Josef Hanzal wrote:

> In this way even if you charge and discharge the capacitor often
> enough, there is still some DC voltage across the solution, causing
> necessarily some plating effect (also called polarization).

  Am I missing something?  Two details seem to me stunningly obvious:
Firstly, that as indeed you want to avoid DC voltage across the
solution, you would ensure this by providing a "blocking" capacitor
between the electrode and the measuring circuit; a polycarbonate of
several microfarads if you could afford the space, or a tantalum
electrolytic configured so that was forward biassed throughout the
waveform.  If it became necessary to compensate for the reactance of
the blocking capacitor, a second capacitor could be introduced
between electrode and AC source:

            Rref     C             C
  VAC >----/\/\/----||-----+------||-----> Metering/ DAC
                           |
            +-----------+  |
            |           |  |
            |       (~~~+~~+~~~)
            =       (   I  I   ) solution
           Gnd      (   I  I   )
                    (__________)
                        ^  ^
                      electrodes

  The second point which I have assumed in the diagram, is that the
reference electrode is grounded, though if another metallic part of
the solution path was also grounded, this could itself lead to
electrolytic problems.  Commonly, some metallic part of the
container already IS grounded, and the "sense" electrode would be
deliberately chosen to be the same material.

  While I realise this is not "set in stone", I am always
uncomfortable to have large ungrounded objects (such as containers
of liquid) connected to small-signal circuitry.  If it MUST be so,
surely the signal circuitry should be "floating" from isolation
devices and shielded in common with the "sense" electrode, by the
reference electrode?

  I am visualising something like a pipe through which the liquid
passes, forming the reference electrode, and containing a central
sense electrode which connects fairly directly with the measurement
circuit, itself encased in a shield attached to the outer reference
"pipe".  Something similar would seem to be in order whether the
shield was grounded or isolated.

  Just some musings,

  Cheers,
        Paul B.