A Reaction-kinetic online determination of free chlorine using a stabilized TMB-derived chromogenic system

Accurate determination of free chlorine is essential for maintaining effective disinfection in drinking-water treatment and distribution systems. Conventional colorimetric methods, particularly endpoint absorbance methods such as the DPD assay, can be constrained in automated analyzers by matrix-dependent optical interference, reagent stability limitations, and relatively slow endpoint measurement cycles. Here, we report a reaction-kinetic method for online free chlorine determination based on time-resolved optical detection of chlorine-driven oxidation of a stabilized TMB-derived chromogenic reagent in a low-water solvent matrix. Instead of relying on endpoint absorbance, chlorine concentration was quantified from the initial reaction rate (dA/dt) extracted from the early absorbance-time profile. The method showed a linear response over 0.01-10 mg/L free chlorine, with a limit of detection of 0.005 mg/L and a repeatability of 2.3% RSD (n = 6). Comparison with a standard DPD method across 45 drinking-water and treated process-water samples showed strong agreement (y = 0.99x + 0.02, R² = 0.996). The reagent maintained analytical sensitivity within ±10% over 24 months under ambient storage and enabled stable operation during 30 days of continuous field deployment in a municipal drinking-water treatment facility. These results demonstrate that reaction-kinetic chromogenic detection is a practical analytical strategy for automated free chlorine determination and that stabilized TMBderived reagent chemistry can support reliable online operation under representative treated-water conditions.