Enthalpy of Formation of Mixed Nickel(II) Complexes with L-Histidine and Oliglycines

Potentiometry and calorimetry are used to study the Ni²⁺–diglycine–L-histidine system in an aqueous solution at T = 298.15 K (KNO₃). The composition and stability of the mixed ligand complex Ni(GG)(His) are defined with potentiometric measurements. The study is performed at ratios Ni : (GG + His) = 1 : 4 and 1 : 6. The concentrations of diglycine and L-histidine are in ratios of 3 : 1 and 5 : 1. The values of \(\log \beta \) obtained at different ratios of diglycine and L-histidine agree well with one another. Thermal effects are measured in the ampoule of a mixing calorimeter with an isothermal shell, a thermistor temperature sensor, and automatic recording of the temperature/time curve. The thermal effects of mixing solutions of Ni(NO₃)₂, diglycine, and L-histidine in the ratio \(c_{{{\text{Ni}}}}^{0}\) : \(c_{{{\text{GG}}}}^{0}\) : \(c_{{{\text{His}}}}^{0}\) = 1 : 3 : 1 are measured to determine the enthalpy of formation of a Ni(GG)(His) particle. The experiments are performed at ionic strengths of 0.2, 0.5, and 1.0 (mol/L). The standard thermodynamic characteristics (Δ_rH°, Δ_rG°, Δ_rS°) of the formation of a mixed-ligand complex are determined. A comparative analysis of the obtained data is performed using thermodynamic characteristics of mixed-ligand complexation in the Ni²⁺–glycine–L-histidine and Ni²⁺–triglycine–L-histidine studied earlier. Structures of forming complexes are proposed.