A stabilized TMB-based kinetic method for low-range online free chlorine monitoring in treated water: method development and preliminary prototype evaluation

Continuous free chlorine monitoring is important for drinking-water treatment and treated processwater control, but online implementation can be limited by reagent instability, multi-reagent fluidics, and maintenance burden. We report a stabilized tetramethylbenzidine (TMB)-based kinetic method for low-range online free chlorine monitoring in treated-water matrices. The method was implemented in prototype online analyzers using a single-reagent architecture and a fixed low-reagent operating condition. During formulation screening, lower-concentration TMB variants were attractive for nominal low-range measurement in simple standards but showed greater susceptibility to matrix-related interference in real treated-water samples. An optimized stabilized formulation was therefore adopted in the final method to improve color-development robustness and practical linearity. Prototype evaluation included a representative on-site point comparison with a handheld DPD-based comparator, a small matrix-comparison dataset covering treated-water-related samples, routine repeated testing at 24 measurements per day over approximately three months, and separate representative higher-frequency runs at a 2.5-minute measurement cycle. Under routine repeated operation, repeatability was around 1% RSD. Sealed-storage stability testing further supported longterm reagent stability under room-temperature and accelerated conditions. Together, these results support the feasibility of stabilized TMB kinetic chemistry for low-range online free chlorine monitoring in treated-water applications.