A polarographic and potentiometric study of metal-ligand equilibria: Instrumentation and investigations of systems with non-reversible electrode reactions

[摘要] New possibilities in collection of polarographic and potentiometric experimental data instudies of metal–ligand systems by automated instrumental methods, and subsequenttreatment of the polarographic data, whereby the degree of reversibility of the electrodeprocesses varies, have been investigated in this work. An automated instrumental set–upwas developed for applications in studies of metal–ligand solution equilibria bypotentiometry and sampled Direct Current Polarography (DCP). The new set–up wasdesigned based on virtual instrumentation principles whereby several commercially–available hardware units as well as custom–built electronic components, were interfacedto a personal computer that was equipped with appropriate hardware and controlprograms. The instrumental set–up was tested and validated by studying the protonationequilibria of the ligand glycine by Glass Electrode Potentiometry (GEP) as well as thecomplexation of the ligand glycine with Cd2+ by GEP and DCP. The new set–up providesincreased versatility, accuracy and convenience in obtaining large numbers ofexperimental points in solution equilibria studies by DCP and GEP as opposed to the useof tedious and time–consuming manual methods. Nonlinear curve–fitting procedures,based on closed–form models that were derived here from suitable theoretical equationsidentified from literature, have been investigated in this work for applications in analysisof DC curves recorded on metal–ligand systems with variation in electrochemicalreversibility. The applicability and limitations of the curve–fitting procedures developedhave been tested in analysis of the DCP data collected on several metal–ligand systemsinvolving Cd2+, Pb2+, Zn2+ and the ligands glycine and sarcosine, whereby the DCPstudies of these systems exhibited reversible, quasi–reversible or irreversibleelectrochemical processes. Information on applicability and limitations of the proposedmethods investigated in this work was derived by comparison of the results obtained fromDCP, using the proposed methods, with either reported literature data and/or resultsobtained in this work by the independent analytical technique of GEP, which wasdeployed wherever it was found to be applicable to study the metal–ligand systemsconsidered.

[发布机构] University of the Witwatersrand