Thermodynamics of Ni(II) Complexation with L-Asparagine in Aqueous Solution

Complexation of the Ni²⁺ ion with L-asparagine (HAsn^±) is studied by potentiometric titration at 298.15 K and at the 0.3, 0.5 and 1.0 ionic strength of the solution (KNO₃). The formation of the NiAsn⁺ and NiAsn₂ complexes was established and their stability constants were determined. The thermodynamic stability constants of the mono- and bis(L-asparagine)nickel(II) complexes were obtained by extrapolation to zero ionic strength. The direct calorimetry method was used to measure the heat effect of the L-asparagine reaction with the Ni(II) nitrate solution in different pH intervals at 298.15 K and at the 0.5, 1.0, and 1.5 ionic strength (KNO₃). The standard enthalpies of the NiAsn⁺ and NiAsn₂ formation were found using extrapolation and the equation with one individual parameter. The enthalpies of the formation of the Ni(II) complexes with L-asparagine in aqueous solution were calculated in the standard hypothetically undissociated state.