Voltammetric Determination of Stability Constants of Iron(iii) - Glycine Complexes in Water Solution

Determination of the stability constants of dissolved iron(III) glycine system in water solution (I = 0.6 mol L in NaClO4, pH = 7.3 at 25±1 oC) using differential pulse cathodic voltammetry (DPCV) and cyclic voltammetry (CV) was performed out on a static mercury drop electrode (SMDE). By choosing voltammetry as analytical tool, measurements with lower iron(III) concentrations (10 mol L), at the pH = 7.3 were enabled, which ensured the formation of higher coordination (1 : 2 and 1 : 3) complexes of iron(III) glycine system. The added concentration of iron(III) ions was 2.5×10 mol L and the concentrations of total glycine varied from 0.1 to 0.5 mol L. Since iron(III) reduction in glycine system, with the techniques employed, was found to be oneelectron reversible process, it was concluded that this system contained labile iron(III) complexes. Thus, it was possible to determine stability constants. The stability constants of [Fe(Gly)2] and Fe(Gly)3 complexes, which had not been reported in the literature so far, were found to be log β23 = 17.21±0.03 and log β33 = 20.19±0.02, respectively. According to the constants found, chemical distribution of iron(III) in glycine water solution, as a function of pH, was calculated and proposed.