The greatest sensitivity is achieved if R1 is approximately equal to the probe resistance. The circuit can be zeroed with R2, while the full-scale voltage is controlled by R5. The correlation between pH and output voltage might not be linear, which would necessitate a shaping circuit.
Ph-probe-and-detector - schematic

A calibration scheme, using solutions of known pH, might prove adequate and more reliable over a period of time because of probe variance.

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