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Author: Subject: Copper Acetate Elephant Toothpaste! (Experiment report)
MidLifeChemist
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[*] posted on 7-11-2020 at 16:31
Copper Acetate Elephant Toothpaste! (Experiment report)


INTRO: In this long-ish experiment report, I go through how I produced copper powder from Aluminum foil, made beautiful Aluminum sulfate crystals, and accidently created a giant copper acetate foam mabchine that rivaled some of the elephant toothpaste demos I've seen. Read on to hear all the details!

HOW TO MAKE, AND NOT MAKE, COPPER:

One of the reasons that I started my home chemistry lab was to teach my kids science and chemistry. And in that respects, it has been a huge success - in fact they have already learned a great deal in a short amount of time.

Our most recent family experiment was producing elemental copper. I filled one beaker with a concentrated solution of CuSO4, and a 2nd beaker with a dilute solution of NaCL. I place aluminum foil in both beakers, we waited but no reaction took place. I then added some of the NaCl solution to the CuSO4 beaker, and immediately the copper started forming on the Aluminum foil. After an hour, there was already an impressive pile of copper at the bottom of the beaker, and the solution was starting to turn clear.

What I find most fascinating about this reaction, is that the exact mechanism of how the chloride ions help the passive layer of Al2O3 on the Aluminum foil get penetrated is not fully agreed upon or understood. What we do know, is that it does not take very many chloride ions to allow the reaction to start, and also that other ions like bromide can also work.

VERY PRETTY Aluminum Sulfate Crystals

The next day, I removed the small amount of copper-coated aluminum foil left floating in the beaker, and gravity filtered the elemental copper with a coffee filter, and washed it once with distilled water. Interestingly, the resulting copper powder greatly resembled used coffee grinds.

I did not dispose of the filtrate, as I had plans for it. I had about 300ml of very clear filtrate. I boiled it down to 50ml, at which point it started to turn a slight olive green color, but I did not see any solids in the liquid. I had picked 50ml as an arbitrary point where I thought I would turn off the heat. I poured the hot solution into an evaporating dish, and left it to cool off and start the evaporation process. 30 minutes later, I noticed 4 or 5 very small non-descript, lily-pad like shapes forming on the surface of the liquid.

Later that day I went to check on the liquid, and it had completely transformed into beautiful crystals. They had a slight green hue - I wonder if was due to Iron impurities in the Aluminum foil? Or perhaps there was a small amount of copper still dissolved in the solution. There did not seem to be any liquid in the dish, but I didn't poke at the crystals so there may be some liquid under them. Which is cool because the super-saturated liquid seemed to "transform" into beautiful solid crystals without much evaporation, merely by cooling down.

IMG_6150.JPG - 1.7MB

Anyways, the copper powder was drying, and it was time for more experiments.

MELTING THE COPPER

I placed a couple small spoonfuls of the copper powder into a small crucible that I had. I placed the crucible onto a fire brick, and blasted both the crucible and the powder inside the crucible with a benzomatic propane torch for about ten minutes. I wasn't able to melt the copper, but I did oxidize the heck out of it! Oh well.

REDISSOVING THE COPPER

As a final step to the experiment series, I thought I would demonstrate redissolving the copper metal to form copper acetate. Several articles reported using a 50/50 mix of vinegar and 3% hydrogen peroxide brought to a boil, for either copper or lead acetate.

In a 1000ml erlenmeyer flask sitting on top of a hotplate stirrer (not turned on yet), I added 320ml of four monk's white distilled vinegar (5% acidity) and 320ml of 3% hydrogen peroxide, for a total of 640ml, which is only around 1/3 of the height of the 1000ml flask.

I expected the reaction to take a while, although I thought that it might commence quicker than expected because the copper was in powder form. I dumped all of the remaining copper into the flask, and the clear solution quickly turned brown. Nothing appeared to happen, but after a few minutes, to our surprise, the solution started to turn a slight blue-ish green color. It appeared to be working!

I decided to speed things up by turning on the hotplate stirrer. Little did I know what would happen next...and this would be the last moment I saw any kind of blue color.

RELEASE THE ELEPHANT TOOTHPASTE!!

I already had a teflon stirbar at the bottom of the flask. I turned on both the hotplate and the magnetic stirring, and was excited to start witnessing further copper acetate production, as was the rest of the family.

About ten seconds after the stirring started, the intensely green-colored foam started, fast and furious. The foam started to steadily rise, and I sensed that it would quickly come out of the flask. Luckily I had a large pan nearby, and I was able to quickly take the flask off the hotplate-stirrer and place it into the pan before any of the foam left the flask. But the green foam kept coming, and coming - pouring out of the flask for god knows how long (more than 30 minutes). "COOL!" exclaimed the kids, as they quickly celebrated, saying they were so happy to see elephant toothpaste. "But, but" I tried to say, to no avail.

IMG_6144.JPG - 2.4MB

IMG_6147.JPG - 2.1MB
This green solution in the beaker is the solution from the flask, diluted with more water. The solution in the flask is darker and has a slightly more blue-ish green tinge than the pure green color in the beaker.

It took hours for the foam to go away. The solution that remained in the flash was a very dark colored. I poured some out into a beaker, and heavily diluted it. It was a beautiful green color, most easily seen from the top of bottom. When viewed from the side, it appeared to be dark green as there seems to be some fine particles suspended in the solution, that did not settle even after 12 hours.

