Understanding Variations in Ferrate Detection through the ABTS Method in the Presence of Electron-Rich Organic Compounds

The chromogenic reaction between 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate)
(ABTS) and ferrate [Fe(VI)] has long been utilized for Fe(VI) content
measurement. However, the presence of electron-rich organic compounds
has been found to significantly impact Fe(VI) detection using the
ABTS method, leading to relative errors ranging from ∼88 to
100%. Reducing substances consumed ABTS•+ and resulted
in underestimated Fe(VI) levels. Moreover, the oxidation of electron-rich
organics containing hydroxyl groups by Fe(VI) could generate a phenoxyl
radical (Ph•), promoting the transformation of Fe(VI)
→ Fe(V) → Fe(IV). The in situ formation of Fe(IV) can
then contribute to ABTS oxidation, altering the ABTS•+:Fe(VI) stoichiometry from 1:1 to 2:1. To overcome these challenges,
we introduced Mn(II) as an activator and 3,3′,5,5′-tetramethylbenzidine
(TMB) as a chromogenic agent for Fe(VI) detection. This Mn(II)/TMB
method enables rapid completion of the chromogenic reaction within
2 s, with a low detection limit of approximately 4 nM and a wide detection
range (0.01–10 μM). Importantly, the Mn(II)/TMB method
exhibits superior resistance to reductive interference and effectively
eliminates the impact of phenoxyl-radical-mediated intermediate valence
iron transfer processes associated with electron-rich organic compounds.
Furthermore, this method is resilient to particle interference and
demonstrates practical applicability in authentic waters.