Sciencemadness Discussion Board

Dissolving Copper

wymanthescienceman - 1-6-2011 at 12:03

Hello fellow forum members, I have a question to ask regarding the dissolving of copper in non-oxidizing acids such as H2SO4 and HCl.

I have done a couple of experiments with both of these acids, using Ammonium Nitrate as the oxidizer to allow the acids to dissolve the copper. My question is, what are the products of this reaction? My thinking is that the reaction goes as such:

2H2SO4 + 2NH4NO3 + Cu ---> CuSO4 + (NH4)2SO4 + 2H2O + 2NO2

And similarly with HCl:

4HCl + 2NH4NO3 + Cu ---> CuCl2 2NH4Cl + 2H2O + 2NO2

Although with HCl I must be getting CuCl as well as CuCl2. When I did the reaction using H2SO4 I got a precipitate of blue crystals which I assume are CuSO4:5H2O. With HCl I get a dark brown/green liquid with no crystal formation.

Is there any way that Cu(NO3)2 is forming in this reaction or is it safe to assume that only the sulfate or chloride is forming? And what would be the best way to separate the copper compounds from the mix, leaving the ammonium behind? It may be as simple as decanting the liquid from the crystals with the sulfate, but I'm not sure whether the Ammonium Sulfate crystallized with it!

#maverick# - 1-6-2011 at 12:34

hot h2so4 is an oxiders i believe

LanthanumK - 1-6-2011 at 12:40

Ammonium sulfate solubility: 70.6g/100mL at 0C. Copper sulfate pentahydrate solubility: 31.6g/100mL at 0C. Copper(II) chloride solubility: 70.6g/100mL at 0C. Ammonium chloride solubility: 29.7g/100mL at 0C. Copper sulfate will crystallize out of mixture 1, copper chloride won't out of mixture 2. What I would do with the copper(II) chloride is evaporate it to dryness (lots of HCl fumes), redissolve it in water, and precipitate the copper as the carbonate. Wash and dry the copper carbonate, redissolve it in HCl, and recrystallize the CuCl2. Someone else may have a more painless way. ;)

ItalianChemist - 2-6-2011 at 02:11

Simply mix conc. hydrochloric acid and some conc. hydrogen peroxide...this solution will dissolve copper in a little time, but be careful because chlorine is evolved.
Another way is to use a mixture of H2O2 30% and H2SO4 96%. But this solution isn't very useful due to decomposition of hydrogen peroxide

Arthur Dent - 2-6-2011 at 03:27

I never thought of trying ammonium nitrate instead of nitric acid with the sulphuric acid to generate copper sulphate. Mhh, i'll try it this weekend.

The peroxide route also produces nice pure copper sulphate, but it's awfully inefficient and the final solution is quite diluted, mind you the most concentrated stuff available to me is 9% H<sub>2</sub>O<sub>2</sub>. My favorite recipe is with the nitric acid, which produces a reaction that's quite vigorous and very quick. The final solution is very concentrated and just a little bit of heat will help it crystallize quite fast.

Robert

ScienceSquirrel - 2-6-2011 at 03:34

Quote: Originally posted by ItalianChemist  

Another way is to use a mixture of H2O2 30% and H2SO4 96%. But this solution isn't very useful due to decomposition of hydrogen peroxide


That is acid Piranha and should be treated with extreme respect.

ItalianChemist - 2-6-2011 at 05:37

Quote: Originally posted by ScienceSquirrel  

That is acid Piranha and should be treated with extreme respect.

Yes, it's very dangerous, and eats trough almost everything!
By using a mixture of concentrated HCl and 35% H2O2 you will dissolve copper very quickly, maybe more quickly than using nitric acid

Arthur Dent - 3-6-2011 at 04:38

Coming back to the recipe of dissolving copper with H<sub>2</sub>SO<sub>4</sub> and AN, I tried yesterday evening with 100 ml of sulphuric acid (96%) and 20g of ammonium nitrate in a 400 ml beaker. The prills dissolved without much reaction. Then I added a few rolls of telephone copper wire to the mixture, there was a modest amount of fizzing, then nothing!

So I added a few squirts of Hydrogen Peroxide 9%. Some vigorous fizzing and then it subsided. Mhhh, not much of a reaction. :(

This morning, I looked at the beaker, completely clear, with the slightest hint of blueish... Now this reaction ain't goin' nowhere! :(

So I put the beaker on the hotplate and put a watchglass over it. 15 minutes later, the solution is a very nice sky blue, and there was an ominous orange cloud of NO<sub>2</sub>. I think the heat kickstarted the reaction because I have shut down the hotplate and the NO<sub>2</sub> gas is gone, but the copper is still gently fizzing and the solution's color is getting more intense.

I'll report back to see how it goes over the day.

Edit:

wow! Something strange happened! I put the heat back on and for about an hour, the reaction continued without much happening. Then all at once, the solution became darker and a MASSIVE amount of NO<sub>2</sub> started forming (it was dark red above the solution) and all of a sudden, the solution became black!!

I stopped the hotplate once again and i'm letting the solution cool down but this is crazy stuff! Never seen such a reaction and i've made copper sulphate often (with nitric acid and no heat though).

The solution has cooled down but remains very dark. Looking up the Copper Chlorate data, I see that the pentahydrate is very soluble in water at high temperatures. Ther appears to be a slightly pale blue precipitate at the bottom of the beaker below the undissolved copper... copper hydroxide? I can assyre you that all my reagents are relatively pure.

