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Author: Subject: Help with Chevreul's salt synthesis
mangoman667
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[*] posted on 9-6-2015 at 18:38
Help with Chevreul's salt synthesis


I tried to do an offhand synthesis of this strange salt, but ended up with puzzling results. Any light shed on this situation would be of great help and highly appreciated. :) Here goes:

I started with a solution of copper sulfate, simply slowly added water to crushed crystals until they were fully dissolved. I also made a strong solution of sodium metabisulfite. I started pipetting the sodium solution into the copper, and I immediately got a color change to green. However, simultaneously, the solution began to behave strangely and much differently from reports I've read; a yellowish brown precipitate was formed with every drop and the solution turned a clear yellow. with continued stirring, the precipitate then slowly dissolved to form a clear yellow-brown solution. Stranger yet, another precipitate then took its place minutes after, and hasn't dissolved since. I am hesitating to move on to the heating step of the process, as I'm not sure what I have in my flask just yet.

Overall, my first experience with this specific copper compound has been intriguing. The behavior has been radically different than what I have read in reports and guides. My best guess is that it's due to the lack of precision measuring instruments such as a hundredth gram scale to measure stoichiometric quantities of the salts. Thus, I had to go with an excess of sodium metabisulfite.

Can anyone help me out with this reaction?
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[*] posted on 9-6-2015 at 19:30


What was the colour of the second precipitate that appeared? I have had a similar yellowish precipitate form when combining a solution of copper sulfate with an excess of sodium sulfite solution. Its identity is still unknown to me though; I suspect it could be merely copper(II) sulfite or even hydrous copper(I) oxide, as solutions of sulfite are alkaline in nature.





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[*] posted on 10-6-2015 at 06:05


That's definitely strange. Maybe concentration played a role? The immediate precipitation makes me think that your 'strong' solutions were very concentrated indeed. You may have already seen my blog post on this compound: http://thehomescientist.blogspot.com/2013/10/chevruels-salt....
Try following that procedure exactly, and see if you still get weird results. Worked perfectly for me several times! If yours is different, I'd have to suspect contamination of your reagents. Try using recrystallized CuSO<sub>4</sub> and distilled water, for a start.
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[*] posted on 10-6-2015 at 06:17


I bet that you added too much metabisulfite and that fully reduced the copper(II) to copper(I) and your precipitate is hydrous copper(I) oxide. I've seen this happen too, when I intentionally added more metabisulfite than necessary to see what would happen. Try using an excess of copper sulfate instead, and a more dilute metabisulfite solution. That should probably solve your problem.



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mangoman667
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[*] posted on 10-6-2015 at 08:30


Wow, a reply from the home scientist himself! I'm honored :D I love your videos and I've followed quite a few to explore chemistry so far, keep it up!

As far as an update, here's a real kicker: the last precipitate that didn't redissolve is actually two precipitates! After I allowed it settle overnight, there is now a clear solution, with a layer of almost granular reddish brown precipitate, as well as a layer of much less granular white precipitate. The red one is more dense, since the white skids around easily on its surface.

Have I actually made the coveted salt? If so, how could I separate these precipitates apart from physical means?

Thanks for all the replies, I feel like we're getting to the bottom of this :)
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[*] posted on 10-6-2015 at 08:37


I know it hurts, but it would probably be best to start over, using an excess of concentrated copper sulfate solution and dilute metabisulfite solution. If you do that you should get a much cleaner result and not have to worry about separating two precipitates. That red stuff might not even be Chevreul's salt, it could be copper(I) oxide. It would be a lot safer to redo it in a more controlled way that's more likely to get you desirable results. If you want, you could still try to salvage your current attempt too and compare the two precipitates to see if they look like the same compound.



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[*] posted on 10-6-2015 at 17:20


I just did this experiment and it was a success. I used 25.0 g Na2S2O5 in 40 mL and 24.6 g of CuSO4.5H2O in 79 mL.

I based my quantities on my equation.
3 CuSO4 + 4 Na2S2O5 → Cu2SO3.CuSO3 + 5 SO2 + 4 Na2SO4

I haven't seen the equation for this reaction anywhere so I wrote down the equation myself to figure out what the stochiometry is suppose to be.

//EDIT: I filtered the end product liquid. The liquid looks almost clear. There is a slight bluishness, probably due to unreacted CuSO4. I had left the Na2S2O5 solution for 5 hours, so some SO2 escaped. I think this stuff is unstable and releases SO2.

[Edited on 11-6-2015 by vmelkon]




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[*] posted on 11-6-2015 at 08:42


Quote: Originally posted by zts16  
I know it hurts, but it would probably be best to start over, using an excess of concentrated copper sulfate solution and dilute metabisulfite solution. If you do that you should get a much cleaner result and not have to worry about separating two precipitates. That red stuff might not even be Chevreul's salt, it could be copper(I) oxide. It would be a lot safer to redo it in a more controlled way that's more likely to get you desirable results. If you want, you could still try to salvage your current attempt too and compare the two precipitates to see if they look like the same compound.


Since I don't have any more copper sulfate on hand at the moment, I'll have to make some more. In the meantime, I managed to separate a nice pile of the brick red precipitate. What are some methods to test it to tell whether it's the real deal or just some oxide?
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[*] posted on 11-6-2015 at 08:46


Try dissolving it in ammonia. If it dissolves to form the distinctive blue copper(II) tetraammine complex, then it's likely Chevreul's salt. Adding hydrochloric acid to it should give you white copper(I) chloride. http://sciencemadness.wikia.com/wiki/Chevreul%27s_Salt

Those two tests prove that it's a (I/II) compound.




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[*] posted on 11-6-2015 at 10:29


So is *everyone* making Chevreul's salt these days? Can't imagine why.....



