Re: Question: Reaction between copper (II) and copper (0) in concentrated HCl

From: Mohammed Farooq (farooq_w_at_hotmail.com)
Date: 08/27/04 

Date: 27 Aug 2004 07:34:50 -0700

photo@woelen.nl (Wilco Oelen) wrote in message
> I'll try this. See if I can get some crystals of K2[CuCl4].

Please do. Do this at a mAcro-scale so that we can compare the
properties of this one and your complex. In the meantime I will search
for a more refined method of preparing K2[CuCl4].

> I prepared a CuCl solution with excess copper (as also described in
> the last URL I have given) and added some CuCl2-solution. Mixing the
> almost colorless CuCl-solution and a green CuCl2-solution results in a
> very dark brown (almost black) liquid at once. This is precisely what
> I expected.

This is interesting, indicating that it is indeed a mixed valency
complex. Our next target is somehow isolate the brown colored species.
Can you do one more thing; To the brown solution add few mL of 33%
H2O2 (to avoid dilution- hence any color change due to dilution) and
note the color change. If there is no color change then heat slowly to
avoid vigorous decomposition of peroxide. If there is no color change
then you will rule out the existence of Cu(I) in it. I believe if any
mixed valency Cu(I)-Cu(II) complex is formed it would not be so stable
to resist oxidation by hydrogen peroxide.

Note that once you added a base to coper salt, first a blue ppt is
formed, when more base is added it becomes dark brown, somthing
similiar to the color your complex. Does that give us a clue?

In your last experiment you describe a colored precipitate which is
soemthing like brown yellow solid. I will tell you of an interesting
experiment that I used to do when I was in 8th grade. If you
electrolyse a concentrated NaCl solution using thin copper wires as
electrodes and use high voltage ie 9 Volt DC current, you would get
exactly the same yellow colored voluminous ppt. sitting in the bottom
of the electrolytic cell. You would see the anode getting green and
after 15 minutes or so something yellow colored settles in the bottom.
I used to prepare metal hydroxides this way, using the required metal
as anode and carbon cathode (taken out from a dry cell, washed and
boiled in water), conc. brine solution and high voltage. Zinc and iron
behave similarly. In due course you might do some experiments with
them. A modification that one can try is to electolyse a relatively
concentrated HCl (in an open room) using copper electrodes, or try
different metals and get nice and new results.

> I'll also try mixing with some ethanol. I do not have pure ethanol
> (this is incredibly expensive in the Netherlands, due to taxes), but
> denatured ethanol. This denatured alcohol is colorless, mixes
> perfectly well with water and on evaporation does not leave any
> residue, just as pure ethanol. I don't know what is used for
> denaturing it, I think some ester, because it has a somewhat fruity
> odour. I hope it is suitable. I do have pure methanol, do you think
> that is suitable?

Methanol is quite toxic, its vapor makes my head ache and so does
alcohol (perhaps allergic to organic solvents) so for the time being
use denatured ethanol. Our purpose is reduce the polariy of solvent
and since methanol is more polar than ethanol so it would not be as
effective as the latter. You can very well use denatured alcohol for
hastening the crystallization process.

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