In situ real-time monitoring of ammonium, potassium, chloride and nitrate in small and medium-sized rivers using ion-selective-electrodes – a case study of feasibility

Abstract

Real-time measurements are particularly important for monitoring especially small and medium-sized highly dynamic rivers. Their results are indispensable for planning (cost)efficient measures to improve the chemical and ecological quality of the rivers. It is therefore expedient to be able to use cost-effective, reliable equipment. In theory, ion-selective electrodes (ISEs) are excellently suited for this purpose due to their cost-effectiveness, their high spatial flexibility and their self-sufficient energy supply for the simultaneous recording of different parameters. In practice, however, malfunctions occur caused by temperature changes, interferences of non-target ions or long-term-drifts. This study investigated the applicability of ISEs for in situ real-time measurement of ammonium (NH₄⁺), potassium (K⁺), chloride (Cl⁻) and nitrate (NO₃⁻) in small rivers. ISEs from three different manufacturers were deployed for five months in a river monitoring station. The measured data were compared with real-time data gained from an on-line photometer, a gas-sensitive analyser and an optical UV probe as well as with grab samples analysed using ion chromatography (IC). Special attention was given to the challenges posed by low concentrations, temperature fluctuations, concentration changes of the analytes, and interfering ions, as well as to the long-term stability and the lifetime of the ISEs. For Cl⁻ and NO₃⁻ good agreements with the comparative measurements were found, and it could be shown that the ISEs are well suited for event detection of all four observed parameters. Temperature compensation is the main challenge that complicates application in highly dynamic rivers. Therefore, further optimisation is urgently needed for reliable quantitative analysis, which is part of future work.