Speciation of iron in river and tap waters by catalytic spectrophotometry using oxidation of o-phenylenediamine with hydrogen peroxide

A new sensitive catalytic method for the determination of labile iron has been developed using iron-catalyzed oxidation of o-phenylenediamine (OPDA) with hydrogen peroxide at pH 4.5 and 40°C. Absorbance (A) of the oxidized OPDA was monitored at 450 nm for a reaction time (t) of 10 min. The calibration graph was constructed by using ΔA/Δt. The same calibration graph with a linear range of 0–20 μg l−1 was obtained for Fe(II) and Fe(III). The detection limit was 0.06 μg l−1. Labile form of iron was characterized by the kinetic and complexation studies for the speciation of iron. The results suggested that Fe(II) was oxidized to Fe(III) at the initial reaction step and Fe3+ and FeIIILi, where i = 1 or 2 for a unidendate and i = 1 for a bidentate ligand (L), were determined as the labile iron. Non-labile Fe(III) hydroxide changed to a labile form by the complexation with humic acid. By the combined use of acid decomposition, the proposed method was successfully applied to the determination of soluble labile and non-labile iron in river and tap water samples.