Chlorine is a gas, which is marginally soluble in water.  The more salts, all
other things being equal, such as pH, the less soluble the Chlorine.  So, you
could be creating Chlorine when the Copper is low, and it is soluble enough
to not be given off.

One key question is do you monitor your ORP, that is to say, actually keep a
continuous record of it??   If you are getting serious spikes in an ORP
chart, you are creating Chlorine, whether you smell it or not.

Another factor, that could be contributing is the viscosity, and thus mixing
speed of the etchant.  If the Copper is low, the mixing speed of the
components is higher, and this will allow the Cuprous ion to come into
contact with the Chlorine gas, and react with it, and make it go away,
faster.

One of the factors that I have seen personally that can contribute seriously
to this problem is the type (and horsepower) of the pump making the
additions.  If the pump is too powerful, and ANY centrifugal pump I have ever
seen is too powerful, It will seriously over-add before the ORP sensor tells
it to shut off.   Best type of  pump is a bellows pump, and they should be
sized so that they are running a lot of the time that panels are going down
the line.  Never less than 20% of the time, and more is better, up to even as
high as 70% of the time.  This ensures slow addition, so that the reactants
can circulate around enough to find the Cuprous ion, and not have time to
say, "Hey, can't find no Cuprous, I'm history!", and escape to the
atmosphere.

Also, speaking of mixing, if you are running more than one chamber, you
should have some serious horsepower mixing the two sumps.  This is best done
by pumping from the last chamber to the first, and cascading back.   I am
going to make a rough estimate that you want one HP of pump circulating for
every etcher sump in a line.

And that is an absolute minimum.

Also, if you have more than one etcher sump, you may want to add Chlorate and
HCl at more than one point.

If you would like more free opinions, which some have said are worth almost
what you pay for them, e-mail, or call.

Rudy Sedlak
RD Chemical Company
415-962-8004

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