[sdiy] Etching PCBs with non-toxic chemicals

[Csaba Zvekan]

Thanks Adam  and thanks David,

This is highly interesting stuff as I live quiet in a remote place and don't have access to the usual chemicals . How ever my question would be rather the reversal of the chemical . Meaning is there a easy neutralization method where I could safely put it back into the environment (disposal) . Perhaps thin it down and somehow separate the copper , natrium and the vinegar acid so I can feed to my plants as fertilizer ? Or is that process to complex ?

Csaba


On 6 Mar 2011, at 08:46, David G. Dixon wrote:

>>> It seems that one can etch PC boards with vinegar, hydrogen 
>> peroxide, 
>>> and salt. I've kind of sworn off etching my own boards, but this 
>>> discovery tickles me enough that I might have to try it.
>> 
>> I use hydrogen peroxide and hydrochloric (a.k.a. muriatic) 
>> acid.  Same basic reaction, and it works a treat.  I've been 
>> using the same bottle of solution for two years.  
>> Occasionally, each time I use it, I add a teaspoon of 30% 
>> hydrogen peroxide, and occasionally I throw in a dollop of acid.
> 
> I went to the weblink provided in the previous post, and read the
> explanation by the guy's sister, who apparently has a PhD in chemistry.
> Well, too bad she didn't have a PhD in metallurgy, because she completely
> misunderstood the role of sodium chloride in that system.
> 
> The etchant is hydrogen peroxide (H2O2), vinegar or dilute acetic acid
> (CH3COOH) and sodium chloride (NaCl).  The sodium (Na+) and acetate
> (CH3COO-) do nothing in this system, and basically form a solution of sodium
> acetate, which is inert and useless.  Vinegar is just a convenient and
> harmless household acid.  However, the chloride is playing a key role: it is
> stabilizing the cuprous ion, Cu+.  You see, the main oxidant in this system
> is not actually the peroxide, as supposed.  It is in fact the cupric ion,
> Cu2+.  However, for the cupric ion to act as an oxidant, its reduced form,
> cuprous Cu+, must be stable.  However, cuprous is unstable in most aqueous
> solutions.  The chloride ion acts here as a complexing agent, which combines
> with cuprous to form the dichlorocuprate(I) complex, CuCl2(-).  This is a
> very stable ion and serves to allow the existence of cuprous in solution.
> Since cuprous is stable, the following oxidation reaction (called
> "disproportionation") can occur:
> 
> Cu2+ + Cu ==> 2Cu+
> 
> This reaction normally could not occur in the absence of chloride (or some
> other complexing agent -- there are many).  The peroxide serves to reoxidize
> the cuprous back to cupric.  Oxygen from the air would do the same thing,
> but the etching occurs much faster with peroxide.  (One other mistake our
> PhD chemist made was to refer to hydrogen peroxide as a "weakly oxidizing
> agent."  It is, in fact, one of the strongest oxidizing agents available.
> The only reason why household peroxide is weak is because it is diluted to
> only 3% in water.  Getting even one drop of concentrated (30% or 50%) H2O2
> on your skin is a very painful experience.)
> 
> The vinegar serves only to provide acid protons, H+, which are required to
> combine with the peroxide to form water during oxidation of cuprous:
> 
> H2O2 + 2H+ + 2e- ==> 2 H2O
> 
> 2Cu+ ==> 2 Cu2+ + 2e-
> 
> Without the protons, the peroxide cannot act as an oxidizing agent, and will
> instead simply decompose to oxygen and water:
> 
> 2H2O2 ==> 2H2O + O2(g)
> 
> This happens anyway, which is why H2O2 forms bubbles.  If this occurs in
> your etching solution, then it implies that you have too much H2O2.  You
> will probably also etch too fast under those conditions, and your traces
> will take on a somewhat ragged appearance as they will etch from the edges a
> little bit.  This probably is not too much of a problem with dilute
> peroxide, but I sometimes over-etch with my concentrated stuff.
> 
> Thus endeth today's chemistry lesson.
> 
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