Letter 33094

Electrolysis of aqueous copper sulphate solution [Australia] 

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Dear Sir,

I am a chemistry teacher. I have on many occasions electrolysed 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 [name deleted for privacy due to age of posting]
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 incresase in metallic copper will eventualy result but the solution more or less remains stable. On the other hand, if inert anodes are used, as in you case, the oxigen 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 emeraldous 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 [name deleted for privacy due to age of posting]
- 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 [name deleted for privacy due to age of posting]
- Macclesfield, Cheshire, UK

Thank you, Ann, An elegant answer.

Ferdinand N [name deleted for privacy due to age of posting]
- Como, Western Australia

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
eabjm - Paris France

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