I did find one schematic so far for a ph probe. The output from the circuit is 0-140mv which corresponds to the ph value. No link to the probe used though.
http://damien.douxchamps.net/elec/ph_meter/
Assembly language! I haven't used that in more than 20 years. I do remember it was fun once I figured out the codes etc. That was for the 8085 microprocessor. Time to start reading!