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Author: Subject: Safely decomposing hydrogen peroxide in solution?
aonomus
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[*] posted on 24-7-2010 at 19:58
Safely decomposing hydrogen peroxide in solution?


So I'm in a bit of a situation, I use CuCl2/HCl/H2O2 copper etchant for circuit board manufacturing for my electronics hobby, and I want to concentrate the etchant so that I can have faster etches and deal with lower volumes of CuCl2.

The problem is that before I can boil it down and distill off water/HCl, I need to get rid of the H2O2. As hard as it is for me to believe that H2O2 can be stable in solution with a high concentration of metal ions, it is, somehow. I did a test prior to distillation and it turned starch/KI paper dark black. I reconfirmed with a positive and negative control to make sure it wasn't the paper, and sure enough there is H2O2 stable in the etchant.

So what can I do to decompose that H2O2, without really contaminating the Cu(II) solution with any major level of other metal ions?

I'm worried that like any concentrated peroxide, looking at it wrong could cause an unexpected exotherm or rapid decomposition. I'm also worried if I attempt to heat or concentrate it before dealing with this, that the H2O2 will also oxidize the excess Cl- into Cl2(g).

Any thoughts in the crowd?
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[*] posted on 24-7-2010 at 21:17


Quote: Originally posted by aonomus  

I'm worried that like any concentrated peroxide, looking at it wrong could cause an unexpected exotherm or rapid decomposition. I'm also worried if I attempt to heat or concentrate it before dealing with this, that the H2O2 will also oxidize the excess Cl- into Cl2(g).


On a very small scale (1-2 mL) heat to boiling is used to decompose H2O2 as part of the analysis of Gp III cations. I have done this and you can see copious bubbles of O2 being released.

My general chem text does indeed say that this is an exothermic reaction (-196 kJ/mole). It also states that H2O2 is a reductant in the presence of Cl2, ie,

Cl2(aq) + H2O2(aq) + 2H2O(l) --> O2 +2H3O+(aq) + 2Cl-(aq)

I would try boiling some of your etchant on a small scale, first, to see how it goes.

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aonomus
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[*] posted on 24-7-2010 at 21:58


Well, I had already started to heat up the batch when I remembered to test for peroxides, at about 50degC, I smelled something funny (might have been the brand new heating mantle breaking in, had a bit of brown tint to it when I pulled it off the flask, could have been old residual oil bath oil burning off). I tested the residue in the beaker for peroxides and it tested positive, so I tested the contents (positive also) and did a +ve/-ve control. The flask was still at around 45degC after 20-30 minutes, so perhaps decomposition was occurring.

Also I was running on the general trend that H2O2 + H3O+ + X- --> X2 + mass balance
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[*] posted on 25-7-2010 at 02:35


"I did a test prior to distillation and it turned starch/KI paper dark black. I reconfirmed with a positive and negative control to make sure it wasn't the paper"

You do know that copper (II) salts will release I2 from KI don't you?
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[*] posted on 25-7-2010 at 08:16


I did not do a test with another copper (II) salt such as CuSO4. Lesson learned.
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[*] posted on 26-7-2010 at 07:06


You can even use copper(II) alone for etching copper-plated printed circuit boards. Dissolve some copper(II) salt in concentrated hydrochloric acid and then use that for etching. The liquid becomes very dark when this is done. Allow contact with fresh air, such that oxygen can be taken up from the air. In fact, the more you have used the liquid, the better it works and the more you get.

The etching reaction is the following:

CuCl4(2-) + Cu --> 2 CuCl2(-)

CuCl2(-) contains copper(I) and this is very easily oxidized by oxygen from air when sufficient HCl is present. So, indirectly the oxygen from air acts as oxidizer. You only need to add some conc. HCl when the liquid is used for etching and then leave it in contact with air for a while. In this way the only chemical you need to pay for is HCl.

If you don't have copper(II) salts to start with, then you can get it started with some H2O2 and then proceed as described above.




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[*] posted on 26-7-2010 at 10:50


Quote: Originally posted by aonomus  
So I'm in a bit of a situation, I use CuCl2/HCl/H2O2 copper etchant for circuit board manufacturing for my electronics hobby, and I want to concentrate the etchant so that I can have faster etches and deal with lower volumes of CuCl2.



William T Harris
Chemical Milling : The Technology of Cutting Materials by Etching
Oxford 1976



Copper-etching-800.jpg - 327kB
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[*] posted on 26-7-2010 at 15:46


You might consider skipping the H2O2 and regenerating your etchant with air -- this will avoid diluting your etchant.

When you use H2O2 the regeneration reaction is:

2CuCl + 2HCl + H2O2 --> 2CuCl2 + 2H2O

But when you use air the regeneration goes like:

2CuCl + 2HCl +(½)O2 --> 2CuCl2 + H2O

Thus the H2O2 regeneration produces twice as much water as the air regeneration. And, of course, any H2O2 you are using [you didn't specify the concentration] usless it's rocket fuel is mostly water which further dilutes the etchant.

I have also etched circuit boards using CuCl2 and I always regenerate by bubbling air thru the etchant -- a small aquarium pump running for a few hours or overnight does real well.





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[*] posted on 8-8-2010 at 17:07


Hello !
I am also a new member. Would a newcomer be warmly welcome here? Good day you guy !


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