DPD as an electron probe in ferrate oxidation: a novel spectrophotometric determination method and the fate of iron intermediates

Abstract

An innovative analytical approach was established for quantifying Fe(VI) concentrations in the 0.47–40 μM range. This technique exploited the electron transfer reaction between Fe(VI) and N,N-diethyl-p-phenylenediamine (DPD), generating a stable radical cation species (DPD˙⁺) with characteristic absorbance at 551 nm in spectroscopic detection. The increase in the absorbance of the formed DPD˙⁺ at 551 nm was linearly related to the added Fe(VI) concentration. The formed DPD˙⁺ was found to be stable in synthetic water samples at pH 5–7 and real water samples. The stoichiometric relationship of the formed DPD˙⁺ to Fe(VI) was 1 : 1 in reaction of Fe(VI) with excess DPD in 300 mM phosphate buffer at pH 6, which was lower than that in lower concentration of phosphate, due to the inhibiting impact of phosphate on the oxidation capacity of Fe(V) with DPD. Demonstrating a molar absorptivity of 2.08 × 10⁴ M⁻¹ cm⁻¹ at 551 nm, the Fe(VI)–DPD method exhibited broad applicability while maintaining accuracy across diverse environmental water matrices. This methodology exhibited superior detection sensitivity, with detection limits established at 0.47 μM (LOD) and 1.57 μM (LOQ). The oxidizing capacity of the complexed Fe(V) followed the order: carbonate–Fe(V) > borate–Fe(V) > phosphate–Fe(V) > pyrophosphate–Fe(V). The ratio of the formed DPD˙⁺ to consumed Fe(VI) decreased with the increasing concentration ratio of pyrophosphate to Fe(VI) (2.5–3.6) at pH 5, indicating that pyrophosphate inhibited the oxidizing capacity of Fe(VI) to Fe(V) using DPD. The developed DPD probe method demonstrated reliable applicability in characterizing Fe(VI) reaction pathways due to its high sensitivity (2.08 × 10⁴ M⁻¹ cm⁻¹) and minimal matrix interference.