Homebrew PCBs

At 06:41 PM 1/15/2007, Stefan wrote:
>On Mon, 15 Jan 2007 23:54:43 +0100, William Carr <Jkirk3279@...>
>wrote:
>
> >
> > I bring this up because I wondered if the process could be used to
> > recharge etchant.   If you put a copper rod in as the sacrificial
> > scrap metal, you'd force copper into the solution.
> > William Carr
>
>
>Actually, you use a graphite rod as one electrode, or both, since you want
>to get copper out of it not into it. I've tried it, but it doesn't work
>very well. Chlorine gas is produced at the electrode (of course! you plate
>the Cu away from CuCl). This chlorine gas would need dissolving in the
>etchant, otherwise you just pollute the air.
><snip>

Some comments:

1. Chlorine gas is very corrosive to metals, including stainless steel. It 
is very soluble in water and very toxic.

2. If your getting chlorine released, you have the wrong electrodes, the 
wrong cell setup or too high a voltage differential.

3. Normally you make two half cells and connect them by a salt bridge. The 
salt bridge consists typically of a U-shaped tube of glass or plastic 
filled with a saturated KCl agar solution (solid, but conducts electricity).

4. If you want to eliminate the salt bridge, you need to make sure the 
right reaction occurs in the two electrodes plus liquid.

5. If one electrode is copper and the other iron, the first reaction would 
have to be Fe -> Fe++. You may end up with a solution that has FeCl2 
instead of FeCl3.

6. You should gradually increase the voltage until current starts flowing. 
If you push too high a voltage, strange reactions will start happening in 
solution.

7. The rate of reaction is determined by the area of the electrodes, the 
conductivity of the solution and temperature.

8. FeCl2 can be converted to FeCl3 via an oxidizing agent, such as H2O2, 
although this also dilutes the solution from the H2O formed.

9. The solution containing FeCl2 can also be converted to FeCl3 
electrolytically, using a salt bridge and another half-cell with a 
sacrificial electrode.

10. If you choose the sacrificial electrode properly, just closing the 
circuit can start the process without external power (you make an 
electromotive cell). E.g., the other half cell could contain iodine or 
permanganate or dichromate plus a carbon electrode.

11. Probably the simplest cell would be a graphite electrode in the etchant 
with a salt bridge to a CuSO4 solution and a copper electrode, and drive 
with sufficient voltage to force the reverse reaction.

All of these techniques involve extra chemicals and cost. One wonders 
whether or not new etchant is not cheaper.

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Robert A. LaBudde                      Email: ral@...
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Virginia Beach, VA 23464-3239          Fax: (757) 467-2947
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