IMG_6148.JPG - 2.1MB

That night (which was last night), I did copious amounts of *further* research on Copper acetate. I read about basic Copper acetate. I read about how an excess of acetic acid is recommended. I even found one post on SM where someone had a good amount of foam - but nothing like this. I looked up the color of both Copper (II) acetate and Copper (II) basic acetate, but neither matched the rich green color that I got.

FYI, there is still unreacted copper powder at the bottom of the flask. And I measured the pH of the liquid in the flask with my recently calibrated pH meter, which gave a reading of 4.8 Reaction of the solution in the flask with Ammonia gave a deeply colored blue-ish purple solution, and reaction with Sodium Carbonate gave a precipitate with the classic color of Copper Carbonate, along with copious bubbles of carbon dioxide. So no surprises with those reactions when testing for Cu+2.

So here are my questions for the SM community:

- What do you think the reaction was that produced the never-ending foam? I think it was most likely H2O2 producing oxygen but that doesn't explain everything.
- What do you think is causing the beautiful green color in the solution? Ok so it could just be Copper (II) Acetate but I've never seen it discussed as being so green in solution.
- What tests can I do on the solution to determine exactly what it is now? (I have some ideas but I want to get your thoughts)
- Who is John Galt?

Ok that's all for now! Please leave your thoughts in the comments below.

I suspect the simplest possibility is just that the solution is copper acetate, and it is just greener than normal, and that the foam was caused by the H2O2 reacting with impurities in the vinegar and releasing O2. But I'm not sure what impurities would cause the foam, some vinegars have yeast remnants but regular white vinegar is not supposed to have any active yeast in it.




[Edited on 11/8/2020 by MidLifeChemist]
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EthidiumBromide
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[*] posted on 8-11-2020 at 00:49


Nice write-up, MidLifeChemist.

The crystals of aluminium sulfate are gorgeous! I love how they've assembled into "cells", with the crystal grains radially diverging from the center of each cell. Very pretty, sodium acetate trihydrate forms very similar crystals when you add a starter crystal into the supersaturated solution. From what I've read, aluminium foil is mostly pure Al, but can contain up to 1% of iron impurity, so there is a chance that this could impart a slight hue. Though some leftover copper contamination seems like the most reasonable explanation.

I suspect the reason for the elephant toothpaste effect to be a Cu catalysed decomposition of H2O2. Finely powdered metals are known to decompose H2O2. And transition metal ions (Fe2+/Fe3+ in particular, but other ions as well) can also break down H2O2. So if you had both finely dispersed Cu metal and Cu2+ ions forming, this was a ripe environment for a catalytic decomposition of H2O2 to occur. What caused the foaming itself is another matter seperately. You say you used "Monks household vinegar". I'm not located in the US (or NA for that matter), so I'm not familiar with this brand, but apparently it's a brand of cleaning vinegar and not food grade vinegar. Perhaps it contains some detergent additives for aiding the cleaning process, and that's what gave rise to the intense foaming? That would be my guess.

The green colour is indeed puzzling. I've prepared copper acetate before by acting on fine electrical copper wire with acetic acid and H2O2 and at no point did the solution go green.
You said you have washed the filtered copper with distilled H2O, so chloride contamination can be ruled out. This is a stretch, but again going on the cleaning vinegar vs food grade vinegar trope, maybe some ingredients of the vinegar cleaning solution could interact with copper to give a green color. An even more implausible theory is that some of the formed copper was in the Cu(I) state, and the mix of Cu(I) adn Cu(II) can give a greenish color. But this is a big stretch, the presence H2O2 should fully oxidize all of the copper to Cu(II).

[Edited on 8-11-2020 by EthidiumBromide]
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[*] posted on 8-11-2020 at 08:07


@ethidiumBromide, thank so much for your insightful comments and nice compliments on the crystals. I was very pleasantly surprised when I saw them.

FYI Reynolds company specifically mentions that their foil contains added Fe and Si, I believe it is around .075% each, based on what I read.

Here is some more info on the vinegar, in case anyone is interested:

From the vinegar container:
"Four monks white distilled vinegar"
Suggested used: Dressing recipes, Clean floors and glass, clean grills and counters
Ingredients: Distilled vinegar from corn, diluted with water to 5% acidity
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draculic acid69
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[*] posted on 8-11-2020 at 17:55


Al foil definitely contains tiny amounts of Si+Fe as an impurity.also did U make the cuso4 or buy it?
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[*] posted on 8-11-2020 at 21:58


Quote: Originally posted by draculic acid69  
Al foil definitely contains tiny amounts of Si+Fe as an impurity.also did U make the cuso4 or buy it?


The CuSO4 was bought at the hardware store.
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[*] posted on 10-11-2020 at 08:01


Quick update:

I re-did the experiment using the exact same bottles of H2O2 and vinegar. Stirring the solution rapidly before adding the copper powder did not produce any bubbles or foam, and neither did adding in small amounts of copper powder left over in excess from the previous attempt. The solutions I produced were a nice blue, the color of Copper Acetate that one would expect.

So at this point, I'm leaning towards either A) the large excess of copper powder somehow causes foaming and decomposition of the H2O2, or B) there was some other impurity in the original batch of copper powder that I hadn't rinsed out that caused the intense foaming. At some point I'd like to try to entire experiment again from the beginning, starting with the aluminum foil, to see if I can narrow it down to A or B.
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