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In this study, a multi-wavelength spectrophotometric method for simultaneous determination of peracetic acid (PAA) and coexistent hydrogen peroxide (H 2 O 2 ) was presented. This method was based on the oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) with the assistance of Fe 2+ /KI to produce a stable green radical (ABTS ●+ ), which could be determined at four characteristic peaks (i.e., 415 nm, 650 nm, 732 nm, and 820 nm). The absorbances of ABTS ●+ at four peaks were well linear (R 2  > 0.999) with concentrations of both total peroxides (PAA + H 2 O 2 ) and PAA in the range of 0-40 μM under optimized conditions. The sensitivities for determining total peroxides at 415 nm, 650 nm, 732 nm and 820 nm were determined to be 4.248 × 10 4  M -1  cm -1 , 1.682 × 10 4  M -1  cm -1 , 2.132 × 10 4  M -1  cm -1 , and 1.928 × 10 4  M -1  cm -1 , respectively. For determining PAA, the corresponding sensitivities were 4.622 × 10 4  M -1  cm -1 , 1.895 × 10 4  M -1  cm -1 , 2.394 × 10 4  M -1  cm -1 and 2.153 × 10 4  M -1  cm -1 , respectively. The concentration of coexistent H 2 O 2 was gained by deducting PAA concentration from total peroxides concentration. The ABTS method was accurate enough to determine PAA concentration in natural water samples. Moreover, the ABTS method was successfully used to determine the changes of PAA and coexistent H 2 O 2 and to distinguish their role on naproxen degradation in heat-activated PAA process. Overall, the ABTS method could be used as an alternative method for the convenient, rapid and sensitive determination of PAA and the coexistent H 2 O 2 in water samples., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

A new spectrophotometric method for measuring permanganate index (chemical oxygen demand using potassium permanganate (KMnO 4 ) as oxidant, COD Mn ) in water was established. The method was based on the rapid oxidation of N,N-diethyl-p-phenylenediamine (DPD) by residual KMnO 4 in digestion solution under neutral pH condition to form the stable pink radical (DPD ●+ ). Only 20 s were enough to form the pink DPD ●+ . The generated DPD ●+ could be quantitatively measured by a visible spectrophotometer at 551 nm. Stoichiometric coefficient of the reaction between KMnO 4 and DPD was close to 1:5 (1:5.07). There was a well linear relationship (R 2  = 0.999) between the change of the absorbance of DPD ●+ at 551 nm and the concentration of COD Mn in the range of 0-4.46 mg L -1 . Limit of detection of the DPD method was as low as 0.02 mg L -1 COD Mn . The DPD method was highly accurate for measuring COD Mn in standard solutions with well recovery rates of 99.17%-102.22%, and was well tolerant to the interference of coexistent Cl - and Fe 3+ . The DPD method was successfully applied for measuring COD Mn in real water samples, including surface water, underground water and drinking water. In comparison to the traditional titration method, the proposed DPD method was more convenient to operate, required less samples and digestion reagents (i.e., KMnO 4 and H 2 SO 4 ) and could be employed for online monitor., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

A low-cost and environmentally-friendly spectrophotometric method for hydrogen peroxide (H 2 O 2 ) determination based on the oxidative coloration reaction of N,N'-diethyl-p-phenylenediamine (DPD) via the Fenton reactions in aqueous water was established. The generated pink radical cation (DPD + ) showed maximum absorption at 551 nm. Importantly, under the optimal conditions (pH 3.0, 20 mM DPD, 1.5 mM Fe(II) and reaction time of 45 s), the increase in absorbance at 551 nm for DPD + generation was linear with respect to the addition of H 2 O 2 (0-12 μM). The sensitivity and the detection limit of the proposed Fenton-DPD method for H 2 O 2 determination at 551 nm were (2.55 ± 0.01) × 10 4  M -1  cm -1 and 0.05 μM, respectively. The stoichiometric factor for the reaction of H 2 O 2 with DPD was 1:1.18. The absorbance of the generated DPD + was found to be stable in different types of water within 20 min. Moreover, the proposed Fenton-DPD method could be used for the analysis of the trace H 2 O 2 in rainwater and determine the rate constants that involved H 2 O 2 with high accuracy., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

In this study, a new spectrophotometric method was proposed for the measurement of hydrogen peroxide (H 2 O 2 ) in aqueous solutions. The method was based on the peroxidase (POD)-catalyzed reaction in which 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) was oxidized to form the stable green radical (ABTS + ). The generated ABTS + could be determined spectrophotometrically. The absorbance of the generated ABTS + at 415 nm, 650 nm, 732 nm and 820 nm were linear with H 2 O 2 concentrations in the range of 0-40 μM. The sensitivities of the proposed ABTS method for H 2 O 2 determination at 415 nm, 650 nm, 732 nm and 820 nm were 6.29 × 10 4  M -1  cm -1 , 2.00 × 10 4  M -1  cm -1 , 2.54 × 10 4  M -1  cm -1 and 1.89 × 10 4  M -1  cm -1 , respectively. The oxidation of ABTS to generate ABTS + in POD-catalyzed H 2 O 2 system at pH 6.0 was so fast that the determination time of the ABTS method was as short as 0.5 min. The stoichiometry of the reaction of H 2 O 2 and ABTS in the presence of POD was calculated as nearly 1:2 (1:1.92). The residual absorbance of the generated ABTS + was also found to be stable within 30 min in natural waters. Low H 2 O 2 concentration in rainwater could be both measured by the ABTS method and the DPD method with high accuracy, however, H 2 O 2 concentration in wastewater contained rhodamine B could only be accurately measured with the ABTS method at 732 nm. Moreover, waste solutions after H 2 O 2 analysis with the proposed ABTS method were non-hazardous towards E. coli., (Copyright © 2017 Elsevier Ltd. All rights reserved.)