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 degrees C) using differential pulse cathodic voltammetry (DPCV) was performed on a static mercury drop electrode (SMDE). Iron(111) concentration of 2.5 x 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(Ill)-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 beta(2) = 16.83 +/- 0.47 and log beta(3) = 18.64 +/- 0.70, respectively. The model that best fitted the data gave two iron (11)-glycine stability constants for [FeGly](+) log K-1 3.69 +/- 0.19 and for Fe(Gly)(2) log beta(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. (C) 2005 Elsevier B.V. All rights reserved.