Chris Fanning wrote:

> The problem is, the solution conducts.  (ie. saltwater)  There will
> also be other probes that will require different offsets.

  My suggestion (the last time round) was to use a third electrode,
perhaps already a metal part of the container, to set the overall
offset.  You then use instrumentation amplifiers with high impedance to
monitor *both* the glass and AgCl (and PO2 etc...) electrodes and
determine the differential.

  Whether you drive (bias) the bulk solution (container) against your
electronics ground or vice versa is a minor consideration.
--
  Cheers,
        Paul B.