Solution Equilibria. A Unified Mathematical Treatment of Potentiometric and Polarographic Data in a Metal-Ligand Equilibrium Study at a Fixed Ligand to Metal Ratio and Various pH Values

It has been shown that experimental data coming from equilibrium technique sensitive towards the free metal ion concentration [M], such as ion selective electrode (ISE), and dynamic technique, such as differential pulse polarography (DPP), can be modelled and stability constants refined by the use of the same set of tools previously used only for a study of metal–ligand equilibria by DPP. Experimental and calculated complex formation curves (ECFC and CCFC) introduced recently in DPP can also be used without any modification in the study of metal complexes by ISE. It is demonstrated that for the same experimental conditions, e.g., the same total ligand [LT] and metal ion [MT] concentrations and when measurements are performed at different pH values, very much the same ECFC and CCFC should be obtained from both experimental techniques. It is demonstrated that when the fully dynamic (labile) and polarographically reversible metal–ligand system is studied then the relationship ΔE vs. log [M] should be linear for both, equilibrium and dynamic, techniques. The application of the relationship ΔE vs. log [M] in modelling of a metal–ligand system is suggested. The combination of the two techniques, ISE and DPP, is advantageous as ISE can be used at low a [LT]:[MT] ratio (such as 2 or 3) and significantly higher [MT], whereas DPP can perform well at the [LT]:[MT] ratios larger than 30 and lower [MT]. This provides the possibility of a study of a metal–ligand system in much broader range of experimental conditions that, in turn, provides more data and information about the metal–ligand system of interest. This new concept was successfully studied on the CdII-glycine system. The formation of ML, ML2 and ML3, reported previously, has been confirmed by the two techniques and stability constants obtained in this work compare well with the literature values. In addition, two new complexes, M(HL) and ML2(OH), and their stability constants obtained from both employed techniques are reported.