Development of a field ammonia monitor using a gas sensing electrode: comparison with colorimetric methods and an ammonium ion selective electrode

An unattended, remote monitor was developed for measuring elevated levels of ammonia (NH₃) in the vicinity of a waste water treatment plant (WWTP), using a commercial flow-through ammonia gas electrode. The instrument performed automatic calibrations and sample analysis hourly. The instrument was deployed continuously for up to 10 days; the results compared favorably with samples collected and run in the lab on a Technicon AutoAnalyzer II using the Berthelot reaction. The NH₃ output of the plants were found to be highly variable on a daily basis, indicating the need for more frequent monitoring than the typical weekly or monthly sampling. An Orion Model 93-18 ammonium ion electrode was mounted in a Hydrolab DataSonde 3 and used to measure the NH₃ levels in the vicinity of the WWTP, compensating for ion activity, electrode sensitivity and NH₃ dissociation using the temperature, pH and conductivity probes. Attempts to compensate for the interference of salinity was only partially successful, confirming that the field use of ammonium ion electrodes is only possible in low conductivity fresh waters. The gas electrode, however, exhibited no salinity interference