[DigiTan]

I'm currently using an opaque Lewis acid to etch out patterns for copper circuitboards.  I want to retrofit the system to work with a different etchant: most likely a Ammonium persulfate solution.

I'm not 100% sure what the products of that reaction will be.  I know it will produce a copper salt, and I need to know the remaining products so I can dispose of the used solution properly.  Can anyone show me how to balance out a reaction beteen copper and Ammonium persulfate?

Also, in case of a spill, what's a safest way to neutralize the ammonium persulfate?

[Alpha-Omega]

Copper is  oxidized by persulfate to form soluble
copper sulfate:


Cu + S₂O₈^-2 ——> CuSO₄ + SO₄^-2

and here is the MSDS sheet for copper(II) sulfate:  http://ptcl.chem.ox.ac.uk/~hmc/hsci/chemicals/copper_II_sulfate.html

Data on Ammonium persulfate:
 http://www.fmcchemicals.com/LinkClick.aspx?fileticket=y%2F0DZcxPM4w%3D&tabid=1468&mid=2563

[DigiTan]

Okay, a few more questions here...

Cu + S₂O₈^-2 ——> CuSO₄ + SO₄^-2

In this notation, I'm guessing the S₂O₈^-2 can represent ammonium persulfate, plus others like Sodium and Potassium sulfates?  Also, where does the ammonium ion fit in on the right hand side of the equation?  

Since these are anhydrous crystals being mixed with water, should I treat the ammonium persulfate as a hydrate?

Also, the brochure on my ammonium persulfate says the solution is non-reusable due to "out gassing."  I'm assuming this is a release of ammonia gas?

Finally, if the solution gets contaminated with a small amount of NaOH, would sodium sulfate be one of the products?

[Borek]

To emphasize on what is going on I would put the reaction equation this way:

Cu + S₂O₈^2- -> Cu²⁺ + 2SO₄^2-

Doesn't matter what is counterion for the persulfate, it is just spectator - it doesn't react.

Since these are anhydrous crystals being mixed with water, should I treat the ammonium persulfate as a hydrate?

No... But to be honest I have no idea what do you mean. Please elaborate, I suppose you are misunderstanding something.

Also, the brochure on my ammonium persulfate says the solution is non-reusable due to "out gassing."  I'm assuming this is a release of ammonia gas?

Ammonia should not evolve here, unless you add strong base. No idea what they mean by "out gassing".

Finally, if the solution gets contaminated with a small amount of NaOH, would sodium sulfate be one of the products?

Interesting things may start to happen. First of all, copper hydroxide will precipitate. Ammonium cation will react with strong base, yielding ammonia. Ammonia can partially dissolve copper hydroxide... But most likely first stage (precipitation of blue hydroxide, that soon decomposes to black oxide) will dominate. So you will end with black precipitate and dissolved sodium sulfate.

[DigiTan]

Okay, I guess you can disregard my question about the hydrate.  I may have had a misconception of what  hydration is.

Ammonia should not evolve here, unless you add strong base. No idea what they mean by "out gassing".

Well, I guess that makes two of us.  All I was able to learn is that the datasheet recommends using the solution within a day of hydrating it.  They say the same of their sodium bisulfate.  My first guess was there was some kind of decomposition at work.  Any other idea of why this solution would go bad in such a short time?

[Borek]

Arghh, decomposition is a key word here. I can't believe I have not realized it earlier

2S₂O₈^2- + 2H₂O -> 4SO₄^2- + O₂ + 4H⁺

It is multistage and goes through Caro's acid and hydrogen peroxide, but in the end solution gets acidified and oxygen evolves, hence degassing.

Sodium bisulfate solution should be stable. Perhaps you mean bisulfite? This one will be oxidized by air oxygen.

[DigiTan]

Okay, I guess I have some redox reactions to practice here.  Can you show me all the intermediate steps used to arrive at the final equation?  I'm still trying to get a handle on balancing multistage reactions.

Sodium bisulfate solution should be stable. Perhaps you mean bisulfite? This one will be oxidized by air oxygen.

Whoops, I meant to say "sodium persulfate" on that one.  How can you tell that sodium bisulfate is stable?

[Borek]

S₂O₈^2- + H₂O -> SO₅^2- + SO₄^2- + 2H⁺

SO₅^2- + H₂O -> SO₄^2- + H₂O₂

Sulfuric acid and sulfates are stable in general.

[DigiTan]

Cu + S₂O₈^2- -> Cu²⁺ + 2SO₄^2-

What is the molecular formula for this going to be?  I'm having trouble learning how to get a molecular equation from the just the ionic equation.  Is it something like...

(NH₄)₂(S₂O₈) + Cu <--> Cu(SO₄) + (NH₄)₂(SO₄)

[Borek]

Cu + S₂O₈^2- -> Cu²⁺ + 2SO₄^2-

What is the molecular formula for this going to be?  I'm having trouble learning how to get a molecular equation from the just the ionic equation.  Is it something like...

(NH₄)₂(S₂O₈) + Cu <--> Cu(SO₄) + (NH₄)₂(SO₄)

That's OK, although some parentheses are superficial:

(NH₄)₂S₂O₈ + Cu -> CuSO₄ + (NH₄)₂SO₄

Note: it is not always simple to came out with molecular reaction, in some cases it may require some stupid tricks. Sometimes it is not worth the hassle.

[DigiTan]

How about this Caro's acid?  It kind of seems like it could also react with the copper to get CuSO₅.  Can that happen?

[Borek]

Doubtfull in the presence of metallic copper.

[DigiTan]

How can you tell for sure?  Does it have something to do with the redox potentials chart?

[PanCerowany]

Checking potential will help but this is strong oxidizing agent in presence of metal that can be oxidized so you will just get more copper sulfate.

[Borek]

That's more or less what I was aiming at.