A Novel Spectrophotometric Method for Determining the Permanganate-Based Chemical Oxygen Demand (CODMn) Using ABTS as an Indicator

Spectrophotometry can efficiently determine permanganate (PM)-based chemical oxygen demand (CODMn) in various waters, but existing methods face challenges, such as large reagent requirements and interference from complex water matrices. Here, we developed a novel spectrophotometric method for determining the CODMn using 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) as an indicator. The kinetics and thermodynamics of the PM + ABTS reaction were investigated, revealing a pseudo-first-order rate constant (k′) of 36.4–48.6 s–1 at [PM]0:[ABTS]0 molar ratios of 1:60–1:100 (pH 6.5 and 298 K) and an Arrhenius activation energy (Ea) of 17.5 kJ mol–1. Compared to the PM + DPD (N,N-diethyl-p-phenylenediamine) reaction, which has a k′ of 29.4–47.5 s–1 under identical conditions and an Ea of 47.4 kJ mol–1, the PM + ABTS reaction demonstrated greater adaptability for rapid CODMn determination. Furthermore, the ABTS method showed higher sensitivity and accuracy for CODMn determination than the DPD method, with a higher slope of the linear calibration curve (0.228 L mg–1 cm–1 vs 0.101 L mg–1 cm–1) and a lower limit of quantification (0.03 mg L–1 vs 0.05 mg L–1). In addition, both methods showed high robustness against different water matrices, including real waters. Although the ABTS method has a higher reagent cost than the DPD method, it offers sensitive, rapid, and accurate CODMn determination, making it suitable for time-sensitive water quality assessments.