Or you could start with a pH probe on its own and a good FET op amp, dual preferably. Op amp needs to have very low bias current. Have a look at MAX407, an old amp, but good. Maxim do a nice application note that covers pH. Use one op amp to do the buffering and the other to do the gain.
Get the amp as close to the probe as possible.
From memory, each pH unit produces an output of 60-mV, temperature dependent. Output is zero at pH 7, then either + 60-mV per pH unit, or - 60-mV depending on acidity or alkalinity. With a single-sided supply (I used 3.7-V lithium) you need to offset the first amp by a simple voltage divider.
You can do the temperature compensation in software or hardware using a temperature sensor.
By getting the probe as close as possible (some probes have the amp in the cap of the probe), you do away with all the complexities of coax and can run the output down some old Black and Decker cable, so useful for remote applications.
A half AA Lithium cell could outlast the probe lifetime (draws 2.4uA) and running on a battery does away with earth loop problems.
Use the same approach for any high impedance source, including other specific ion electrodes (nitrates, fluoride etc.) as a pH probe is another specific ion electrode, specific for hydrogen ion.