Edward:

Nobody adds aqueous ammonia, it adds too much water to the etchant.  What you
are proposing, controlling pH by addition of ammonia, is a very good idea,
and often done by sparging in anhydrous ammonia.  The key is to use a sparger
(to create tiny bubbles of ammonia) and to do it slowly, and only when the
pumps are on.

The conversion of ammonia gas to ammonium hydroxide is essentially
instantaneous, but you still do it slowly, to make sure the ammonia bubbles
do not rise to the surface before they are absorbed.

Good luck

Rudy Sedlak
RD Chemical Company

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