A stabilized single-reagent TMB method for free chlorine monitoring in drinking water-related matrices

Continuous free chlorine monitoring supports drinking water and treated water management, but practical implementation depends on analytical performance as well as reagent handling and fluidic simplicity. This short communication evaluates a stabilized single-reagent tetramethylbenzidine (TMB) method designed as a measurement layer for simplified online monitoring. Quantitative comparison used a controlled benchtop fixed-time absorbance-at-650-nm (A650) readout at 30 s. The dataset comprised 32 paired measurements, including 20 tap-water or finished-water samples and 12 controlled matrix-concentration conditions spanning three defined mineral-background matrices evaluated at four free-chlorine levels each. TMB and N,N-diethyl-p-phenylenediamine (DPD) comparator measurements were performed within 5 min and in triplicate. Deming regression yielded a slope of 0.985 and an intercept of 0.007 mg L⁻1 as Cl₂. The mean paired difference (TMB-DPD) was + 0.001 mg L⁻1 and the mean absolute difference was 0.013 mg L⁻1. Precision assessed from the triplicate TMB readings gave a median RSD of 2.4% and a mean RSD of 3.6%. Sealed storage observations further supported the reagent-handling rationale for a stabilized single-reagent format. Together, these results show close agreement with routine DPD measurements across the evaluated drinking water-related matrices and provide a quantitative basis for further translation into a compact monitoring platform.