Determination of Thermodynamic Stability of a Ni (II) Glycine Complexes in aqueous solution: Potentiometric and Spectroscopic Studies

The stability constants of Glycine and Ni(II)Glyx complexes (x = ۰-۳) in aqueous solution were studied by potentiometric titration at ۲۵°C. The data analysis afforded insight on speciation, which was further confirmed by measuring absorbance in the UV-VIS region, covering a pH range from ۲ to ۱۱ to establish the Ni(II)Gly complex extinction coefficients. The potentiometric and spectroscopic data were analyzed using the computer program ESTA. It was shown that the formation of Ni(II)/glycinate complexes are strongly pH dependent. The Ni(II) competes with the protons bound to Glycine at the amine and carboxylate moiety. The successive formation of the NiL, NiL۲ and NiL۳ was observed as the pH was increased. Free Ni(II) complexes with one Glycine ligand above pH ۴ followed by further association of another Glycine ligand above pH ۶ and again at pH ۸ to form the NiL۳ complex. The stability of the complexes is proportional to the number of glycine ligands bound. The analysis of the spectroscopic data supports this trend as an increase in Glycine bound to the Ni(II) center. However, it increases its stability in need of shorter wavelength (higher energy) to excite the electron, suggesting stronger field splitting exerted by the glycine ligand due to higher stability. Thermodynamics study proposed that the increase in stability of the Ni(II) complexes was in this direction: NiL۳ > NiL۲ > NiL Good correspondence between theoretical and experimental data, low standard deviations in the stability constants and low RH lend reasonable confidence in all obtained results.

کلیدواژه ها:

Glycine ، Ni(II) complexes ، Potentiometric ، UV-Vis Spectroscopy