Voltammetry as a virtual potentiometric sensor in modelling of a metal-ligand system and refinement of stability constants. Part 4. An electrochemical study of Ni-II complexes with methylene diphosphonic acid

摘要

The Ni-II-MDP-OH system (MDP = methylene diphosphonic acid) and stability constants of complexes formed at ionic strength 0.15 M at 298 K were established by direct current polarography (DCP) and glass electrode potentiometry (GEP). The final M-L-OH model could only be arrived to by employing recent concept of virtual potentiometry (VP). VP-data were generated from non-equilibrium and dynamic DC polarographic technique. The VP and GEP data were refined simultaneously by software dedicated to potentiometric studies of metal complexes. Species distribution diagrams that were generated for different experimental conditions employed in this work assisted in making the final choice regarding the metal-ligand model. The model established contains ML, ML2, ML(OH) and ML(OH)(2) with stability constants, as log beta, 7.94 +/- 0.02, 13.75 +/- 0.02, 12.04 (fixed value), and 16.75 +/- 0.05, respectively. It has been demonstrated that virtual potential must be used in modelling operations (predictions of species formed) when a polarographic signal decreases significantly due to the formation of polarographically inactive species (or formation of inert complexes). The linear free energy relationships that included stability constant logK(1) for Ni-II-MDP established in this work together with other available data were used to predict logK(1) values for Sm-III and Ho-III with MDP. The log K-1 values for Sm-III-MDP and Ho-III-MDP were estimated to be 9.65 +/- 0.10 and 9.85 +/- 0.10, respectively. (c) 2005 Elsevier Inc. All rights reserved.