Reflections\nand Recommendations on the Accuracy of\nABTS Colorimetric Methods in Environmental Analysis

Colorimetric methods using 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonate)\n(ABTS) as a chromogenic reagent are widely used for oxidant detection\nin water samples. However, the coexisting natural organic matter,\nespecially those having phenolic hydroxyl groups, can consume the\nformed ABTS·<sup>+</sup> radical, leading to a reduced measured\nconcentration using the ABTS method. Using phenols as model substances,\nthe rate constants between ABTS·<sup>+</sup> and deprotonated\nphenols could reach as high as 10<sup>7</sup>∼10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup>, approximately 4–6\norders of magnitude higher than those with protonated phenols. These\nreaction rates could decrease with an increase in ABTS concentration.\nLowering the chromogenic pH and increasing the initial ABTS concentration\ncan reduce the consumption of ABTS·<sup>+</sup> by matrix components,\nbut does not fully eliminate the reductive interference caused by\npara- or ortho-phenolic hydroxyl groups. Similar measurement deviations\nof the ABTS method were also observed during the analysis of real\nwater samples. Relatively, utilizing 3,3′,5,5′-tetramethylbenzidine\nas the chromogenic reagent, the TMB method exhibits stronger anti-interference\nperformance toward matrix components in both synthetic solutions and\nnatural waters, making it suitable for the quantitative analysis of\nmultiple oxidants. This work investigates the deviation characteristics\nof the ABTS method and provides several suggestions for the accurate\nand sensitive determination of oxidants in complex aqueous environments.