Sciencemadness Discussion Board - Copper carbonate and some strange complex

Copper carbonate and some strange complex

Mixell - 22-6-2011 at 12:27

I prepared a solution of K2CO3 and a bit of NaHCO3, and used copper electrodes in an electrolysis to yield copper carbonate, the results weren't so great. So I added a concentrated solution of copper sulfate to the first solution and obtained a bit of copper hydroxide, but what is strange, the solution became dark blue, much as the [Cu(NH3)4(H2O)2]2+ ion, and I added quite a small amount of copper sulfate solution.
Any thought about this?
Also, is there any procedure to generate fairly pure copper carbonate/hydroxide that later will become copper acetate/formate?
Because adding a solution of sodium hydroxide to copper sulfate does yield copper hydroxide, but I am afraid there is a fair amount of sodium sulfate suspended in those particles.

m1tanker78 - 22-6-2011 at 14:07

Cu(s) will slowly dissolve in dilute acetic acid (vinegar). After evaporating, there'll be copper acetate crystals. Not sure about the dark blue complex you observed, though...

Tank

Mixell - 22-6-2011 at 14:16

How much dilute it needs to be? Because I got glacial acetic acid. And what is the mechanism of this process? Because I'm very curious how can a noble metal like copper dissolve in weak, non oxidizing acid.

m1tanker78 - 22-6-2011 at 14:22

Sorry, I forgot to mention that you need to add H2O2. I used 5% 'vinegar' and roughly one quarter of the volume of 15% H2O2.

I never tried glacial; you'd have much less water to evaporate.

Tank

barley81 - 22-6-2011 at 14:26

The dark blue complex could be tetrahydroxycuprate, formed if Cu(OH)2 dissolved in excess hydroxide. To make copper carbonate, you can react copper sulfate with 2 mol. eq. of baking soda. I haven't tried it, but you could detect sodium contamination in your product with a flame test.

Mixell - 22-6-2011 at 14:41

I reacted copper sulfate with potassium carbonate, now all is left to wait for the precipitate to settle, while washing from time to time and drying it out.

ScienceHideout - 22-6-2011 at 15:20

Copper hydroxide solutions ARE deep blue.

Arthur Dent - 23-6-2011 at 04:03

Quote: Originally posted by ScienceHideout

Copper hydroxide solutions ARE deep blue.

Wait, isn't Copper Hydroxide insoluble in water?

It forms a light greenish/blue precipitate...

Robert

blogfast25 - 23-6-2011 at 05:54

Copper is slightly amphoteric: in excess strong alkali a bit of Cu(OH)2 does indeed dissolve into a deep blue (cobalt blue) solution (cuprate). It was my first thought too but Mixell's conditions don't seem to support cuprate (Cu(OH)42-

ions being formed (not alkaline enough).

No matter what the concentration of acetic acid, dissolving copper metal in acetic acid + H2O2 mixtures is a slowboat to China. Being a weak acid, acetic acid (HAc) doesn’t deprotonate much:

HAc(aq) + H2O(l) < === > H3O+(aq) + Ac-(aq)

For fairly dilute solutions: [H3O+] = SQRT (Ka x Ca

With SQRT the square root,

Ka: the Bronstedt acid constant of acetic acid (about 10-4.76

Ca: the molar concentration of the acid.

No, much better to precipitate the copper as Cu(OH)2.nH2O or as basic copper carbonate and wash carefully with copious amounts of COLD water, then dissolve the precipitate in glacial AA (or strong AA), making sure there is a good excess of the acid. Then evaporate carefully, making sure there’s always some excess acid right up to the end, to prevent the Cu(Ac)2 from hydrolysing. Ditto for copper formiate.

[Edited on 23-6-2011 by blogfast25]

ScienceHideout - 24-6-2011 at 07:05

Quote: Originally posted by Arthur Dent

Quote: Originally posted by ScienceHideout

Copper hydroxide solutions ARE deep blue.

Wait, isn't Copper Hydroxide insoluble in water?

It forms a light greenish/blue precipitate...

Robert

Try it. Mix copper sulphate with sodium hydroxide. I get a deep blue solution.

[Edited on 24-6-2011 by ScienceHideout]

unionised - 24-6-2011 at 07:31

Quote: Originally posted by ScienceHideout

Quote: Originally posted by Arthur Dent

Quote: Originally posted by ScienceHideout

Copper hydroxide solutions ARE deep blue.

Wait, isn't Copper Hydroxide insoluble in water?

It forms a light greenish/blue precipitate...

Robert

Try it. Mix copper sulphate with sodium hydroxide. I get a deep blue solution.

[Edited on 24-6-2011 by ScienceHideout]

Lots of other people don't.

blogfast25 - 24-6-2011 at 08:13

Quote: Originally posted by unionised

Lots of other people don't.

Your alkali has to be quite strong: 5 M or higher I'd imagine. I've done it, no worries.

m1tanker78 - 24-6-2011 at 08:44

My first (and only) attempt at copper acetate pre-dates my practice of taking notes and measurements. IIRC, I was experimenting with copper electroplating solutions at that time. As was pointed out, it's a slow reaction. It was left to react overnight then evaporated in a hot water bath. My intention was to evaporate some of the water but I let it go too long and got some nice acetate crystals.

Quote:

...or as basic copper carbonate and wash carefully with copious amounts of COLD water, then dissolve the precipitate in glacial AA (or strong AA), making sure there is a good excess of the acid. Then evaporate carefully, making sure there’s always some excess acid right up to the end, to prevent the Cu(Ac)2 from hydrolysing.

That's probably what I would try now-a-days if I wanted to make some copper acetate. Most of the Na2CO3 would be washed out in the cold water rinses, exploiting the insolubility of the copper counterpart. I'd imagine that a very fine filter media would be crucial to minimize loss.

Tank

blogfast25 - 24-6-2011 at 08:50

Quote: Originally posted by m1tanker78

I'd imagine that a very fine filter media would be crucial to minimize loss.

Tank

Basic copper carbonate is quite dense: a series of sedimentations and decantations (always replacing the decanted solution by fresh water) is just as effective and quite 'hands'free'. Takes a little longer though...

[Edited on 24-6-2011 by blogfast25]