Chromatography Forum

For her Science Fair project my granddaughter is trying to use Paper Chromatography to analyze the colors in crayons. She has not been able to identify a liquid that will dissolve the crayon and separate the molecules. She has actually tried several different types of solvents (acetone, ammonia, bleach, alcohol, Rapid Remover (an adhesive remover), DX330 (a wax and grease remover), lacquer thinner, Goof Off, and turpentine) with no success.

We believe it is the paraffin that is resisting us, but we don't know what else to try. Any help or suggestions would be greatly appreciated.

Thank you very much.

Stephanie

Gasoline?

Her dad did try gasoline, along with WD-40 and tire cleaner. Obviously thinking like a guy. Everything we've tried has been full strength, and we've let the samples set for 15 minutes. I can't believe it's not possible, just running out of ideas.

Thanks.

Try xylene, toluene, or methyl ethyl ketone (MEK). I saw an article (http://www.informaworld.com/smpp/conten ... 570&db=all) that used a 60:20:20 MEK:Benzene:Toluene mix to dissolve parrafin, but you're probably not going to be using benzene, as it is expensive, and carcinogenic, and unavailable. I saw another article that used an acetone:toluene mix, so you can try that, but keep the toluene ratio high (3:1). Try pure solvents first, it will be easier that way.

Toluene or MEK are my bets - might try those first. Oh, and hexane or dichloromethane if you can get any.

Oh, and you're probably already doing this, but you should probably break up the crayons into small pieces to make dissolution easier.

Oh, and make sure you're doing all of this in a very well ventilated area.

I've used chloroform or chloroform:methanol to dissolve the wax that was coating apples.

I don't want to send this in too much of a different direction, as I'm sure she's put a lot of work into this already, but it would be A LOT easier to do something like various colors of Sharpie markers than trying to work with crayons.

Good luck.

Yea, but where's the challenge for an eleven year old to get sharpie all over the place? It's much cooler to watch a crayon dissolve in some stinky solvent.

Here's a thought - IF she gets the crayon dissolved... then she's got a solvent/pigment/paraffin mixture. How does she separate the pigment from the paraffin? Add water to the mix, see which layer the pigment stays in?

http://www.madehow.com/Volume-2/Crayon.html

This experiment is only half of her project... her question is how do the colors in crayons compare to how they taste? (What can I say; she's an interesting child.) She has already done the taste test with a group of kids, so she's stuck with crayons.

The method she is using is to cut coffee filter paper into narrow strips, smear crayon about 1-1/2" from the end, and suspend the strip in a container with the end in the solvent. The paper wicks the solvent up, and is supposed to dissolve the crayon and carry the pigment with it, separating into the different colors. We've tested the procedure with marker, so we know it works... just not with crayon!

We've been trying to stick with readily available products, so maybe next we should try nail polish remover that has ethyl acetate in it instead of acetone.

Is it possible that the solvent is moving too fast? Everything we've used has traveled past the crayon mark within 2 to 3 minutes. And how would you slow it down?

I appreciate the advice and links very much. I now know more about crayon production and the nature of solvents than I did before!

Well, xylene, toluene and MEK are readily available at hardware or paint stores. And a taste test? That is definitely interesting. Maybe not so safe, but interesting.

I think, because the pigments start as a dry material mixed into paraffin and homogenized, and the solvents she's using are too polar to dissolve the paraffin and separate the pigments, that she might have better luck doing this a different way - separate the pigment from the paraffin first, then spread a water/pigment mix on the filter paper and dip it into the solvents she has been using.

A standard technique in an organic extraction lab is to use a non-polar solvent like hexane or dichloromethane to extract non-polar compounds from water, so I think I'm proposing the reverse - use a non-polar solvent to dissolve the non-polar paraffin, then use a polar solvent like water (or maybe rubbing alcohol?) to extract the pigments from the paraffin. Then she'll have a pigment slurry, and she can do the TLC experiment more easily.

I do understand this is a science fair project, and what I've suggested might be too involved, so you might have some luck trying the solvents I suggested (I used to have xylene and MEK in my garage as a kid, they aren't too 'exotic') to do the TLC experiment. She could try thicker paper than the filter paper - that would slow the travel of the solvent up the paper - is there such a thing as white construction paper? That might work.

Is it obvious I don't have kids and have never had to help with a science fair project before?

Ahh, thicker paper! That should help slow it down. And the taste test was very interesting. Crayons are non-toxic, and she did get consent forms from the parents. The results surprised me; I thought they might mostly be tasteless, but that wasn't the case at all. Apparently purple and red are the favorites, while green, blue, and black are the least favorite. Yellow, orange, and white are the other colors she used, total of 8.

