Voltammetry as a virtual potentiometric sensor in modelling of a metal/ligand system and refinement of stability constants - Part 2 - Differential-pulse- and sampled-current-polarographic and virtual free-metal-ion potentiometric study of a bismuth(I

摘要

A mathematical conversion of data from nonequilibrium and dynamic voltammetric techniques (direct-current-sampled (DC) and differential-pulse (DP) polarography) into potentiometric, free-metal-ion sensor-type data is described and employed in the study of Bi-III complexes with the ligand picolinic acid (pyridine-2-carboxylic acid) (labile and dynamic metal/ligand system). A novel procedure that allows evaluation of experimental data collected at very low pH values (acid-base titration) is proposed. Software ESTA dedicated to potentiometry was successfully employed in the refinement operations performed with virtual potentiometric (VP) data obtained from DC and DP polarography, the latter being performed at fixed pH (ligand titration) as well as at fixed [L-T]/[M-T] ratio (acid-base titration). It was possible to refine stability constants either separately from VP-DC or VP-DP, or simultaneously from any combination of VP-DC and VP-DP obtained from ligand and/or acid-base titrations. The concept of VP-DC or VP-DP is employed for the first time in the study of an unknown Bi-III/picolinic acid/OH system, and numerous documented and possible advantages are discussed. Five complexes of bismuth, ML, ML2, ML3, ML4, and ML3(OH) and their stability constants (as log beta) 7.48 +/- 0.01, 13.94 +/- 0.01, 18.10 +/- 0.04, 20.47 +/- 0.25, and 26.65 +/- 0.03, respectively, are reported at 0.5m (Na,H)NO3 ionic strength and T 298 K. The proposed procedure can be easily utilized also by non-electrochemists who are interested in, e.g.. ligand-design strategies.