Abstract
Analytical methods based on the Berthelot reaction were recently adapted for determining monochloramine (MCA: NH2Cl) in freshwater. The specificity of the Berthelot reaction with regard to MCA is related to the need for two exchangeable hydrogen atoms to form indophenol blue. MCA can thus be distinguished from organic N-chloramines, which have only one exchangeable hydrogen atom. Monobromamine (MBA: NH2Br) may be formed during chlorination of seawater containing ammonium ions. Quantifying MBA is quite challenging and no method has been reported for its specific determination in seawater. As MBA also has two exchangeable hydrogen atoms, its reactivity might be analogous to that of MCA, but this hypothesis has never been investigated. The aim of this study was to examine the applicability of the so-called “indophenol method” for the determination of the MBA in freshwater and seawater samples. The reaction between MBA and Berthelot reagents was studied in both ultrapure water and artificial seawater. The reaction products were characterized by using gas chromatography coupled to mass spectrometry (GC–MS), Fourier transform-ion cyclotron resonance mass spectrometry (FT–ICR MS), and UV–vis spectroscopy. Results showed that colorimetric methods based on the Berthelot reaction were not suitable for measuring MBA in freshwater or seawater, since NH2Br reacts with alkaline phenol derivative via electrophilic substitution to form ortho- and para-brominated phenols instead of forming indophenol.
Highlights
Chlorine (Cl2 /HOCl/ClO− ) is widely used for water treatment in various fields, such as disinfection of drinking water and wastewater, the food industry, and biofouling control [1]
The culmination of this research was to examine the applicability of the colorimetric methods based on the Berthelot reaction to quantitate MBA in natural water samples
Instead of indophenol formation as with MCA, MBA reacts with alkaline phenol by aromatic electrophilic substitution, forming brominated phenols
Summary
Chlorine (Cl2 /HOCl/ClO− ) is widely used for water treatment in various fields, such as disinfection of drinking water and wastewater, the food industry, and biofouling control [1]. While chlorine chemistry has been extensively studied in freshwater, it remains poorly documented in seawater. Bromide and ammonia largely determine the fate of chlorine in seawater. Average bromide concentrations in seawater generally range from 65 mg L−1 to more than 80 mg L−1 in confined sea areas [2]. When chlorine is added to seawater containing ammonia, competition takes place between two reaction pathways: bromide oxidation into bromine versus haloamines formation [3]. Chlorine can react with nitrogenous organic compounds, usually amino acids, proteins and amines, to form a variety of organic N-haloamines. It has been shown that chlorine can bind to some nitrogenous organic compounds more quickly than ammonia [4,5,6]. Oxidizing species likely to be formed during seawater chlorination vary widely, depending on chlorine dose and water quality
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