Revision of iron(III)–citrate speciation in aqueous solution. Voltammetric and spectrophotometric studies

A detailed study of iron (III)–citrate speciation in aqueous solution (θ = 25 °C, Ic = 0.7 mol L−1) was carried out by voltammetric and UV–vis spectrophotometric measurements and the obtained data were used for reconciled characterization of iron (III)–citrate complexes. Four different redox processes were registered in the voltammograms: at 0.1 V (pH = 5.5) which corresponded to the reduction of iron(III)–monocitrate species (Fe:cit = 1:1), at about −0.1 V (pH = 5.5) that was related to the reduction of FeL25−, FeL2H4− and FeL2H23− complexes, at −0.28 V (pH = 5.5) which corresponded to the reduction of polynuclear iron(III)–citrate complex(es), and at −0.4 V (pH = 7.5) which was probably a consequence of Fe(cit)2(OH)x species reduction. Reversible redox process at −0.1 V allowed for the determination of iron(III)–citrate species and their stability constants by analyzing Ep vs. pH and Ep vs. [L4−] dependence. The UV–vis spectra recorded at varied pH revealed four different spectrally active species: FeLH (log β = 25.69), FeL2H23− (log β = 48.06), FeL2H4− (log β = 44.60), and FeL25− (log β = 38.85). The stability constants obtained by spectrophotometry were in agreement with those determined electrochemically. The UV–vis spectra recorded at various citrate concentrations (pH = 2.0) supported the results of spectrophotometric–potentiometric titration.

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► Combination voltammetry-spectophotometry best portrayed Fe–citrate speciation in aqueous solution.
► 4 redox processes of Fe(III)–citrate species were registered: at 0.1, −0.1, −0.28 and −0.4 V.
► Biochemically most important Fe(III)–citrate mononuclear species, were described.
► log βFeLH=25.69,   log βFeL2H2=48.06,   log βFeL2H=44.60,   log βFeL2=38.85.log βFeLH=25.69,   log βFeL2H2=48.06,   log βFeL2H=44.60,   log βFeL2=38.85.