I was performing copper (II) sulfate electrolysis (graphite electrodes; 10V; 0,01A) today and I observed black precipitate falling from around cathode. I suppose it's copper(II) oxide. I couldn't find information about this reaction. Have you ever read somewhere about this reaction or do you know it?

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    $\begingroup$ How do you know this is not graphite, to begin with? $\endgroup$ – Ivan Neretin May 9 '17 at 19:09
  • $\begingroup$ I thought about it but I used the same electrode in the electrolysis of a few solutions and precipitate occurred only when copper was reduced. I used it as a cathode in the electrolysis of dilute hydrochloric acid when it could be mechanically damaged by hydrogen bubbles (the same voltage) and nothing happened. I am aware that graphite electrodes decay but in this case, it seems to fast and as I mentioned occurred only in this solution. $\endgroup$ – Naunus May 9 '17 at 22:07
  • $\begingroup$ When done without proper procedures, electrochemical reduction of copper salts produce copper as a fine dust. Under water and in presence of air this dust easily oxidizes, forming black copper oxide. $\endgroup$ – permeakra Mar 9 '18 at 13:49

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