Doing bad things to copper



Once again, folks, please don’t try this at home.

I’ve read that certain household cleaners are very bad for copper, but being me, I wanted to see for myself. I decided to apply some forbidden cleaners to copper: hydrogen peroxide, bleach, and muriatic acid. (Okay, muriatic acid is not a household cleaner, really, but I happen to have some for the swimming pool and I wanted to see what it does.)

My first objective was to see what each chemical did to the copper.

  • I expect the hydrogen peroxide, H2O2, to react with the copper to produce copper oxides, CuO and Cu2O. I’m using the 3% concentration stuff you buy at the pharmacy so it’s not too strong.
  • I expect the chlorine bleach — sodium hypochlorite, NaClO, with caustic soda, NaOH — to react with copper to produce cupric chloride, CuCl2, and possibly cupric hydroxide, Cu(OH)2. I’m using concentrated Clorox bleach at 8.25% concentration.
  • Muriatic acid is a dilute form of hydrochloric acid, HCl. I’m using a pool chemical that is labeled at 31.45%. It’s pretty nasty stuff, but the Internet tells me that HCl will not react with copper at all. We’ll see.

I just did a whole post on all the various copper compounds that can form with photos of their colors. My hope is that I can identify the products of each reaction by the colors I see.

My second objective was to see how serious the effects were. Did the reactions produce surface tarnish that I could subsequently erase? Or would there be permanent effects, like etching or pitting?

Here’s how I ran the experiment.

  • I cleaned the base of a copper pot with copper cleaner to remove tarnish and washed it with soap and water to remove the cleaner.
  • I placed hydrogen peroxide, bleach, and muriatic acid on sections of the base of a copper pot. I did not allow the acid and bleach to come into contact.
  • I let them sit for an hour, taking photos every ten minutes or so.
  • I rinsed and washed the areas with soap and water.
  • I cleaned the areas with copper cleaner to remove surface compounds.

Starting point

As above, I cleaned tarnish from the copper and then cleaned chemical residue from the surface. Here’s the cleaned and marked pot. (The black spots are just carbonized grease — this is not one of my better pots.)

Doing bad things to copper


Here we go!

Hydrogen peroxide (H2O2, 3%)

Chlorine bleach (NaClO, 8.25%)

Muriatic acid (HCl, 31.45%)

End states

Hydrogen peroxide

Doing bad things to copper
End state: H2O2
The hydrogen peroxide results are about what I expected. I predicted that the H2O2 would produce CuO (a red compound) and Cu2O (a black compound), and the ring around the reaction area is reddish and brownish.

Chlorine bleach

Doing bad things to copper
End state: Bleach
I expected that the chlorine in the beach would produce copper chloride, and I think that is indeed what is showing up. Copper(II) chloride is a blue-green compound, but the bright green had me stumped until I did some more research (see below).

Muriatic acid

Doing bad things to copper
End state: Muriatic acid
The Internet reassures me that copper and muriatic acid don’t react, so I’m not sure what’s going on here. The acid is certainly etching the heck out of the copper. I also saw some bubbles forming along the edge of the puddle; I’d cleaned the tarnish off the copper, but I suspect copper oxides were spontaneously forming and then reacting with the acid. But I still don’t know what was going on under the puddle.

What’s the bright green stuff?

I noticed a bright acid-green color around the perimeter of the bleach spot, and it didn’t match up with the color of copper(II) chloride, which is blue-green.

I saw the same color again when I neutralized the muriatic acid puddle with baking soda (NaHCO3). The spot fizzled and bubbled as the acid and base worked on each other, and it turned blue-green with copper(II) chloride) but also, again, bright yellow-green. What could it be?

I did some research and what I think is happening is that the chemical reactions are producing water (H2O) in addition to the copper(II) chloride, and the water enables a second chemical reaction to occur. The copper(II) chloride dissolves into the water and if there are more chlorine atoms floating around, they join up too and create CuCl4, an aqueous variant of copper(II) chloride that is green, not blue-green. The more chlorine, the more CuCl4, and the greener the liquid gets.

What I think is happening is that the bleach reaction and the muriatic acid + baking soda reaction both produced H2O. Some of the copper(II) chloride dissolved into the water, and then sucked up more chlorine and shifted to green.

Soap and water wash

After an hour I rinsed everything off and washed the surface with soap and water. Some of the surface stuff wiped right off — here’s how it looked.

Doing bad things to copper

Cleaning with copper cleaner

But there was also true tarnish — copper compounds on the surface that wouldn’t wipe or rinse off. I used copper cleaner to remove them.

Final results

Doing bad things to copper

Overall, each of the chemicals reacted with the copper, which explains the prohibitions against using them. A good copper cleaner reacts with what’s on top of the copper and not the copper itself.

The hydrogen peroxide produced tarnish but not lasting damage. The reaction I saw produced brown copper oxides but aside from the copper atoms stolen to form these there was no lasting damage.

The bleach area produced blue-green tarnish and also light etching. This light etching is evidence of some loss of copper but is not permanent, as it can be erased with an abrasive polish.

But the muriatic acid area is severely etched. It is rough to the touch and unlike anything I’ve seen on a copper pot. I’m honestly bewildered at what happened chemically, because multiple sources on the Internet reassure me that HCl on its own doesn’t react with copper. Acid etching of copper with HCl is a thing but requires hydrogen peroxide to activate the reaction. I was very careful not to let any of the test chemicals touch each other and I have a hard time imagining that the hydrogen peroxide could have snuck over into the acid.

I’m going to keep researching this. The etched spot doesn’t bother me — this was a sacrificial pot — but not knowing the chemistry bugs me. Any ideas?

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