title: Revision of iron(III)-citrate speciation in aqueous solution. Voltammetric and spectrophotometric studies.
abstract: A detailed study of iron (III)-citrate speciation in aqueous solution (θ=25°C, I(c)=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 FeL(2)(5-), FeL(2)H(4-) and FeL(2)H(2)(3-) complexes, at -0.28 V (pH=5.5) which corresponded to the reduction of polynuclear iron(III)-citrate complex(es), and at -0.4V (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 E(p) vs. pH and E(p) vs. [L(4-)] dependence. The UV-vis spectra recorded at varied pH revealed four different spectrally active species: FeLH (logβ=25.69), FeL(2)H(2)(3-) (log β=48.06), FeL(2)H(4-) (log β=44.60), and FeL(2)(5-) (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.
authors.affiliation: Ruđer Bošković Institute, Department for Marine and Environmental Research, Zagreb, Croatia.
authors.forename: Petra
authors.initials: P
authors.lastname: Vukosav
authors.forename: Marina
authors.initials: M
authors.lastname: Mlakar
authors.forename: Vladislav
authors.initials: V
authors.lastname: Tomišić
mesh_terms: D000465:Algorithms; D019343:Citric Acid / Q000032:analysis*; D055664:Electrochemical Techniques; D005290:Ferric Compounds / Q000032:analysis*; D006863:Hydrogen-Ion Concentration; D007502:Iron Chelating Agents / Q000032:analysis*; D007700:Kinetics; D008024:Ligands; D010084:Oxidation-Reduction; D012996:Solutions; D013056:Spectrophotometry, Ultraviolet; D014867:Water / Q000737:chemistry*
chemical_list: D005290:Ferric Compounds; D007502:Iron Chelating Agents; D008024:Ligands; D012996:Solutions; D014867:Water; D019343:Citric Acid; C025314:ferric citrate