RE: Etching Solution PCBs..how to handle it?

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From: Brad Hernlem (alihernlem@hotmail.com)
Date: Wed Jun 05 2002 - 09:20:01 CDT


Leon Heller (leon_heller@hotmail.com) writes:

>...BTW, I don't understand all the stuff about adding acid to water (or
>vice-versa) in this context. FeCl3 isn't an acid, as such, like sulphuric
>or hydrochloric, and generates very little heat when mixed with water. ...

JOSE VICENTE (vicente@supernet.com.br) writes:

>.... Ferric Chloride is not an acid, it is a salt not to dangerous,...

Leon, Jose, et al.,

>From my experience using the ready made solutions
(ferric chloride etchant is also sold as solid FeCl3),
there is not enough exothermicity to worry about
diluting one way or the other. I suppose that the
original poster meant dissolving the solid form.

Anyway, on whether or not "ferric chloride" is an
acid or not, YES and YES. Yes, it is an acid in its
own right. Ferric ion is a "Lewis acid" and has a
strong "desire" to rip water apart, surrounding
itself with hydroxyls and liberating protons (H+,
those things that in high concentration define
low pH). If it manages to get enough of the hydroxyls
around itself it will precipitate. It is the same
with copper and most metals but to a weaker
extent. Metal ions in solution are more of a problem
environmentally and so that is why I recommended
neutralizing the etchant (which increases the pH,
i.e. lowers the H+ concentration, reducing the
competition for OH- ions, so the metals can more
easily glom on to them and fall out of solution).

And the second YES is because some commercial
etching solutions include a little bit of
hydrochloric acid ... probably to help keep everything
in solution. See MSDS at:

http://www.mgchemicals.com/msds/415.html

Brad KG6IOE

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