Comparison of continuous flow analysis including photometric detection and ion-selective electrode potentiometry for the measurement of ammonium nitrogen in wastewater

Abstract

Continuous flow analysis (CFA) with photometric detection is widely used to quantify . However, low-molecular-weight amines interfere with CFA measurements. Therefore, we compared CFA and ion-selective electrode (ISE) measurements using an ISE25NH4 electrode from Radiometer Analytical for determining in wastewater streams in a plant where protein separation media are manufactured. Approximately 50% of the trimethylammonium chloride in one of the samples was recorded by CFA as ammonium nitrogen. Furthermore, CFA spikings using 6 mg N/L additions to the wastewater showed less accuracy compared with equivalent ISE spikings. Direct ISE measurements of levels in wastewater across the concentration range 0.1–1400 mg N/L typically showed >90% accuracy, with <1% RSD repeatability (both within- and between-day). Furthermore, due to the excellent selectivity of the electrode for over Na + (logK NH4 − N, NaPot) = –3.11, the presence of potentially interfering Na+ ions in selected wastewaters did not affect the readings. ISE measurements obtained by multiple standard addition methodology showed similar levels of accuracy and repeatability, except that the accuracy of the determinations was affected by the non-linear electrode responses to samples with low concentrations (<1 mg N/L). Direct ISE and CFA measurements on two types of wastewater streams revealed significant differences (p < 0.05). The ISE results showed on average 18 and 112% higher concentrations compared with CFA results. The known occasional ammonium increase in one of the streams was not detected by the CFA method.