What happens when performing electrolysis on a copper coin and a carbon rod in?

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Offline Noah Håkansson

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Hello,

I would like to know what happend here:

I performed an experiment where i used electrolysis on a copper coin and and a carbon rod in normal tap water with salt (saltwater). I cant remember what was on the negative/positive side of the electrolysis but the coin got really dirty covered in black, and the water went to a darkish green color.

Would be really happy if womeone could explain the reaction and logic behind whats going on here!

Thank you!

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Offline chiralSPO

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Hello Noah,

I'm happy to help out, but you will see I also have a lot of questions that will aid us to figure out what happened and why...

The greenish color produced in the solution is almost certainly dissolved copper chloride.

The simplest way this could have formed is if the copper electrode was the positive electrode (anode), where the following half reaction could have occurred:

Cu → Cu2+ + 2 e

The Cu2+ ions would then react with chloride in the solution (from the salt):

Cu2+ + 4 Cl → CuCl42–

It is the CuCl42– ion that is so green.

The final green compound could also be formed if the copper was the cathode (negative electrode). The chloride ions could be oxidized at the carbon (graphite?) anode:

2 Cl → Cl2 + 2 e

Then the produced chlorine could react with copper and chloride:

Cl2 + Cu + 2 Cl → CuCl42–


I don't have a good answer about the coin turning black. When I have done similar experiments in the past, my copper anodes always ended up looking really clean and shiny because the copper dissolved off exposing fresh copper metal. However, it is possible that, due to different experimental conditions, you observed a reaction in which the copper anode dissolved unevenly, producing a very rough structure that would appear dark despite being pure copper.

Or, if the copper were the cathode, it could be reducing dissolved copper ions back to metallic copper, which could form a very rough surface on the coin, that would appear dark.

Alternatively, the black surface could be a copper (I) compound (unlikely) or have nothing to do with copper. Was the coin very clean to begin with? Do you know that it was copper? (For instance US pennies are mostly zinc now, and only have a thin skin of copper.) Was there black material coming off of the other electrode? Was there also a black solid present in the water? Did the "darkish" part of the darkish green water settle if left alone for a while?

What was your electric source? Was is at AC or DC? Do you know the voltage or current? I am assuming the electrodes were attached with alligator clips (did either of the clips corrode?)

Also, what happened at the carbon electrode? Did is fizz or bubble?

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Offline Noah Håkansson

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Hallo and thanks (:

Ye, Im from Sweden so im not to good with the chemistry terms in english but yes it was graphite i meant.

The swedish 50-öring coin consists of an alloy of 97 percent copper, 2.5 percent zinc and 0.5 percent tin. It weighs 3.70 grams.

The graphite rod did fizzle and bubble, and the black darkish part of the water was like little particles/bits in the water.

The black did come of the graphite rod, yes.

im not really sure what you mean about AC or DC but the voltage was at 10volts.

The coin was attatched to an alligator clip, dont know if they corroded.

Thanks again!


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Offline chiralSPO

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Hej Noah :-) (and that's all the Swedish I know)

There are multiple types of carbon. Graphite is most common, but there are also vitreous carbon, and boron-doped diamond electrodes.

I think 2.5 % zinc and 0.5 % tin shouldn't change the chemistry too much. The zinc probably gets consumed first, and then the copper begins to oxidize.

Was the bubbling at the graphite electrode particularly violent (enough to physically break the electrode apart)? Otherwise the graphite was probably corroded electrochemically. If very violent, I would guess that you were producing hydrogen at a graphite cathode:

2 H2O + 2 e → H2 + 2 OH

If not particularly vigorous, it could be oxidation of a graphite anode.
C + 2 H2O → CO2 + 4 H+ + 4 e

This would generate carbon dioxide gas (some of which would dissolve in the water) and would corrode the graphite electrode significantly.

AC means alternating current (each electrode is positive and negative at different times), and DC means direct current (one electrode is positive and one is negative, for the whole time).

10 V is very high for this type of reaction. I am not surprised that you see decomposition of one or more of your electrodes. Voltages between 1.5 V and 3.0 V should offer more controlled (and slower) reactions.

I would also recommend against using sodium chloride (table salt) as the electrolyte because chloride can be oxidized to chlorine or hypochlorite, which will corrode your electrodes, and could be dangerous if you make enough. Sodium bicarbonate (baking soda) can serve as a weak electrolyte. You can also use salts of sodium phosphate (which you should be able to find at a gardening store. Also dilute sulfuric acid or sodium hydroxide are excellent electrolytes, but you have to be careful with such strong acids and bases, even when they are dilute.

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Offline chiralSPO

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You can try the experiment again, keeping track of which one was + and which was – to see what is going on.

But it sounds like oxidation of the coin and reaction with chloride caused the green color, and the black stuff came from the graphite electrode.

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Offline Noah Håkansson

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Hej igen (: (hey again)

The bubbling was not particularly vigorous.

Can confirm it was DC now that i know what it is (:

Have no possibility of redoing the experiment, it was done in school in the beggnining of december.

