Voltammetric determination of stability constants of iron(III)–glycine complexes in water solution

Abstract

Determination of the stability constants of dissolved iron(III)–glycine system in water solution ( I = 0.6 mol L −1 in NaClO 4 at 25 ± 1 °C) using differential pulse cathodic voltammetry (DPCV) was performed on a static mercury drop electrode (SMDE). Iron(III) concentration of 2.5 × 10 −5 mol L −1 and the pH range from 9.05 to 6.36 ensured the formation of enough concentration of iron(III)–glycine higher coordination complexes (1:2 and 1:3) to be measured by the applied method. The concentrations of total glycine varied from 0.1 to 0.5 mol L −1. Cyclic voltammetry (CV) measurements were used to investigate reversibility of the iron(III)–glycine complexes which showed one-electron reversible character. The stability constants of iron(III) [Fe(Gly) 2] + and Fe(Gly) 3 complexes, which had not been reported in the literature so far, were found to be log β 2 = 16.83 ± 0.47 and log β 3 = 18.64 ± 0.70, respectively. The model that best fitted the data gave two iron(II)–glycine stability constants for [FeGly] + log K 1 = 3.69 ± 0.19 and for Fe(Gly) 2 log β 2 = 5.08 ± 0.60. According to the constants found, chemical distribution of iron(III) in glycine water solution, as a function of pH, was calculated and proposed.