes with only one type of molecule? Do they all have the same Rf?

morgan marin wrote:So if the question in my problem is about the markers being hetero vs. homo why do I need the rf values?

Good question. I'd say you really don't need Rf to answer your main question. If you wanted to do some additional hypothesis testing, you could, for example, use the Rf values to see if the same compounds are used in different markers. For example, if those red bands have the same Rf, it would support the hypothesis that the colors are the same chemical compound. If the red bands have very different Rf values, that would suggest that they aren't the same chemical compound. Keep in mind, there are many chemicals used to produce colors:
http://www.ihcworld.com/_technical_tips ... _chart.htm

Hi Morgan,

1. The red dye may be an impure compound; it may contain a number of closely related molecules that are all red, but which separate slightly differently on the paper because the molecular structure is slightly different. If the red dyes in the pens contained just one red molecule, it would probably travel as a distinct peak.

2. The size of the dye spots is related to the spot of ink you put on the origin. You will get the best separation by using the smallest possible dot at the origin. Did you control the size of the dye at the origin of the chromatogram? If not, this variable could provide an explanation for some of your results.

3. You have answered your question to satisfy the requirements of the science fair project. However, science fair judges will generally award a higher number of points if the results are quantitative. Since you have the chromatograms, you can calculate the Rf values and then compare results with different sources of dye. For example, if all of the red dyes have the same Rf value, you might conclude that the red pens all have the same dye. You can't compare the acetone and water chromatograms, only the ones that are run with identical conditions. But I am suggesting that you do this because it will improve your project.


I hope this helps! Don't forget to include a bibliography on your board.

Donna Hardy