So, I was doing a little experimental electrolysis in my basement. I used 2 (presumably) galvanized steel screws as my electrodes and ran 5A through tap water with a lot of NaCl dissolved in it (to decrease resistance). It worked great: I got a lot of bubbles from the - side. Problem is, my cup started filling up with this black crud.

Now, I have a little plastic disposable cup with really salt water, 2 screws (black with what I presume is some corrosion), and a lot of black and red insoluble crud. How do I get rid of it?

Also, what it that crud (most likely)?

EDIT: I figured out that putting NaCl in an electrolytic cell means that I was actually generating chlorine gas instead of oxygen gas. That means that I have a bunch of iron and zinc chlorides in that water. It's also probably pretty basic (I saw no bubbling at that electrode). Any disposal problems?

  • $\begingroup$ most likely iron oxides, pretty safe to dispose. $\endgroup$
    – permeakra
    May 26, 2016 at 21:12
  • $\begingroup$ @permeakra I update the question to reflect my current research: it turns out that I was generating Cl2 which reacted with electrodes (I think). $\endgroup$
    – dpdt
    May 26, 2016 at 21:17

1 Answer 1


The black one is probably $\ce{ZnFe2O4}$; insoluble compound of very weakly toxic metal ions, or $\ce{Fe3O4}$. The red one is $\ce{Fe2O3}$ In terms of disposal all above counts as non-toxic mineral materials, similar to sand.

It is also possible to have some carbon in this setup, but I doubt it.

Anyway, you have nothing too toxic in this setup, you can safely dump it into ground without any meaningful lasting environmental consequences.

The problem may arise, if the steel used was stainless steel or another iron alloy, but it doesn't seem to be the case from your description (and they are rarely galvanized anyway)


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