Re: re:Mystery compound from electrolysis

From: Wilco Oelen (photo_at_woelen.nl)
Date: 03/07/05 

Date: 7 Mar 2005 07:42:39 -0800

farooq_w@hotmail.com wrote:
> I think that we should exclude the presence of any Cu(I) compound. As
> you know the solution formed after electrolysis of saturated NaCl has
> bleaching properties from dissolved chlorine in alkaline medium.
> Hypochlorite, no matter how little, is fairly strong oxidizing agent,
> chances of Cu(I) are quite low. Wilco, what do you think about it?
When a graphite rod is used, then you see bubbles of chlorine appearing
(and smell it). With copper wire, no smell of chlorine can be detected
at all. I think that there is a direct oxidation of the copper metal to
either copper (I) or copper (II), without the intermediate formation of
chlorine. This can explain the formation of the dirty yellow stuff as
follows:
- The liquid becomes somewhat alkaline due to formation of OH(-) at the
cathode.
- At the anode, copper (I) and copper (II) are formed.
- The copper (I) and copper (II) react with the hydroxide in the
solution. The color of the observed stuff is somewhere between dirty
brown/yellow and brown/green, depending on the voltage and current
density applied.
Probably the yellow stuff is mostly hydrous copper (I) oxide and the
green stuff probably also is hydrous copper (I) oxide, contaminated
with a relatively large amount of copper (II) hydroxide.

<some text snipped>
> Interesting. Does the color change slowly to brick red (on heating
> perhaps?). You seem to have partially solved my mystery compound.
Since
> I don't know of any other copper compound which is bright yellow.
I've never tried heating, on standing the color does not change, at
least not within several hours. I can imagine two reasons for the
yellow color. Either it is strongly hydrated, or it is yellow, due to
the small size of the particles.
>
> >Crystalline copper (I) oxide, however, is brick-red.
>
> Fehling's test shows the same brick red color of Cu2O.
Yes, I'm familiar with that. An important difference between this test
and the experiment, I did with the soluble ammonia-copper (I) complex
is that in the Fehling's test the copper (I) oxide is formed very
slowly, allowing nice buildup of fine crystals (in that way I prepared
some nice powdery copper (I) oxide for experimental purposes), while in
the experiment with the yellow compound, the precipitate is formed
quickly.
>
> >
> > Wilco

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