[balaruman]

Short version :
what happens (chemically) when you mix acetic acid, hydrogen peroxide, sodium chloride and copper ?

Long version :
I've been trying to make a non-smelly, non-toxic, blue(ish) wood stain with household ingredients. I need to apply indoors in a child's room -  commercial woodstains are too stinky while drying.
I've searched for a recipe, found one that said "copper coins plus vinegar" and tried it : a week later - no results. Search some more : "add salt" - did it, another week , still nothing (the coins got really shiny though). Even more google : "add hydrogen peroxide". Instant results, solution starts coloring green. A few minutes of vigorous stirring, let sit for a few hours and the liquid became a beautiful deep emerald. Not blue, but good enough , or so I thought until I opened the jar and realized I failed the "non smelly" objective : it had an strong bleach odour. I tried to find out at least if it's safe to use, but couldn't find a clear answer.  The internet says NaCl doesn't react with either acetic acid or peroxide- but unless my nose is lying to me, the table salt  did react somehow since there was no other source of chlorine.

So, wise chemists, my question is what happened in the jar ?
Was I right to dump it fearing a health hazard ?

Thank you.


Details :
Reaction took place in a glass jar, covered but not airtight.
Acetic acid : wine vinegar 9%, aprox. 100ml.
NaCl : table salt, dissolved to saturation.
Peroxide : regular pharmacy stuff, 3%, 30-50ml.
Coins : copper plated steel. Completely lost their shine and became porous after the reaction, looked and felt like freshly broken brick.

[Hunter2]

You have an acidic solution and peroxide will produce chlorine if chloride is present. This solution will etch the copper. The result is the green colored solution of Cu²⁺ Ions. Chloride make it more green, water more blue.

[Intanjir]

The most straightforward way to get copper ions into solution is to just use a simple electrolysis set up with a copper electrode.

Anyways I would question whether a copper salt is sufficiently non-toxic.
The tolerable upper intake level is a mere 1 mg for the small crawly-type offspring(1-3 years).

Copper salts are a rather soluble form of a metal which is normally rather resistant to dissolution as you experienced.

[balaruman]

Thanks for the quick replies guys.

So it was indeed chlorine I smelled. Well, too bad, it was a nice colour but I don't want that in my kid's room.

@Intanjir : I mostly worry about released gases - that's why I don't use commercial solvent based woodstains. My "crawly-type" (I like the expression ) can't reach to lick it - it's a high shelf. I will also apply a water based laquer after staining.

Maybe I'll try the electrolysis when I got the time to set up and supervise it - the vinegar methods have the advantage they require almost no time, which is in short supply for me (small baby and stuff...)

After the experiment described above I tried another one, this time without the salt - just vinegar and peroxide, same type of coins. Now (three days later) the solution is a murky yellow, with some black stuff oozing from the coins when stirred. The copper seems unaffected (still shiny) but some of the coins developed black spots, so I assume the chemicals went straight for the steel core. I tested the staining properties - not very good, two coatings and it's barely visible.

Now after reading Hunter's post I think I'll try again with NaCl, but just a little, and a more diluted solution. Also with pure copper  -some electrical wire - don't really know why the folk wisdom recommends "pennies" which usually have other metals in them as well.

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Slightly offtopic, but I'll just add this here as a reference, in case anyone looks for "homemade woodstain" : an older but more succesful experiment of mine : Vinegar + salt + stainless steel kitchen scrubber, let soak for about 2 weeks until it turns a deep purple.
It's  very effective at staining wood, one coating is usually enough for a dark bluish-grey colour (exact nuance depends on type of wood).
Not sure if the salt is required, but that's how I did it.
Why stainless steel ? Most resources on the internet use steel wool, but I didn't have any on hand. Dish scrubber works just fine, but requires more patience. Also it's probably why it turned purple instead of red (chromium or nickel compounds ?).
 

 

[billnotgatez]

@balaruman

Out of curiosity which country's copper coated coins are you using?

I wonder if the chlorine smell will go away after the reactions are complete and the chlorine is entirely evaporated?

Why not stain the wood with blue berry juice?

Is there a link you can post that exemplifies your proposed staining preparation?

[balaruman]

@billnotgatez :

"Out of curiosity which country's copper coated coins are you using?"

Coins : https://en.wikipedia.org/wiki/Five_bani

"I wonder if the chlorine smell will go away after the reactions are complete and the chlorine is entirely evaporated?"

Possibly. That's why I will try it again with less salt to make sure the chlorine source is exhausted and not run the risk of it still reacting and releasing gases after applied to the wood shelf. My meager chemistry knowledge doesn't allow for me to compute exact quantities required to ensure all reagents are balanced for a complete reaction, so I'll just try a practical "use less, wait and sniff" approach.

"Why not stain the wood with blue berry juice?"

I've read about this method as well, but not sure if fruit juices are as stable as copper compounds over time when exposed to air. I won't be using regular laquer over the stain (due to solvent smell) but hobby-type children-safe laquer, so sealing won't be very good.

"Is there a link you can post that exemplifies your proposed staining preparation?"

Sorry, can't remember the exact locations I used as reference. Just type "penny wood stain" or some such on google and click randomly on links : that was my "research methodology"  plus some quick wikipedia reading whenever I came upon some more specific chemical reaction descriptions.

One example : http://www.moneypit.com/question-and-answer/penny-stain-how-make

What I can tell you about my "research" is that it left me somewhat confused. For example  some sources stated that vinegar+peroxide will result in peracetic acid wich will etch copper (it doesn't appear to do so, see my second salt-less experiment), others that it is the HCl that will form (if salt is added) that dissolves the copper.