Sciencemadness Discussion Board

Grey copper hydroxide

Romix - 26-6-2015 at 18:16

I did electrolysis of pure copper in tap water.
First time I did it, I'v got blue copper hydroxide on anode, and hydrogen on cathode. also blue copper compound swimming on the top of cathode.

How I understand it,Cu = Cu2+ + e2 , Cu+2 + 2OH- = Cu(OH2)
2H+ + 2e = H2

Second and third electrolysis I did, grey hydroxide formed on anode. what is it? and why it changed colour? I used same electrodes, same electrolyte(filtered).


[Edited on 27-6-2015 by Romix]

[Edited on 27-6-2015 by Romix]

Volanschemia - 26-6-2015 at 18:21

Was there a temperature difference the second time? Copper(II) Hydroxide decomposes pretty easily to Copper(II) Oxide (a black/grey insoluble compound) and water:

Cu(OH)2 = CuO + H2O

Romix - 26-6-2015 at 18:28

Quote: Originally posted by TheAustralianScientist  
Was there a temperature difference the second time? Copper(II) Hydroxide decomposes pretty easily to Copper(II) Oxide (a black/grey insoluble compound) and water:

Cu(OH)2 = CuO + H2O


Same temperature, not oxide, it's light grey.

[Edited on 27-6-2015 by Romix]

DraconicAcid - 26-6-2015 at 19:41

A bit of impurity might turn the light blue hydroxide grey. Iron, perhaps?

Romix - 26-6-2015 at 19:56

Quote: Originally posted by DraconicAcid  
A bit of impurity might turn the light blue hydroxide grey. Iron, perhaps?


Electrodes are pure copper, no impurities


[Edited on 27-6-2015 by Romix]

99chemicals - 26-6-2015 at 20:43

It most likely is copper II oxide being formed. The local heating of the solution around the electrode (if you were running enough current) could have caused the oxide to be formed.

Romix - 29-6-2015 at 09:18

Yes you right, it is CuO.
I poured boiling water in a beaker with light blue copper hydroxide and it decomposed into same looking dark-grey oxide.

AJKOER - 1-7-2015 at 06:11

My guess actually is some Cu2O, Cuprous oxide, as well (hence the gray, not black per pure CuO). Here is a supporting source, to quote:


"Cuprous oxide is also formed at the anode in the electrolysis of a solution of cupric sulphate,12 and by heating cupric oxide in steam. "

Your boiling water/steam test is also supportive of the above statement.

Source link:
http://copper.atomistry.com/cuprous_oxide.html

[Edited on 1-7-2015 by AJKOER]

Amos - 1-7-2015 at 06:14

Quote: Originally posted by AJKOER  
My guess actually is Cu2O, Cuprous oxide. Here is a supporting source, to quote:


"Cuprous oxide is also formed at the anode in the electrolysis of a solution of cupric sulphate,12 and by heating cupric oxide in steam. "

Your boiling water/steam test is also supportive of the above statement.

Source link:
http://copper.atomistry.com/cuprous_oxide.html

[Edited on 1-7-2015 by AJKOER]


This formed at the cathode according to the OP, and the color is neither red, brown, or yellow.

AJKOER - 1-7-2015 at 06:18

Amos, I since edited my comment (actually while you were forming your response) to be more inclusive (CuO + Cu2O) based on the coloration observed.

The CuO I once prepared by heating Cu(OH)2 was intensely black. One gets grey by mixing black with a light color.

[Edited on 1-7-2015 by AJKOER]