How to measure copresent ammonia and inorganic chloramines reliably: Some fundamental issues and novel solutions

Ammonia (NH₄⁺) and chloramines often coexist in disinfected water, and accurate determination of their concentrations is important in understanding the status of treatment process. However, their measurements are prone to interference from each other and many operational variables. To select proper approaches, we assessed several representative methods commonly used for measuring NH₄⁺, monochloramine (NH₂Cl), and dichloramine (NHCl₂) in water dosed with varying molar ratios of free chlorine (FC) to NH₄⁺ (Cl₂:N). Results indicate some fundamental issues that were overlooked before. For example, neither Nessler's reagent method nor ion chromatography (IC) can analyze NH₄⁺ reliably when NH₂Cl was present, because the reaction NH₄⁺ + OCl^- ⇌ NH₂Cl + H₂O is reversible and whenever NH₄⁺ was depleted by its reaction with Nessler's reagent, which is a rapid and reversible process, NH₂Cl immediately reverts to NH₄⁺. For the same reason, quantifying FC using N, N-diethyl-p-phenylenediamine (DPD) reagent was also problematic, because NH₂Cl spontaneously converts into FC whenever FC was consumed by its reaction with DPD reagent, which is also rapid and irreversible. Moreover, copresent NHCl₂ was able to interfere with the analysis of NH₂Cl by the Indophenol reagent method, because NHCl₂ can partially become NH₂Cl under strongly alkaline condition, which is required for the Indophenol method. To address these issues, we propose to estimate NH₄⁺ as the difference between total negative three valent inorganic nitrogen (TIN(-3)) and chloramines. Liquid chromatography (LC) appears to be a suitable method for chloramine analysis because it is derivatization-free and exhibits strong resistance to interferences, although its minimum detection limits (MDLs) could be further optimized. The proposed methods demonstrated superior reasonableness over conventional methods in both synthetic and real samples, and agreed better with the theoretical estimations. These methods might help not only reveal hidden information but also rectify misleading information in the ammoniacal chlorination system in the future.