ESD protection for pH measurement electrode input
I don't know about PH sensors.  I'd start by "cheating" and see how other people had done it.  If you do end up needing a low leakage diode then you might want to look into using a small transistor.  The c-b diode (open emitter) is low leakage (1-10 pA) but slow (100's of ns) The c+b - e (diode connected transistor, c/b shorted) is faster, but has a lower breakdown voltage.  I've measured a few 2N3904/6's.  and Bob Pease recommends the 2n930, 2N3707 and 2N4250.  But you won't find these leakage number on the spec sheet.  (For B. Pease reference search the web for B. Pease "Bounding and Clamping".)