Arthur Dent
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Copper Chloride question
I have an odd little problem, I wanted to synthetize some copper chloride, so I used fairly pure chemicals... fine copper wire from stripped telephone
wire, and concentrated technical grade HCl.
I put a ball of Cu wire in a beaker and topped off with HCl. The reaction was slow but after a week, all the copper was gone.
Now my problem is... CuCl2 and/or CuCl solutions should have a light blue to blue/green color, but my solution right now is a very dark brown color?
I looked-up various resources and no Cu salt should be brown in a solution, especially with only Cu and HCl as the ingredients?
I have no doubt as to the purity of the ingredients, and even if there were impurities, it wouldn't affect the color that much! I succesfully
synthetized large quantities of CuSO4 with that same wire, and the acid produced very clean-looking solutions of Zinc Chloride, ferrous/ic chloride
and Manganese chloride.
Could it be that my synthesis formed some weird copper oxychloride complex?
Oh, one last bit, when I neutralized the remaining acid from the paper filter, I poured a bit of sodium bicarbonate solution on it and the resulting
foam and solution was a beautiful sky blue, which is what a chloride or a carbonate should look like. 
Robert
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Random
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Oxygen oxidized copper in HCl so a small amount of CuCl2 formed. CuCl2 reacts with copper to make two molecules of CuCl (Copper (i) chloride). CuCl is
then oxidized by HCl and air to make CuCl2 again. And the cycle starts again until all copper is gone. In the end you have CuCl(which is brown) mixed
with CuCl2. You need to bubble air through it or just stir it or leave it in the air with excess hcl to oxidize it to green CuCl2.
When I was dissolving copper and this happened (actually the solution had some brown CuCl mixed with Cu metal) I put into the solution small amount of
NH4NO3 so it dissolved copper during the night. I the morning i had CuCl2 mixed with small amount of Cu(NO3)2
[Edited on 27-11-2010 by Random]
It could be also because of high concentration, check this page:
[Edited on 27-11-2010 by Random]
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ScienceSquirrel
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The colour is almost certainly due to to the CuCl42- ion formed from the copper chloride and excess acid.
When you neutralised the caid the complex decomposed to form the normal copper II compounds.
http://en.wikipedia.org/wiki/Copper(II)_chloride
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ThatchemistKid
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CuCl is a white solid not a brown solid and it rather quickly in moist air gets oxidized to Cu2+ contaminating the white CuCl with a greenish yellow
compound.
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Arthur Dent
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Success!!!
I had a feeling I needed to oxidize somehow the solution...
So I quickly pulled out a few test tubes (shattering some in the process, argh!) and put about 3 ml of my dark brown solution in two tubes. I added
very gently a bit of H2O2 to the second tube and voila!! The solution became instantly a bright emerald green, as seen in the picture.
@ Random: The addition of Ammonium nitrate, (in the 1st test tube) did nothing, the solution remained murky brown, and from what resulted in test tube
2 with the peroxide, there isn't any elemental copper surpended in the solution.
Robert
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woelen
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Quote: Originally posted by Arthur Dent  | I put a ball of Cu wire in a beaker and topped off with HCl. The reaction was slow but after a week, all the copper was gone.
Now my problem is... CuCl2 and/or CuCl solutions should have a light blue to blue/green color, but my solution right now is a very dark brown color?
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Nice to see that there are more people who find this nice riddle 
I have devoted multiple webpages and hours and hours of experimenting to this:
http://woelen.homescience.net/science/chem/riddles/copperI+c...
Have fun reading all of this and hopefully it gives an explanation to what you observed.
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Arthur Dent
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Wow! Thanks Woelen!
A fascinating series of experiments, showing how Cu reagents with Cl give unusual results! I have added your site to my favorites! 
I found in my selection of vintage reagents an antique bottle (Fischer ACS, circa 1950) of CuCl that unfortunately turned into a rock solid, very
light green mass... Can't even scrape some off because it's so hard!
Anyway, i'll try to slowly dessicate the CuClx solution I just made, and see what color it turns into. 
Robert
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bbartlog
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I also have seen the dark brown solution (dissolving copper pennies in HCl) and erroneously thought it was due to a shortage of acid and formation of
oxychlorides.
Very nice exploration of the topic, woelen. It sounds like there might be a Cu2Cl3 compound involved. In my experience the color of anhydrous CuCl2 is
similar to the brown seen here; what that means I don't know.
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bquirky
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yeah ive seen that too. I always assumed my HCL had some Fe contaminates that where oxidising
I have allso repetadly seen zinc desolving in HCL to form a pink/purple solution while trying to make zinc chloride. what that means i dont know
either
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Random
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Interesting, I had the same brown solution that contained some copper metal, excess hcl and some that brown stuff. I added NH4NO3 and the copper was
dissolved next morning and the solution was green like when you added h2o2. Maybe air oxidized it or small amount of nitric acid that was produced
with nh4no3 in hcl. There could be also oxygen included in all this.
It's interesting though that you are researching this
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kmno4
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Brown colour is caused by charge transfer between Cu(I) and Cu(II), it is known for a long time.
Some literature about Cu : DOI: 10.1039/J19680001838; DOI: 10.1039/J19680001835
Charge transfer is also responsible for deep colour of prussian blue (Fe(II)-Fe(III)) and many other compounds contaning some metals in different
oxidation states.
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