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Letter 33094
Electrolysis of aqueous copper sulphate
solution
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Dear Sir,
I am a chemistry teacher. I have on many occasions electrolized an
aqueous royal blue copper sulphate solution with my students to
observe changes such as reduction of copper onto cathode and
evolution of oxygen from anode. I also find that the solution turns
green over time. The electrodes are "inert" carbon. None of my
colleagues and I are able to effectively explain this colour
change...can you explain the green colour change for us?
Ferdinand N
Como Secondary College - Perth, WA, Australia
++++
The colour change may come from other source, but as a clue,
kindly check the pH readings before and after. If you operate such a
galvanic cell with "active" copper anodes (phosphorized), a slight
increase in metallic copper will eventually result but the solution
more or less remains stable. On the other hand, if inert anodes are
used, as in you case, the oxygen that evolves at the anode will tend
to raise the pH (neutralize the acidity of the solution).
Guillermo Marrufo
Monterrey, NL, Mexico
+++++
I have ALSO left royal blue
copper sulfate [link is to product info at Amazon] solution in
an open PET bottle rather than pour it down the sink...I had no top
to seal it. I left it inadvertently for months on a shelf, without a
top(lid) and it eventually turned emerald green! Is it a case of
CuCl4- ions building up as the solution evaporates???.(There may have
been Cl- ions originally in the bottle from storing tap water in
it)...I am aware that I will need to set up an investigation with
controls to try to reproduce the outcome....
I am very interested in any enlightening replies...I have commented
on this to my colleagues and some academia from the local university
but they're not forthcoming with answers at this stage...
Yours faithfully,
Ferdinand N
- Como, Western Australia
++++++
An aqueous solution of copper(II) sulphate will contain the
hexaaqua ion and the pentaaquamonohydroxyl ion, one of which is
violet and one green (Having been retired many years, I've forgotten
which is which, but the answer was contained in Dr. Alan Sharpe's
Inorganic
Chemistry, published many years ago. It may be included in one of
his later textbooks.) Is it possible that the violet ion changes to
the green ion under the conditions discussed? The commonly
encountered blue solution contains a mixture of the two ions.
Ann B
- Macclesfield, Cheshire, UK
++++++
Thank you, Ann, An elegant answer.
Ferdinand N
- Como, Western Australia
February 5, 2008
Have you considered that CO or CO2 may be involved? Metallic
copper is known to form greenish deposits after atmospheric exposure
for some time. The hydroxy complexes certainly seem probable.
Adrian THOMSON
- Paris France
August 14, 2009
the green colour come from mixing of two simple colors first is
blue of original solution and the second is yellow of finely divided
copper metal which produced
Mohamed Naser
- Egypt
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