does the circuit suit your requirements ..
make sure you know what you want to make before finalizing a particular circuit
for your question .. the 2 batteries are meant to give positive and negative supply to the opamp
and the potentiometer (you wouldnt ask this question if you had enough knowledge of opamps) provides offset trimming.
hi thanks for your help . yes this circuit suits my requirement but iam not still not sure how I will tune it to a particular PH value. can you please guide ?