Sciencemadness Discussion Board

facile CuBr prep

chemrox - 10-5-2012 at 21:26

I found the following:

http://lanthanumkchemistry.over-blog.com/article-how-to-make...

Copper(I) bromide: React copper(II) sulfate with an equal amount of sodium bromide. Add ascorbic acid or sodium metabisulfite. White copper(I) bromide will precipitate.



What do you think he means by"equal amount?" equal weight or mole/mole? I have the materials to make this (in excess) but before I invest any reagents I'd like to hear from anyone who has done this. btw- I searched before posting..

Thanks,
crx

woelen - 11-5-2012 at 00:49

One mole of CuSO4 uses up one mole of NaBr, the Cu and the Br combine 1 : 1. Use a slight excess of NaBr, to be sure that enough bromide is present and all copper precipitates as bromide and not as something else, making your product impure. Do not use a large excess, because that could keep copper in solution as CuBr2(-), especially when you work with concentrated solutions.

The S2O5(2-) ion in theory is capable of reducing 4 copper ions. Just use a fairly strong excess amount of this, to assure that the reaction completes. I would take 1 mole of copper sulfate for half a mole of S2O5(2-), which is a two-times as much amount as theoretical. Bisulfate is dirt cheap, so do not save on that too much.

I do not know how many electrons ascorbic acid produces per molecule when it acts as reductor, so first try to find that and then use a slight excess of ascorbic acid, based on that. Only use a slight excess amount, because ascorbic acid is good at making complexes with copper and these may keep part of the copper in solution.

I would prefer the bisulfite for this reaction. It is much cheaper and you can more safely use a large excess amount of this to drive the reaction to completion quickly.

chemrox - 11-5-2012 at 11:46

Thanks so much for clearing this up and providing the detail on how to do this. I have bisulfite, NaBr and CuSO4 so it seems better than ordering something that runs $100/1/4# when I need it as a catalyst.

Sooo.. that appeared to have worked rather well. The white ppt is the CuBr and the aqua color is the hydrated Cu++ ions ?

[Edited on 11-5-2012 by chemrox]

and I like this too:

The submitter used commercial cuprous bromide. The checkers prepared cuprous bromide by dissolving 600 g. (2.4 moles) of commercial copper sulfate crystals and 350 g. (3.4 moles) of sodium bromide in 2 l. of warm water; the solution was stirred while 151 g. (1.2 moles) of solid sodium sulfite was added over a period of 10 minutes. Occasionally a little more sodium sulfite was required to discharge the blue color. The mixture was cooled, and the solid collected on an 8-in. Büchner funnel, washed once with water, pressed nearly dry, and then dried in the air overnight. The yield of cuprous bromide was 320 g. (93%).
http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv3...

[Edited on 11-5-2012 by chemrox]

DJF90 - 11-5-2012 at 15:05

The issue is not with making it, but getting it out of the reaction in a dry and pure state. Whilst wet it oxidises very quickly (at least the chloride does, and I'm certain the bromide does too), so filtering it out and getting it dry without it going green is likely to be a big challenge. See Inorganic syntheses vol. 2 for the prep of CuCl.

chemrox - 11-5-2012 at 15:20

Thanks DF, check above the edit I made adding an orgsyn note. Seems like the br is less sensitive than the Cl

That said, I'd like to start with some cleaner CuSO4.6H2O. Mine has visible crap in it. I think they kind of exaggerated the definition of technical..

[Edited on 11-5-2012 by chemrox]

blogfast25 - 12-5-2012 at 05:57

Quote: Originally posted by woelen  

The S2O5(2-) ion in theory is capable of reducing 4 copper ions. Just use a fairly strong excess amount of this, to assure that the reaction completes. I would take 1 mole of copper sulfate for half a mole of S2O5(2-), which is a two-times as much amount as theoretical. Bisulfate is dirt cheap, so do not save on that too much.





I recently tested that hypothesis when I was recovering some gold from a mobile phone (sodium metabisulphite is the reducing agent used to precipitate the gold from solution). Since as my solution also contained copper, and concerned the copper would co-precipitate, I added quite a bit of Na2S2O5 to a separately prepared copper solution: no reaction or precipitation ensued.

I then added metabisuphite to my copper/gold solution (somewhat neutralised first) and obtained a precipitate of gold powder. On re-dissolving this (tiny bit of) gold in (a tiny bit of) Aqua Regia it was clear it contained no copper whatsoever (solution was slightly yellowish, not a hint of blue or green).

It seems metabisulphite isn’t capable of reducing Cu (II) to Cu (0).


[Edited on 12-5-2012 by blogfast25]

woelen - 12-5-2012 at 09:56

My excuse for not being very clear in my previous post. I meant reduction of copper(II) to copper(I), not reduction to metallic copper.

Indeed, metabisulfite (or just sulfite if pH is higher) is not capable to reduce copper(II) to copper(0), but it can reduce to copper(I). Just a simple experiment:
- dissolve some copper sulfate in water
- add some sodium chloride
- add some solution of sodium sulfite or metabisulfite
The color changes from greenish/blue to pale yellow. All copper is reduced to copper(I) and remains in solution as a coordination complex.
You can precipitate the copper(I) by acidifying somewhat and pouring the solution in water. It precipitates as CuCl. I expect this works with NaBr as well as with NaCl and then you get CuBr.

Isolating CuCl (or CuBr) is very hard. As soon as it comes in contact with air it turns green due to partial oxidation and the material becomes contaminated with a basic copper(II) chloride.

[Edited on 12-5-12 by woelen]

blogfast25 - 12-5-2012 at 11:04

Woelen:

Ah yes, it’s all coming back to me, I made some CuCl that way years ago. I guess those who recover gold from ewaste solutions in Aqua Regia are lucky that when they add the sodium bisulphite the copper is reduced and complexed to Cu (I) and doesn't precipitate as CuCl...