Re: copper etching

From: Wilco Oelen (photo_at_woelen.nl)
Date: 12/28/04 

Date: 28 Dec 2004 10:53:21 -0800

farooq_w@hotmail.com wrote:
> Wilco Oelen wrote:
> > farooq_w@hotmail.com wrote:
> > > Wilco,
> > > The chance of chlorine evolving from a iron-HCl-H2O2 mixture is
> > almost
> > > zero becuase of the instant reaction, similiar to
> > hypochlorite-peroxide
> > > reaction, between gaseous Cl2 and H2O2. The noxious vapors are
> oxygen
> > > from peroxide decomposition loaded with HCl vapors from the heat
of
> > the
> > > reaction.
> > With this I have to disagree. I have experience with mixing 30%
H2O2
> > and 30% HCl. If you do this, then a strong smell of Cl2 can be
> noticed.
> > Not so much, that the liquid starts bubbling, but sufficient to be
> > choking. The smell of Cl2 is quite different from the smell of HCl.
> Cl2
> > especially is formed, when the liquid is heated somewhat. This is
why
> > the fumes are so corrosive, much more than plain HCl-fumes.
>
> Wilco,
> Somehow I didn't feel convinced that the HCl-H2O2 mixture could
evolve
> chlorine gas so I checked a very comprehensive monograph on hydrogen
> peroxide; it did not mention the reaction of HCl with H2O2
> (conductivity measurements of HCl in hydorgen peroxide were given
> though), instead it was written that elemental chlorine is reduced to
> chloride by hydrogen peroxide. If you remember the experiments with
> copper where I used concentrated (~12M) HCl, copper turnings with
pure
> 35% H2O2. No odor at all, except vigorous bubbling due to oxygen was
> noticed.
> Lets take the electrode potentials of two half cells:
> Cl2 + 2e ----> 2Cl(-) +1.36 V
> H2O2 + 2H(+) + 2e ----> 2H2O +1.77 V
>
> Then theoretically the oxidation of chloride to chlorine is possible
as
> E for the overall reaction (1.77 - 1.36)V is positive though the
> difference is quite small. But remember that H2O2 has quite weird
> reactions.
>
> Why didn't I notice chlorine from HCl-Cu-H2O2 mixture, using much
more
> concentrated reagents? You mixed concentrated HCl and 30% H2O2, and
> immediately noticed chlorine odor or did it take some time for the
odor
> to be noticeable?

I did the following:

Mix up H2O2 (30%) with HCl (somewhere between 30% and 35%). The ratio
is something like 2 volumes of conc. HCl to 1 volume of 30% H2O2, total
volume was somewhere between 30 and 50 ml. I don't remember exactly
anymore, but much less volume of H2O2 than volume of HCl. Immediately
after mixing, there was no odour of Cl2. I added copper wire, which
quickly dissolved. There was a lot of bubbling of oxygen and the liquid
became quite hot. After some time, I smelled Cl2.

So, what I think is that H2O2 and HCl do react to form Cl2, but only
small amounts, as I already mentioned in the earlier posts and slowly.
But it is sufficient to be a problem when storing the mixture of H2O2
and HCl.

I know that with hypochlorite the reaction is the other way around
(hypochlorite and H2O2 give O2 and water, very nice red light effect!).
In acid environments, the situation is quite different.

But, in order to settle the problem for once and for all, I'll do a
little (and careful) experiment with 30% H2O2 and 37% HCl. I'll let you
know the result in a follow up post. It might also be that the presence
of certain metal ions influence the reaction.

Wilco

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