That's odd stuff, folks! The magic of copper chemistry!

Robert




[Edited on 3-6-2011 by Arthur Dent]

smuv - 3-6-2011 at 07:27

the copper probably passivated, next time try it with more dilute acids. Something like 40% nitric acid is best for dissolving copper, so adjust the amounts of H2SO4/NH4NO3 to produce something like that in situ.

Arthur Dent - 3-6-2011 at 08:02

After a couple of hours cooling down and settling, I am left with a deep blue solution of copper sulphate, which I diluted a bit with distilled water, and a good quantity of black, insoluble powder. I am in the process of washing and filtering the powder, which appears more and more to be CuO.

@ Smuv: The powder could also be elemental copper, but wouldn't it be reddish, and wouldn't it dissolve in the still acidic solution?

I am encouraged by the very deep blue color of the solution, I think it will make some nice crystals.

What really made me curious is that precise moment where the deep blue solution turned black and evolved a lot of NO<sub>2</sub>, it looks like the nitrate went out of the solution all at once in the span of 3 or 4 seconds, oddly enough, the temperature had remained constant throghout the heating cycle for some 40/45 minutes before this neat chemical magic trick! ;)

Edit:

Oh and I believe something overheated because I noticed a tiny crack at the bottom of the beaker. Shame. It was a good beaker. RIP: Rest in pieces.... ;)

Robert

[Edited on 3-6-2011 by Arthur Dent]

Jor - 3-6-2011 at 08:36

The reason your copper dissolves so slowly in the H2SO4/NH4NO3 is the absence of water. Just add one volume of water for every volume of H2SO4, dissolve the NH4NO3 and add copper. This reaction should proceed without difficulty.
Fuming HNO3 also very slowly attacks copper, while the azeotropic acid dissolves it very vigorously. It seems the water ligands are needed in dissolving it (to form Cu(H2O)6(2+)-ions). When using fuming acid, only a very small amount of water is present and the copper passivates.

Arthur Dent - 3-6-2011 at 11:03

You're right. As I mentioned, I added some water to the black solution I got this morning and it gave ample amounts of blue solution. I have already harvested a dishful of crystals, There's still some of that black powder left, i'll analyse it as soon as it dries.

Robert


[Edited on 3-6-2011 by Arthur Dent]

wymanthescienceman - 5-6-2011 at 12:07

Wow, thanks for all your replies guys!

The other day I harvested the crystals I got from my first experiment to find that it must indeed be a mix of CuSO4 and (NH4)2SO4 :( The crystals are a mix of blue and colorless, the only way I can describe them would be a blue sno-cone where the color has run out near the top ;)

I was using new, fresh battery electrolyte for my sulfuric acid, it has a specific gravity of 1.265 making it roughly 35% strength. I really want to get a more pure solution of copper sulfate, so I'm thinking the nitrate route may not be the best option, I don't want to have to go through the hassle of recrystallizing a bunch of times to separate the copper out. The route LanthanumK suggested may be viable but that's still more work than I want to deal with. I had planned on dissolving a lot of the copper scrap I had laying around to make useful copper compounds out of it, so I need a cheap and easy way to get it dissolved.

I may try the piranha acid solution when I find a supply of strong peroxide, but I have none available locally. I really don't want to deal with HCl fumes anymore either so I think that acid is out, plus the battery electrolyte is about half the price of HCl.

The other route that may work for me is electrolysis, but I have yet to try it and I have a feeling it will take a long time. I'm going to try and dig out a power supply and give it a shot soon to see how that option goes.

Arthur Dent - 5-6-2011 at 17:33

@ wymanthescienceman: I think your experiment with Ammonium Nitrate and Sulphuric Acid was sound. The only thing you needed to complete the reaction is a bit of heat.

In my case, I put on just a bit too much heat and the solution started decomposing, and I ruined a beaker. So if you reproduce your experiment and add a bit of heat, you'll get better results. Be careful of the noxious NO<sub>2</sub> fumes though. The heat completes the process and gets rid of the nitrogen left.

The crystals I harvested are really nice and very pure. Maybe you could try it again and use moderate heat to get the reaction going.

Robert

MrHomeScientist - 7-6-2011 at 12:16

Quote:
Quote: Originally posted by wymanthescienceman  

The other day I harvested the crystals I got from my first experiment to find that it must indeed be a mix of CuSO4 and (NH4)2SO4 :( The crystals are a mix of blue and colorless, the only way I can describe them would be a blue sno-cone where the color has run out near the top ;)

That sounds very similar to something I observed when making potassium nitrate via the reaction
NH4NO3 + KCl == KNO3 + NH4Cl

I decanted most of the solution from the potassium nitrate crystals, which were beautiful clear/white spikes, and the ones at the bottom became frosted with pure white snowflakes of ammonium chloride where the excess liquid evaporated. Sounds like your experience was about the same!
Quote: Originally posted by wymanthescienceman  

I may try the piranha acid solution when I find a supply of strong peroxide, but I have none available locally. I really don't want to deal with HCl fumes anymore either so I think that acid is out, plus the battery electrolyte is about half the price of HCl.

Be extremely careful if you do this. Piranha solution is frighteningly powerful, and must not be taken lightly. It can react very violently with just about anything. In glassware cleaning at least, they usually recommend only using this as a very last resort. I have never used it myself, and from what I've read I'd recommend avoiding it if another method will work for you.