PREDOMINANCE-ZONE DIAGRAMS OF FE(III) AND FE(II) SULFATE COMPLEXES INACIDIC MEDIA - VOLTAMMETRIC AND SPECTROPHOTOMETRIC STUDIES

A thermodynamic study based on concepts of generalized species and equ ilibria, was used to understand the distribution of Fe(III) and Fe(II) species in the Fe(III)/Fe(II)/H2SO4/H2O system. The two-dimensional p redominance zone diagrams (TDPZ) and Pourbaix type diagrams thus obtai ned permitted the selection of optimum experimental conditions, to dif ferentiate the chemical species involved in this system. The existence of the different predominant chemical species for Fe(III): Fe3+, FeSO 4+ and Fe(SO4)(2)(-) was evidenced by spectrophotometrical studies for pSO'(4) values from 4 to 0 units in a buffered solution of pH 0.5. Ad ditionally, voltammetric studies performed on Pt, Au and carbon paste electrodes confirmed that the electron exchange between Fe(III)/Fe(II) in H2SO4 media occurs by an inner Helmholtz layer mechanism. It was a lso possible to show that the representative couples at pSO'(4) = 0 (b uffered) are: (a) for pH < 0 FeSO4+/FeHSO4+ and (b) for pH > 1.0: Fe(S O4)(2)(-)/FeHSO4+. The last couple presents a coupled chemical reactio n in the electrochemical mechanism; this reaction is associated with t he different coordination numbers of Fe(III) and Fe(II).