Analytical Methods Applications of 1,3,5-trimethoxybenzene as a derivatizing agent for quantifying free chlorine, free bromine, bromamines, and bromide in aqueous systems Journal: Analytical Methods Manuscript ID AY-ART-07-2019-001443.R1 Article Type: Paper Date Submitted by the 10-Sep-2019 Author: Complete List of Authors: Dias, Ryan; Towson University, Chemistry Schammel, Marella; Towson University, Chemistry Reber, Keith; Towson University, Chemistry Sivey, John; Towson University, Chemistry Page 1 of 32 Analytical Methods 1 2 3 4 Applications of 1,3,5-trimethoxybenzene as a 5 6 derivatizing agent for quantifying free chlorine, free 7 8 bromine, bromamines, and bromide in aqueous 9 10 systems 11 12 13 Ryan P. Dias,a‡ Marella H. Schammel,a Keith P. Reber,a and John D. Sivey*ab 14 15 16 a Department of Chemistry, Towson University, 8000 York Road, Towson, Maryland 21252, 17 18 United States 19 b 20 Urban Environmental Biogeochemistry Laboratory, Towson University, 8000 York Road, 21 Towson, Maryland 21252, United States 22 23 24 * Corresponding author contact information: phone: 1-410-704-6087; e-mail: [email protected] 25 26 ‡ Current affiliation : Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, 27 Edmonton, Alberta, Canada T6G 2G2 28 29 30 31 Submitted exclusively to Analytical Methods 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Analytical Methods Page 2 of 32 1 2 3 4 Abstract 5 6 Free chlorine and free bromine (e.g., HOCl and HOBr) are employed as disinfectants in a 7 8 9 variety of aqueous systems, including drinking water, wastewater, ballast water, recreational 10 11 waters, and cleaning products. Yet, the most widely used methods for quantifying free halogens, 12 13 including those employing N,N-diethyl-p-phenylenediamine (DPD), cannot distinguish between 14 15 16 HOCl and HOBr. Herein, we report methods for selectively quantifying free halogens in a variety 17 18 of aqueous systems using 1,3,5-trimethoxybenzene (TMB). At near-neutral pH, TMB reacted on 19 20 the order of seconds with HOCl, HOBr, and inorganic bromamines to yield halogenated products 21 22 that were readily quantified by liquid chromatography or, following liquid-liquid extraction, gas 23 24 25 chromatography-mass spectrometry (GC-MS). The chlorinated and brominated products of TMB 26 27 were stable, and their molar concentrations were used to calculate the original concentrations of 28 29 HOCl (method quantitation limit (MQL) by GC-MS = 15 nmol L–1 = 1.1 μg L–1 as Cl ) and HOBr 30 2 31 –1 –1 32 (MQL by GC-MS = 30 nmol L = 2 μg L as Cl2), respectively. Moreover, TMB derivatization 33 34 was efficacious for quantifying active halogenating agents in drinking water, pool water, 35 36 chlorinated surface waters, and simulated spa waters treated with 1-bromo-3-chloro-5,5- 37 38 39 dimethylhydantoin. TMB was also used to quantify bromide as a trace impurity in 20 nominally 40 41 bromide-free reagents (following oxidation of bromide by HOCl to HOBr). Several possible 42 43 interferents were tested, and iodide was identified as impeding accurate quantitation of HOCl and 44 45 HOBr. Overall, compared to the DPD method, TMB can provide lower MQLs, larger linear ranges, 46 47 48 and selectivity between HOCl and HOBr. 49 50 51 52 53 54 55 56 57 58 59 60 Page 3 of 32 Analytical Methods 1 2 3 4 1. Introduction 5 6 Free chlorine and free bromine serve as disinfectants (or are generated in situ) in drinking 7 8 9 water, wastewater, spas, pools, ballast water in ships, as well as in household and industrial 10 11 cleaning products.1-3 Free chlorine (i.e., free available chlorine) describes all aqueous chlorine 12 13 species in the +I or 0 oxidation state (except chloramines).4 Examples of free chlorine species 14 15 – 4,5 16 include HOCl, ClO , Cl2, and Cl2O. Solutions of free chlorine are commonly generated by 17 18 dosing water with NaOCl(aq), Ca(OCl)2(s), Cl2(g), or stabilized chlorine, including various 19 20 organic chloramines.1,6 Free bromine (i.e., free available bromine) refers to all aqueous bromine 21 22 species in the +I or 0 oxidation state (except bromamines).7 Example of free bromine species 23 24 – 7 25 include HOBr, BrO , Br2, BrCl, Br2O, and BrOCl. Free bromine can be produced via hydrolysis 26 27 of liquid bromine (Br2) or organic bromamines, as well as by in situ oxidation of bromide (e.g., by 28 29 HOCl, eq 1).1,7,8 30 31 32 HOCl + Br– ⇌ BrCl + OH– ⇌ HOBr + Cl– (1) 33 34 Ozone and hydrogen peroxide/peroxidase systems are also capable of oxidizing bromide into free 35 36 9,10 o 11 37 bromine. At pH 7, hypochlorous acid (HOCl, pKa = 7.58 at 20 C) and hypobromous acid 38 o 12 39 (HOBr , pKa = 8.70 at 20 C) are the most abundant forms of free chlorine and free bromine, 40 41 respectively. 