A polarographic method of speciation for labile metal-ligand systems based on mass-balance equations. A differential pulse polarographic study at fixed ligand to metal ratio and varied pH

A new equation for speciation of labile species by polarography is derived. It relates the observed shift in a peak potential and a decrease in a peak height with a change in the concentration of a free metal ion in a solution. The free metal ion concentration is controlled by the formation of complexes of the metal ion with ligands and is calculated from mass-balance equations written for the metal-ligand system. This allows an incorporation into the ligand-metal system any number of complexes, those which are known and those which are thought to be formed and for which formation constants are calculated. As a test of this method, complexes of Cd(II) with the ligand N,N,N′,N′-tetramethylethylenediamine are studied by differential pulse polarography (DPP) at fixed ligand-to-metal ratio and varied pH. All known hydroxide species of cadmium, including polynuclear species, were incorporated into the metal-ligand system. A good agreement was found between formation constants calculated in this work from DPP and those reported previously from potentiometry. In addition, a new complex, viz. CdL2(OH)+, was found and its formation constant, as log β, was estimated to be 8.94 ± 0.03.