>Right. That's probably what is happening now. A question tho:  Is it volts
>that changes the solution, or amps? What I've seen suggests it's amps, so
>if I can limit the current below 1mA then no changes should occur. I
>think?

Both, maybe charge is more precise. There is a big difference in conducting
electricity in metals and solutions. While electrons carry the current in
metals, ions do that in solutions. At the boundary of metal and solution can
various things happen. The very first ions close to the metal form so called
double-layer. From an electrical point of view, it acts as (big) capacitor
in series with the resistance of the solution. When you force current thru
the solution, you charge this capacitor. But when you overcharge it, then
positive ions will combine with electrons from the metal and form neutral
molecules, negative ions will give away its electrons and also become
neutral molecules - the solution undergoes electrolysis. This is boring
theory, but it concludes, that you have to pass thru the solution the same
(and not too big) charge in both directions. One way to ensure that is
symetrical (no DC) waveform with limited voltage.

Yes! Maybe there is a way how to do it. Remove the 8.2k resistor to RA3.
Keep RA0 always as output. Connect big enough resistor R in series with
probe. It should have about 5 times higher resistance than your least
conductive solution - you will limit voltage across the solution to less
than 1V. Although you loose some resolution, your need is to distinguish
4-36 units. Maybe reduce the capacitance of the .47uF capacitor - it depends
on the surface of your probe electrodes, need to try. Charge and discharge
the capacitor only thru PROBE + R + Pot. In this way you pass exactly the
same charge thru the solution in both direcitions. But your probe may suffer
from leakage current at TMR0 - small but continuous in one direction. Need
to try. Maybe some SMD FET operating amplifier from the solder side as a
voltage folower, or better as a comparator (but two more resistors) will help.

                           ---      +--------------------+
                   R      |   |     |                    |
                          |  \ /    |                    |
RA0   ---PROBE---/\/\/\-----/\/\/\--+                   ---
                             1.6k   |                   --- .47uF
                                    |                    |
                                    |                    |
                                    |                  -----
TMR0   -----------------------------+                   ---
                                                         -


>Maybe I could use a RMS measuring technique to convert the square wave to
>a DC value, and get a PIC with an ADC on it to do the conversion...?

I guess not...

>I'm gonna keep that in mind for the future. Right now tho, I'm trying to
>do it with very little modifications to the original circuit; because we
>already have over 250 boards printed and populated.

I can imagine, how difficult amending those bords could be.

Good luck,

Josef