Thermodynamic Characteristics of the Formation of Iron(II) and Iron(III) Complexes with L-Alanine in Aqueous Solutions

The Clark–Nikolsky oxidation potential is used to study the formation of iron(II) and iron(III) coordination compounds in aqueous solutions of α-alanine at a temperature of 298.15 K and an ionic strength of the (Na(H)ClO4) solution of 1.0 mol/L. Experimental curves of dependences of the EMF of the system on the concentration parameters of hydrogen, iron(III), iron(II), and α-alanine ions (pH, рСох, рСred, and pCL, respectively) are recorded. The curves show that complexation in the studied system proceeds stepwise in the wide range of pH 0.5–9.0. Mononuclear coordination compounds [FeHL(H2O)5]3+, [Fe(HL)2(H2O)4]3+, [Fe2(HL)2(OH)4(H2O)6]2+, [FeHL(H2O)5]2+, [Fe(HL)(OH)(H2O)4]+, and [Fe(HL)(OH)2(H2O)3]0 and a heterovalent [FeIIFeIII(HL)2(OH)4(H2O)6]+ complex form. The type and number of coordinated ligands and cations and the total composition of the resulting complex compounds are determined. Chemical models complexation are compiled, and the regions of their dominance are determined.