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−1 in NaClO4 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 β2 = 16.83 ± 0.47 and log β3 = 18.64 ± 0.70, respectively. The model that best fitted the data gave two iron(II)-glycine stability constants for [FeGly]+ log K1 = 3.69 ± 0.19 and for Fe(Gly)2 log β2 = 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.