Selective sensing of submicromolar iron(III) with 3,3′,5,5′-tetramethylbenzidine as a chromogenic probe

•A colorimetric method was developed for the determination of Fe(III).•Fe(III) directly oxidized 3,3,′5,5′-tetramethylbenzidine to produce blue solution.•The method benefited the characteristics of low cost, sensitivity and selectivity.•As low as 1.0 μmol/L Fe(III) could be perceived by the naked eyes.•Iron content in milks was successfully determined.

The development of highly selective and sensitive method for iron(III) detection is of great importance both from human health as well as environmental point of view. We herein reported a simple, selective and sensitive colorimetric method for the detection of Fe(III) at submicromolar level with 3,3,′5,5′-tetramethylbenzidine (TMB) as a chromogenic probe. It was observed that Fe(III) could directly oxidize TMB to form a blue solution without adding any extra oxidants. The reaction has a stoichiometric ratio of 1:1 (Fe(III)/TMB) as determined by a molar ratio method. The resultant color change can be perceived by the naked eye or monitored the absorbance change at 652 nm. The method allowed the measurement of Fe(III) in the range 1.0 × 10− 7–1.5 × 10− 4 mol L− 1 with a detection limit of 5.5 × 10− 8 mol L− 1. The relative standard deviation was 0.9% for eleven replicate measurements of 2.5 × 10− 5 mol L− 1 Fe(III) solution. The chemistry showed high selectivity for Fe(III) in contrast to other common cation ions. The practically of the method was evaluated by the determination of Fe in milk samples; good consistency was obtained between the results of this method and atomic absorption spectrophotometry as indicated by statistical analysis.

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