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[*] posted on 11-6-2015 at 15:25


Success! After performing the ammonia/dilute HCl tests, I have gotten positive results on both! It turns out that even though the reaction wasn't totally clean, it still yielded a decent amount of the salt! I'm really happy that this turned out a success, and will probably try to do it again cleanly.



Quote: Originally posted by DraconicAcid  
So is *everyone* making Chevreul's salt these days? Can't imagine why.....


As far as why I chose to do it, I had the reagents laying around and I decided to mess around with it haha
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[*] posted on 11-6-2015 at 15:32


Quote: Originally posted by mangoman667  
Success! After performing the ammonia/dilute HCl tests, I have gotten positive results on both! It turns out that even though the reaction wasn't totally clean, it still yielded a decent amount of the salt! I'm really happy that this turned out a success, and will probably try to do it again cleanly.



Quote: Originally posted by DraconicAcid  
So is *everyone* making Chevreul's salt these days? Can't imagine why.....


As far as why I chose to do it, I had the reagents laying around and I decided to mess around with it haha
Nice job! And I think what DA was hinting at was this. :)



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[*] posted on 11-6-2015 at 20:09


Hmm... now that I've rinsed the flask, I'm noticing a layer of black crud/thin, barely noticeable copper on the bottom. Any suggestions on how to clean it? It's an Erlenmeyer, and I can't just sick my fingers in :P haha I've been hearing piranha solution, but I would like to try something a little less drastic
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[*] posted on 11-6-2015 at 20:24


How about soaking it in dilute nitric acid for a while? That should take care of it. Hydrochloric acid with a bit of peroxide would probably be a suitable substitute if you don't have any nitric or it's too dear to you. If it's just copper/CuO, either one should handle it pretty easily. Piranha solution would be a bit overkill for it.



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[*] posted on 12-6-2015 at 07:51


Quote: Originally posted by zts16  
How about soaking it in dilute nitric acid for a while? That should take care of it. Hydrochloric acid with a bit of peroxide would probably be a suitable substitute if you don't have any nitric or it's too dear to you. If it's just copper/CuO, either one should handle it pretty easily. Piranha solution would be a bit overkill for it.


Yea, I will try the HCl method when I get a chance, since I don't even have any nitric haha... I picked up a pound of potassium nitrate and the next experiment on the board is making some decent quality nitric acid. Hope that goes a bit better than the last synthesis, but I have to gather more supplies like stoppers and tubing first :P Any ideas on where to get these on the cheap, considering I'm fresh out of high school?
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[*] posted on 12-6-2015 at 09:21


If hydrochloric acid doesn't work by itself, add a few grams of potassium nitrate to it. Since both acids are strong, it will be akin to adding dilute nitric acid.

[EDIT] Regardless of how you plan on prepairing a nitric/water solution, rubber stoppers will degrade. Nitrogen oxides and of course nitric acid vapors will ruin nearly any plastic (except HDPE and LDPE) and any rubber (except PTFE). The former two will melt at azeotropic boiling temps (120 C) and the latter is rather pricy. Just a buy GG rig.

[Edited on 12-6-2015 by Molecular Manipulations]




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[*] posted on 12-6-2015 at 09:26


eBay! Also Dr. Bob here on the forum has supplied me with tons of very cheap equipment and glassware, so definitely check him out. He's got a sticky thread in the Reagents subforum. Remember though that nitric acid attacks rubber stoppers, so to distill halfway decent strength acid you need an all glass setup. Allow me to plug another one of my videos: https://www.youtube.com/watch?v=PYDFplw8iKg :)

Congratulations on making Chevreul's salt and good job testing it! Also thank you for the compliments on my work. Always glad to meet a fan :cool:


Edit: That's an old video above, and now I see some errors I made. I'm pretty sure I didn't actually make RFNA, just some strong acid contaminated with lots of NO<sub>2</sub>. Also the white crust produced in the end is most likely ammonium hydrogen sulfate rather than ammonium sulfate.

[Edited on 6-12-2015 by MrHomeScientist]
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[*] posted on 12-6-2015 at 16:21


Well, I was thinking of just going the route of a NO2 generator and bubbling the gas into some hydrogen peroxide. I don't feel like my purposes would call for super high concentrations, so I don't think I would need to distill anything. What is the consensus concerning what a good stock concentration would be?
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[*] posted on 12-6-2015 at 19:49


Quote: Originally posted by mangoman667  
Well, I was thinking of just going the route of a NO2 generator and bubbling the gas into some hydrogen peroxide. I don't feel like my purposes would call for super high concentrations, so I don't think I would need to distill anything. What is the consensus concerning what a good stock concentration would be?
I just tried that yesterday actually, using some NO2 left over from my last nitric acid distillation. The acid produced was very dilute, and not good for much of anything although it could (very slowly) dissolve copper. Unless your H2O2 is more concentrated than the generic 3% pharmacy stuff, you're likely better off doing the distillation of a nitrate salt and sulfuric acid. You can dilute the sulfuric acid a bit, so that the nitric acid that you distill will be the azeotropic concentration (~68%) rather than something more concentrated. That's what I did, and the resulting acid has been satisfactory.



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[*] posted on 12-6-2015 at 20:03


God, I really hate being on this ledge of a point in life... The thing is, I'm moving out to uni in a few months, and I obviously can't move my lab with me (not that I would need to as a ChemE major, but still). I'm quite sad that I'll have to give up my hobby for a while, even though all of my apparatus will be at my older brother's house for the time being (my mother and father will be moving out of the country, so the lab will need to be moved). Should I continue building it up, as I don't really have anything advanced such as condensers, hotplates, or scales? I guess my question is, will apparatus become any harder to get as time goes on, or should I just save my money as of now?

I can't thank you enough for the quick and continued replies, guys, you've been really helpful! :)
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