mangoman667
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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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blargish
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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.
BLaRgISH
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MrHomeScientist
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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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Texium
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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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Wow, a reply from the home scientist himself! I'm honored  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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Texium
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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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vmelkon
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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]
Signature ==== Is this my youtube page? https://www.youtube.com/watch?v=tA5PYtul5aU
We must attach the electrodes of knowledge to the nipples of ignorance and give a few good jolts.
Yes my evolutionary friends. We are all homos here. |
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mangoman667
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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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Texium
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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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DraconicAcid
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So is *everyone* making Chevreul's salt these days? Can't imagine why.....
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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mangoman667
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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.
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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Texium
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| 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.
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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mangoman667
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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  haha I've
been hearing piranha solution, but I would like to try something a little less drastic
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Texium
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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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mangoman667
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| 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 Any ideas on where to get these on the
cheap, considering I'm fresh out of high school?
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Molecular Manipulations
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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]
-The manipulator
We are all here on earth to help others; what on earth the others are here for I don't know. -W. H. Auden
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MrHomeScientist
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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 
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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mangoman667
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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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Texium
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| 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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mangoman667
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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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