About a month ago, I ran a small electrolysis experiment in an old fish bowl with a $\ce{MgSO4}$ electrolyte and copper electrodes. It ran for about 3-4 hours.

Now, a month later, I am left with about 40% of the initial volume of water and a blue copper compound on the bottom of the bowl. I believe this is $\ce{Cu(OH)2}$. I've provided a picture.

This has been sitting outside for the entire month under a little greenhouse type thing (but air can still get at it). As winter is coming, I was hoping to collect what I've formed and dispose of it properly. So

1) Could it be $\ce{Cu(OH)2}$?

2) Is there a way I can get the solids out? Or should I just pour everything into a plastic container for disposal?


  1. Unlikely. Copper hydroxide adsorbs carbon dioxide from air rather fast, forming basic carbonates. if you want to test, try to boil it. Copper hydroxyde looses water under mild heating, forming black copper oxide.

    Most likely it is either basic copper carbonate or one of basic copper sulfates or a mix of both. I highly doubt that it is a pure copper sulfate, as Mg had to go somewhere.

  2. If you want to be a responsible citizen (which is not actually much a point with relatively safe copper salts), add some soda, leave overnight and then filter off the precipitate with filter paper. If no filter paper is present, cotton white, toilet paper, and similar should work, just be careful to not tear them. The precipitate then may be heated until black and disposed any way suitable.


It's probably copper sulfate ($\ce{CuSO4}$) residue (the electrolysis of magnesium sulphate with copper electrodes would yield a decent amount of copper and sulphate ions which would precipitate in absence of solvent i.e. water). A good way to test if it's $\ce{Cu(OH)2}$ would be to use a pH strip in a dissolved solution of your blue salt, and see if the pH strip indicates base (blue color). If it's decently neutral, you likely have copper sulfate.

Copper sulfate is used in stump remover as a common household product. It's fine to dispose of in a plastic bottle etc.

You can also dry it (for example by heating) and see if it turns white. The anhydrous form of $\ce{CuSO4}$ is white. Copper hydroxide wouldn't change from it's blue-green color upon heating.

P.S. nice moth.

  • 1
    $\begingroup$ This does not look like copper sulfate to me at all, nor does it resemble pure hydroxide (both should be way brighter). Some form of basic carbonate, maybe. Also, heating copper hydroxide does not leave it unchanged - it turns to oxide (black). $\endgroup$ – Ivan Neretin Sep 29 '15 at 5:56
  • $\begingroup$ It indeed looks more like copper carbonate en.wikipedia.org/wiki/Basic_copper_carbonate . Copper sulfate has a more intense blue color! I think you can dispose it down the drain. If you really don't know, visit your local high school. They probably have a heavy metal waste container craving for some copper ;-) $\endgroup$ – JR_ Sep 29 '15 at 8:23
  • $\begingroup$ I'd say both of you are right, by appearance. That leaves the question of how the OP got such a substantial amount of carbonate in the sample in the first place, considering he used a sulphate salt to begin with. $\endgroup$ – khaverim Sep 30 '15 at 15:03

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