Thermodynamic Parameters of the Mixed-Ligand Complexation of Cobalt(II) with Triglycine and L-Histidine in an Aqueous So
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ICAL CHEMISTRY OF SOLUTIONS
Thermodynamic Parameters of the Mixed-Ligand Complexation of Cobalt(II) with Triglycine and L-Histidine in an Aqueous Solution G. G. Gorboletovaa,*, S. A. Bychkovaa, E. A. Gorbashovaa, and K. O. Frolovaa aIvanovo
State University of Chemical Technology, Ivanovo, 153000 Russia *е-mail: [email protected]
Received October 12, 2019; revised December 5, 2019; accepted December 10, 2019
Abstract—The Co2+–triglycine–L-histidine system is studied in an aqueous solution at Т = 298.15 K (KNO3) via potentiometry and calorimetry. The formation of mixed-ligand CoL2Y− complex is established, and the standard thermodynamic characteristics (ΔrH°, ΔrG°, ΔrS°) of the complexation reaction are determined. A structure is proposed for this complex. Keywords: mixed-ligand Co2+ complexes with triglycine and L-histidine, thermodynamic characteristics DOI: 10.1134/S0036024420090083
INTRODUCTION The compatibility of ligands in mixed MLY complexes, where L and Y are amino acids and peptides, is of great interest because of their scientific and practical importance. Complexes of amino acids and oligopeptides are used in analytical chemistry, agriculture, industry, and medicine [1, 2]. Metal complexes containing amino, carboxyl, and amide groups are also used as pharmaceuticals [3, 4]. Information about the factors influencing the compatibility of two different ligands in the same coordination sphere of a central ion can be acquired from the enthalpy and entropy characteristics of coproportionation reactions. However, there are usually no reference thermochemical data on reactions of the formation of mixed-ligand complexes of amino acids and peptides. The thermodynamic characteristics of complexation processes in the Co2+–glycine–L-histidine system were determined in [5]. In this work, we performed a potentiometric and calorimetric study of the processes of Co2+ ion complexation with triglycine (L−) and L-histidine (Y−). EXPERIMENTAL Glycyl-glycyl-glycine (Sigma) and L-histidine of chemically pure grade were used in this work without additional purification. Solutions of amino acid and peptide were prepared with precisely weighed samples of crystalline substances dried to constant weight at 343 K. The Co(NO3)2 solutions were prepared from reagent of chemically pure grade, and their concentration was determined complexonometrically. The specified ionic strength was maintained with potassium nitrate of chemically pure grade.
The composition and stability of the mixed-ligand CoL2Y− complex were determined from potentiometric measurements. The total concentration of ligands was varied from 3.2 × 10−2 to 4.8 × 10−2 mol/L. Our studies were performed at ratios Co : L : Y = 1 : 3 : 1, 1 : 4 : 1, 1 : 5 : 1 (I = 0.2). These ratios corresponded to the maximum yield of the studied complex. Potentiometric titration was performed by standard means. The electromotive force (EMF) of the circuit ±
Ag, AgCl, KCl sat | HL , KNO3, Co(NO3)2 | glass electrode was measured on an IPL-311 Multitest pH-meter. The absolute error of pot
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