I would also recommend against using sodium chloride (table salt) as the electrolyte because chloride can be oxidized to chlorine or hypochlorite, which will corrode your electrodes, and could be dangerous if you make enough.

I guess this is why our teacher, a bit concerned asked us to stop the electrolysis |:



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Offline Noah Håkansson

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Hey, finally got my notes back and the Copper coin was indeed the positive electrode.

I understand from your first answer what happend to the water and the copper, but what reaction is behind the black stuff from the electrode going onto the coin?

Thanks again!


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Offline chiralSPO

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I'm not exactly sure what is causing the black stuff on the copper coin, but perhaps it is just black material from the graphite rod that gets stuck to the copper as it corrodes, or a very rough copper surface (as suggested above)...

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Offline Discouraged101

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Seen this post so thought id also make my enquiry here aha.

Okay so ive been in the process of making copper oxide and copper II oxide and i have too come across this issue

To start i was using 120 grams of salt to 440ml of tap water (i know distilled is better), took ages to dissolve then ages to filter and remove all salt traces.

I was also using 12 VDC at around 3-5 amps and while leaving it to dissolve over night it had turned black at a point and ruined the solution (so i think currently). I noticed that when removing the copper anode that had been dissolving that its entire surface had this black type soot over it and that's what had caused this solution to become ruined.

So i rethought it and dropped the voltage, re set up and went again, It seemed like a success after 4 hours at 5 VDC 4-5 Amps, so i removed my good solution set up again and let it run over night again.... Black solution again when i woke. So i disposed of the solution and kept going,

Still in the process of working out if its the NaCl content thats dropping over the period and causing this to occur or if something else is occuring here

But its all pure 99.8% copper for my anode and cathode and i do use alligator clips which stay out of the solution so no contaminants except for reactions between the solution and metal

Kinda trying to get back into chemistry currently aha

(down to 15 grams of salt to 400ml of water now and the black soot reaction seems to be more frequent)

Will keep it updated

Can any copper reactions turn black from lack of electrolyte or over extended periods of electrolysis?

Thanks guys

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Offline Discouraged101

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Also forgot to mention that i also noticed that the copper anode will gain a white like soot first then black after.

Also i have been left with quiet a few different coloured copper oxides which has me thinking aha

Could that be a higher copper concentration? Or is it impurities like id imagine

I have a brownish copper oxide

An orange(copper) copper oxide

And a red copper oxide

They all turn green when introduced to a little HCl

Thanks again

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Offline chiralSPO

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Discouraged101: The black (and brown, red, orange) soot could be copper nanoparticles. It is important to remember that there are reactions that occur at both the anode and the cathode. A common outcome is that the anode is oxidized, causing copper ions to go into solution, and then these same copper ions are reduced back to copper metal at the cathode. At the high currents you reported, there is presumably also significant generation of hydrogen bubbles at the cathode--this will knock the deposited copper off of the cathode, and could generate a suspension of copper particles (the different colors are representative of different shapes and sizes of particles).


Are you trying to make copper (I) oxide (Cu2O) or copper (II) oxide (CuO)?

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Offline Discouraged101

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Well see i used to do electroplating for 3-4 years and vaguely remember the zinc balls gaining a black soot type passavate over them that prevented them from breaking down any further into the KOH solution, we would either up the KOH levels or bring the zinc cages out, dip them in the HCl and then once cleaned of the crap, they would be rinsed and back in they went

My knowledge of nanoparticles is almost nonexistant except for a couple of things.

Would the nano particles just be more copper though? and if that were the case the anode should still dissolve into the solution no as its still copper?

The black soot is the only soot there, them other colours are just the end dried CuO, so i have a red, orange and brown CuO and the black is what mad the orange process turn brown so it makes me thing contaminates or some sort aha

Ive seen the the copper in the solution accumulating on the cathode which is something im used to with electroplating just not on such a thickness but i just knock it off back into the solution to be processed.

Hydrogen is definitely being created as i know that lung closing smell anywhere, not to much of the Na bath but the KOH bath i have just started it releasing Hydrogen like all sh1t which is again something id expect

Im fully aware there are quiet a few reactions going on in there with Na, Cl, H, O and Cu, just trying to figure out if the process is the same where it is going wrong and going black, which going off so far im thinking lack of Na in the Solution is causing the black soot as i had seen it a few times on the zinc cages at work when low on KOH would would either top up with more KOH or drop the zinc cages in the HCl for a minute or 2 to clean them up nice then rinse and back into the tanks and that black soot also used to make it almost impossible to break the metal down further

So a red, orange and a brown CuO are the same just different sizes? I mean i dont know but going off how these colours came about i just dont see that being the case as the brown CuO was actually Orange until the black soot stuffed the anode up and then leached into the solution causing it to go black and turned the orange CuO brown once rinsed and processed.

Trying to make CuO but worse comes to worse i believe CU2O will suffice

Would an electron microscope be what im after if trying to solve this sh1t myself?

Thanks mate
« Last Edit: 18/02/2017 06:06:44 by Discouraged101 »