How to interface a pH probe?
Like miceuz says, what sets a pH sensor apart is its high impedance, which calls for a FET opamp or instrumentation amplifier for minimum error due to input bias current. Next to the parts miceuz already mentioned I would add:  

LMC6041: the lowest cost opamp listed at Digikey with input bias current less than 10 fA (2 fA typical).

INA116: the only InAmp listed by Digikey with an input bias current less than 10 fA (3 fA typical, 2 mV offset maximum).