Temperature Dependence of Indophenol Method Color Development: Monochloramine, Free Chlorine, or Free Ammonia Concentration Impact
Alexander, M., T. Waters, AND D. Wahman. Temperature Dependence of Indophenol Method Color Development: Monochloramine, Free Chlorine, or Free Ammonia Concentration Impact. Presented at 2022 Water Quality & Technology Conference, Cincinnati, OH, November 14 - 17, 2022.
The environmental or health problem addressed by the study: Measurement of monochloramine in drinking water A general description of the work and results: Experiments have been conducted to evaluate a new methodology to measure monochloramine in drinking water The long-term importance or significance of the findings: Provides new method monochloramine in drinking water Who would be interested in or could apply the results (e.g. program or regional partners, general public, local communities): Researchers, engineers, regions, and drinking water utilities measuring monochloramine in drinking water systems.
The Surface Water Treatment Rule (40 CFR 141.74 (a)(2)) requires a detectable disinfectant residual in the distribution system, measured as free or total chlorine, chlorine dioxide or ozone. Total chlorine includes free and combined chlorine. Combined chlorine includes inorganic and organic chloramines. Monochloramine (NH2Cl) is the dominant inorganic species formed at typical pH values and chlorine to ammonia-nitrogen mass ratios (i.e., < 5:1) used in drinking water treatment. Free and total chlorine are commonly measured with S.M. 4500-Cl G, which is a N,N-diethyl-p-phenylenediamine (DPD) colorimetric method. Organic chloramines formed in the presence of dissolved natural organic nitrogen are not effective disinfectants and are known to interfere with the total chlorine DPD method potentially overestimating the effective disinfectant residual. This is problematic in the context of protecting public health and properly documenting a detectable disinfectant residual. A commercially available NH2Cl indophenol method specifically quantitates NH2Cl (i.e., the most effective disinfectant in chloramine systems) and is not impacted by the presence of other inorganic or organic chloramines. This method has been used for process control in chloraminated systems and chloramine-related research for several years. The NH2Cl indophenol method is not an EPA-approved method; therefore, it cannot be used for compliance monitoring. EPA’s Office of Ground Water and Drinking Water and Office of Research and Development built on the existing indophenol method and published EPA Method 127, a new analytical method for the determination of NH2Cl as a disinfectant in drinking water. During EPA’s method development, sample temperature’s impact on reagent reaction development was investigated using a spectrophotometer equipped with a Peltier temperature control unit. NH2Cl solutions of known concentration and temperature (5-30°C) were prepared. Indophenol reagent was then added, and absorbance was repeatedly measured until reaching full color formation. Color formation time increased as temperature decreased and could be described by an Arrhenius relationship. At < 20°C, color development times were greater than reported in the commercially-available method, reaching to nearly three times as long at 5°C. Based on the results from this study, new color formation times were determined. In addition to the commercially-available method and EPA Method 127, the determined temperature dependence may also be applicable to the free ammonia (NH3) and free chlorine indophenol commercial methods because they use the same reagent and chemistry. Overall, the indophenol method can both minimize potential high bias in NH2Cl measurement when organic chloramines are present and reduce the risk of low bias measurement of NH2Cl, free chlorine, or free NH3 concentrations in water samples (particularly < 20°C) when reaction times determined in the current study are followed.