42 43 44 Accurate quantification of free halogen residuals in disinfected waters is important for 45 –1 46 protecting human and environmental health. At doses on the order of 1 – 4 mg L , free halogens 47 48 are capable of inactivating pathogens in drinking water, wastewater, and recreational waters.1 49 50 Higher concentrations (e.g., 10 to >10,000 mg L–1) of free chlorine are employed for disinfection 51 52 53 processes associated with food service, medical/mortuary settings, and decontamination of 54 55 biological warfare agents.13-16 56 57 58 59 60 Analytical Methods Page 4 of 32 1 2 3 Monitoring of free halogens in disinfected waters is also important due to the propensity 4 5 6 of free halogens to react with organic matter to form chlorinated and brominated disinfection 7 8 byproducts (DBPs).17,18 Brominated DBPs are generally more toxic than their chlorinated 9 10 analogues,19 which underscores the need for analytical methods capable of selectively quantifying 11 12 free chlorine and free bromine as precursors of DBPs. Quantification of free halogens is also 13 14 15 important in laboratory settings, including experiments investigating disinfection efficacy and 16 17 DBP (trans)formation.20-22 Such studies frequently employ solutions containing mixtures of free 18 19 chlorine and free bromine generated via oxidation of bromide (e.g., from NaBr) by excess free 20 21 23 22 chlorine. Free chlorine/bromine mixtures are also generated in waters (e.g., spas and toilet tanks) 23 24 disinfected with 1-bromo-3-chloro-5,5-dimethylhydantoin (BCDMH, Scheme 1),1 as well as in 25 26 chlorinated solutions amended with salts (e.g., NaCl) in which bromide is present as an impurity.24 27 28 29 30 Br H N 31 O N O 32 + 2 H2O + HOBr + HOCl 33 N N 34 O Cl O H 35 Scheme 1. Hydrolysis of 1-bromo-3-chloro-5,5-dimethylhydantoin (BCDMH) yielding HOBr 36 37 and HOCl. Whether BCDMH exists principally as 1-bromo-3-chloro-5,5-dimethylhydantoin or as 25 38 3-bromo-1-chloro-5,5-dimethylhydantoin has been discussed in the literature. 39 40 41 Several methods exist for quantifying free halogen residuals in aqueous samples, including 42 43 iodometric titration, amperometric titration, and, perhaps most commonly, colorimetric methods 44 45 employing N,N-diethyl-p-phenylenediamine (DPD).26,27 Methods of quantifying free chlorine 46 47 28 29-31 48 involving chemiluminescent reactions and quantum dots have also been reported. None of 49 50 these methods, however, are able to selectively and concurrently quantify free chlorine and free 51 52 bromine in samples containing mixtures of these analytes. 53 54 55 56 57 58 59 60 Page 5 of 32 Analytical Methods 1 2 3 To distinguish between free chlorine and free bromine, electron-rich aromatic compounds 4 5 6 can serve as derivatizing agents (i.e., probes) for these free halogens. Such methods leverage the 7 8 propensity of aromatic compounds containing “activating” substituents (e.g., hydroxy and 9 10 methoxy) to form organohalides via electrophilic substitution.32,33 When selecting among possible 11 12 probes, several characteristics of the candidate probe and of its halogenated products need to be 13 14 15 considered, including water solubility, nucleophilicity, stability, ease of analysis, molecular 16 17 symmetry, acid/base behavior, and toxicity. Probes (and their halogenation products) must be 18 19 sufficiently water soluble so as to preclude precipitation. Probes should demonstrate adequate 20 21 22 nucleophilicity in reactions with free halogens so as to promote stoichiometric conversion of free 23 24 halogens to the corresponding organohalides in reasonable time frames (ideally on the order of 25 26 seconds). Candidate probes must generate stable halogenated products amenable to selective 27 28 29 quantitation (e.g., by liquid chromatography (LC) and/or gas chromatography (GC)) so that non- 30 31 halogenated, chlorinated, and brominated probe species can be individually quantified. Probes 32 33 with higher degrees of symmetry have fewer unique substitution positions, generate fewer 34 35 products, and thereby simplify analysis of product mixtures compared to probes of lower 36 37 38 symmetry. Acid-base behavior (if any) should be chosen so as to enhance (or, at a minimum, not 39 40 diminish) the probe’s performance in terms of water solubility, nucleophilicity, and ease of 41 42 analysis. Importantly, probes should also be selected so as to minimize toxicity hazards. 43 44 45 Previous studies have demonstrated the ability of electron-rich aromatic compounds 46 47 (including phenol, 2,6-dimethylphenol, 2,6-dichlorophenol, and 3,5-dimethyl-1H-pyrazole 48 49 (DMPyr), Figure 1) to serve as probes of free chlorine and/or free bromine via quantification of 50 51 34-39 52 their halogenated products by LC or GC.