I see a problem with the circuit of my last post. (BMP file).

When we are not measuring the Ri of the Nernst-cell, the output voltage of the cell +2.95V (+2.5V +0.45V) goes through the two 10K resistors and the 82.5 ohm resistor back to the 2.5V of the virtual ground. (We have 20082.5 ohm connected in parallel with the cell).
The circuit will need analog switches (like the 4066) in order to insulate the Nernst-cell.

Two years ago I helped my cousin on a similar project.
He married, his wife sold the car, the project never started.

Luciano