Brian and I beat this question seriously into the ground some time back, and we do not need to do it again, but I do believe that we can all agree that the best type of automatic monitoring will vary with the type of chemistry, and perhaps even the type of soil.  If you are lucky enough to be able to get hold of a sample of cleaner solution that is functioning only marginally, or not functioning, and can characterize it, and perhaps even try to "revive" it, then re-characterize it, you might get some good clues on how best to control it.

In the world of PCB bareboard fabrication, wherein I learned my lessons about control, it is legendary that photoresist stripper is almost always monitored by titration... and that a new stripper bath always far out-performs an old stripper bath, even if the old one titrates full strength.  Yet, when you monitor the pH of the new vs. the old, the new stripper bath will always be much higher in pH...and in general, we find that new, old, or Friday night, a correctly made up stripper bath will always strip the same at the same pH, whereas that is not the case with titration, as discussed previously.  This accounts for my love of pH as a control method.

Rudy Sedlak
RD Chemical Company

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