Predominance-zone diagrams of Fe(III) and Fe(II) sulfate complexes in acidic media. Voltammetric and spectrophotometric studies

Abstract

A thermodynamic study based on concepts of generalized species and equilibria, was used to understand the distribution of Fe(III) and Fe(II) species in the Fe(III)/Fe(II)/H(2)SO(4)/H(2)O system. The two-dimensional predominance zone diagrams (TDPZ) and Pourbaix type diagrams thus obtained permitted the selection of optimum experimental conditions, to differentiate the chemical species involved in this system. The existence of the different predominant chemical species for Fe(III): Fe(3+), FeSO(+)(4) and Fe(SO(4))(-)(2) was evidenced by spectrophotometrical studies for pSO'(4) values from 4 to 0 units in a buffered solution of pH 0.5. Additionally, voltammetric studies performed on Pt, Au and carbon paste electrodes confirmed that the electron exchange between Fe(III) Fe(II) in H(2)SO(4) media occurs by an inner Helmholtz layer mechanism. It was also possible to show that the representative couples at pSO'(4) = 0 (buffered) are: (a) for pH < 0 FeSO (+)(4)FeHSO (+)(4) and (b) for pH > 1.0: Fe(SO (4)) (-)(2)FeHSO (+)(4). The last couple presents a coupled chemical reaction in the electrochemical mechanism; this reaction is associated with the different coordination numbers of Fe(III) and Fe(II).