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

The NiII-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 β, 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 log K1 for NiII-MDP established in this work together with other available data were used to predict log K1 values for SmIII and HoIII with MDP. The log K1 values for SmIII-MDP and HoIII-MDP were estimated to be 9.65 ± 0.10 and 9.85 ± 0.10, respectively.