Sciencemadness Discussion Board

Copper precipitation from a CuCl2 solution using Aluminum - the powder is all grey

RogueRose - 11-2-2020 at 15:36

I'm trying to work on a procedure to separate very thin, chopped copper and aluminum foil and one of the things I was trying was to make a solution of CuCl2 by dissolving only the copper foil in some fairly dilute HCl solution. After a few days the copper had turned into a powder in the bottom of the vial and the solution was very light blue. The powder looked mainly grey with a little copper color mixed in. It was really strange. I decided to try to dissolve more and added more Cu, HCl and some H2O2 and I ended up getting a very nice blue solution with some copper that had not dissolved and there was a slight excess of H2O2, so the limiting factor was HCl.

I the added some of the Al foil and it started dissolving furiously and at first I thought the powder looked brown or copper colored and the foamy area around the metal was grey/black while it dissolved (at a temp of about 120-130F). The powder eventually turned to a very light grey powder, almost white and the solution was green with a slight brown tint. I had extra solution (CuCl2 solution) and I took a soda can tab and put that in and the exact same thing happened but the tab ended up getting coated with copper. The reaction seemed to stop with a green/slight brown color, which seems very odd for an aluminum chloride salt.

now I'm trying to figure out what this white/grey powder could be. I figured it would precipitate pure copper but thought maybe if there was an excess H2O2 it might make a CuO/Cu2O, CuO2 or some other copper oxide/salt. The only thing I can think of is CuCl which does give a white powder but it sure doesn't seem like it is possible to obtain from this method, at least what is listed in Wiki, it doesn't seem possible.

Does anyone have any idea what might be going on here? I'm thinking of trying the same thing with sulfuric acid or maybe even acetic acid (IDK if Al will displace the Cu in a copper acetate solution though...). I don't think nitric acid would be a good choice and I don't have any now anyway. If you have another acid suggestion, I'd like to hear.

Finally, my next step is that I'm going to look at other methods of seperating Cu & Al and am trying to find some liquid (salt solution probably) that has a density of ~2.7g/ml so that the Al will float and the Cu will sink.

I'd appreciate any ideas on what is going on here. Thanks!

clearly_not_atara - 11-2-2020 at 16:01

The natural guess is CuCl; the problem with this guess is that CuCl is typically soluble in solutions containing excess chloride. But CuAlCl4 has been described, so you could get a precipitate containing CuCl and CuAlCl4, I suppose.

In order to avoid the precipitation of copper (I) salts when reducing copper (II) solutions, I think you can just add ammonia, which will ensure that Cu+ stays in solution as Cu(NH3)2+.

RogueRose - 12-2-2020 at 09:34

Quote: Originally posted by clearly_not_atara  
The natural guess is CuCl; the problem with this guess is that CuCl is typically soluble in solutions containing excess chloride. But CuAlCl4 has been described, so you could get a precipitate containing CuCl and CuAlCl4, I suppose.

In order to avoid the precipitation of copper (I) salts when reducing copper (II) solutions, I think you can just add ammonia, which will ensure that Cu+ stays in solution as Cu(NH3)2+.


Thanks, I'll try adding a little ammonia and see what it looks like for another trial.

I thought that you could precipitate copper powder from almost any copper salt solution by adding any of the following: iron, zinc, aluminum or magnesium and I'm pretty sure I've done that before certainly with iron but also I think Al. . I know I did this with old nails but maybe it was copper sulfate, so IDK if sulfate works differently than chlorides. the problem with the nails was the copper would coat the nails to a fair thickness and ome would come off as a shell of he nail body, it was really odd. I'm going othave o try ,this again with different salts.

wg48temp9 - 13-2-2020 at 00:25

Quote: Originally posted by RogueRose  
Quote: Originally posted by clearly_not_atara  
The natural guess is CuCl; the problem with this guess is that CuCl is typically soluble in solutions containing excess chloride. But CuAlCl4 has been described, so you could get a precipitate containing CuCl and CuAlCl4, I suppose.

In order to avoid the precipitation of copper (I) salts when reducing copper (II) solutions, I think you can just add ammonia, which will ensure that Cu+ stays in solution as Cu(NH3)2+.


Thanks, I'll try adding a little ammonia and see what it looks like for another trial.

I thought that you could precipitate copper powder from almost any copper salt solution by adding any of the following: iron, zinc, aluminum or magnesium and I'm pretty sure I've done that before certainly with iron but also I think Al. . I know I did this with old nails but maybe it was copper sulfate, so IDK if sulfate works differently than chlorides. the problem with the nails was the copper would coat the nails to a fair thickness and ome would come off as a shell of he nail body, it was really odd. I'm going othave o try ,this again with different salts.


Copper metal will reduces a solution CuCl2 to insoluble CuCl usually coating the metal and forming a sludge . If copper metal can do the reduction I would expect iron, aluminium and even stainless steel to do the same. CuCl is soluble in HCl and can be redeposited when diluted.

I used a stainless steel spoon the remove some CuCl sludge from the bottom of a beaker. The spoon was pit corroded in minutes. Yes that was daft but the sludge had hardened so I could not use my disposable plastic spoons.