I need to measure voltage on PH electrode, where output impedance is very high. So I need Op Amp with input impedance 1 TOhm or higher, that I could put in front of AD converter. Could you please recommend me suitable Op Amps for this purpose? How can I determine input impedance of Op Amp if I know only input current from datasheet? I need sampling rate only up to 10 Hz. I am not very familiar with analog technics.