Lab subject: ammonia and nitrate nitrogen?

Question

When measuring pH the voltage (mV) gets more positive as {H+} increases. Do the voltage always get more positive as { } increased for ammonia and nitrate ISEs? If not, please, explain why.

Answer 1

Generally speaking, no, they do not.

For a typical Ammonia ISE the approximate Calibration Voltages are High (100 mg/L) 2.1 V Low (1 mg/L) 1.3 V

For many Nitrate ISE the order is reversed with approximate Calibration Voltages of High(100 mg/L)1.6, Low (1 mg/L) 2.4 V

All ISE's measure vs. a standard electrode (often a Ag/AgCl system). The potential for the measuring electrode can be either greater or less than the standard.