I think you're right about her solvents being too polar, so we will try the ones you suggested. Hopefully that will do it... if not she may have to try separating the pigment from the paraffin first.

Thanks so much for your help.

I'm just guessing, but the colours might be insoluble solid pigments rather than soluble dyes. If this is the case even a solvent that dissolves the matrix of the crayon may not dissolve or separate the colours. You could try askng the manufacturers of the crayons.

You probably know that there has been a lot of grown up research on the relationship between perceived flavour and colour of foods like jelly (jello) and soft drinks.

Good luck with it, and be sure to let us know how the final project goes.

Peter

Thanks Peter, I will let you know how it goes. Hopefully we'll figure out a way to separate the colors!

I did forget to say that the kids were blindfolded for the taste test so the color wouldn't influence their opinion. And they were tested separately, not in a group. They were also told it wasn't necessary to actually chew and swallow the piece of crayon, but my grandson, who was the first "guinea pig", was determined to actually eat it. (Each piece was about 1/4".) Anyhow, he gagged and almost chucked over the green, blue, and black. We laughed so hard... it was hysterical. I think he was trying to prove something to his big sister. The rest of the kids followed instructions a little better and didn't have any problems.

Stephanie

PeterApps talk words of wisdom. Certainly ask the crayon manufacturer. But they may throw a wobbly about the dangers of feeding crayons to children. They will certainly have considered toxicity and may have some sort of quality control on their pigments. They should certainly know what the pigments are (and there's nothing like knowing the answer before you plan the experiment...)

Instead of writing w/ crayon across the bottom of a paper, make a slurry of crayon in your chosen solvent and spot the bottom of the paper w/ a toothpick, capillary pipet or fountain pen nib. That will (hopefully) leave the insoluble spectator moieties out of the process (and tip you off to the use of any lakes, which will probably not migrate up the paper).

Another suggestion - what about using the Prang brand of crayons by Dixon-Ticonderoga? They make a soy-oil based crayon that might have different solvent solubilities than the paraffin-based crayons.

https://www.dixonusa.com/index.cfm/fuse ... dIndex=139

I like Peter's advice - call Crayola or Dixon-Ticonderoga and ask if the pigments are water-soluble. I wouldn't tell them your granddaughter is doing taste tests, though - the seventh-grade TLC of pigments should suffice to extract information.

On the paper: the advantage with the coffee filter is that there is not a lot of clay in the paper. Construction paper may have a lot of clay in it - for density and surface characteristics. The chromatography will change.

I would not worry about the speed of the chromatography. The trick is to get rid of the paraffin so you can get pigments to move. And the composition of crayons includes stearic acid, paraffin and polymers - some of which may not be soluble in any solvent you are likely to have at home. The fact that you are not getting the crayons to dissolve tells me that you are most likely running into problems with the stearic acid or one of these polymers.

Remembering the rule that like dissolves like - there is a lot of non-polar character to the approved dies for food coloring and for plant pigments. Most will be more like wax than they are like water, alcohol or acetone. I suspect that these compounds will partition selectively into the wax - and thus will not move along the paper or in a moderately polar solvent. So, trick number one is to get the colors away from the "wax."

My first reaction is to go get some crayons and try some things before giving advice - but my kids are grown and it might take a while to dig to the bottom of the correct sofa...

So, what I would try is melting the crayons. They should melt below the boiling point of water. If the color stays in solution in the melted crayon, I would try to extract the colors with water. This will get ugly because if the colors prefer to stay in the wax, it will take a significant ratio of water to crayon to get any material out of the wax. Then, the water will have to be evaporated away. (Don't try heating to dryness - the temperature of the nearly dry pan will rise and you are likely to char any color you extracted.) And salts in the water will leave a lot of residue - so you will need distilled water - which can be purchased at the drug store or grocery. I would do the evaporation in a flat glass pan – like a cake pan. After the water evaporates, take a few drops of alcohol and run them around the pan and see if you pick up some color…

The down side is that some stearic acid should disolve in the water and you may still have to do a bit more to separate the colors from it.

This technique must be attempted with great care -because a small accident can result in a big scald.

As has been pointed out already, some coloring materials are solids – so you may find that when you melt some crayons, the color sinks to the bottom (or floats to the top) and will not dissolve in anything.

If you can melt the crayon, you may be able to blend in some mineral oil and then when that cools, but remains liquid, you may be able to add in other solvents - such as toluene. But stearic acid has low soluabity in a wide range of solvents, so it may drop out of solution on you. (If it drops out without taking the color, it would be nice - but I would not count on it.)