10N NaOH, 10% v/v MeOH) until a stable potential value (mV) was obtained (at least 2 or 3 minutes). For the quantification, a calibration curve was daily constructed plotting the mV value recorded against the Log Concentration, to verify the slope (varying in the range -54 ÷ -60 mV). The ammonia concentration in the sample was further assessed by two standard additions. Figure reports an example of mean calibration curve (0.25 mg L -1 ÷ 10 mg L -1 as NH3).