pH sensor with DS18B20 temp compensation on PIC
Look up my answer on PH probe interfacing. 

I've used LMC6001 and it works, it has a reference design in a datasheet. Be aware that PCB board has some leakage too, so that could ruin your measuremetns. 

I've desoldered an input lead of DIP package of the frontend opamp and soldered it directly to BNC so it does not touch anything and there is only air and BNC isolation around the input signal. 

Otherwise, I was getting wrong results. This is how I measured opamp input current. No matter how vigoriously I cleaned the PCB (the friend whom I was helping with this probe is a chemist, he knows some things about cleaning stuff), too much current was going into opamp input untill I connected the input lead directly to BNC.