Formation constant calculation for non-labile complexes based on a labile part of the metal-ligand system. A differential pulse polarographic study at fixed ligand to metal ratio and varied pH: application to polarographically inactive complexes

A new method of speciation by differential pulse polarography (DPP) is described. It is focused on non-labile complexes on the DPP time scale. By solving mass-balance equations - which are written for the labile part of a metal-ligand system - free metal, free ligand and non-labile complex concentrations are calculated. These are used for calculation of formation constants of non-labile complexes for each datum point. Each datum point is obtained during titration of a single sample solution by an addition of a standard sodium hydroxide solution. Where possible, the determinations were carried out in such a way that results from polarography and glass electrode potentiometry have been simultaneously obtained from a single experiment. The theoretical and analytical procedures as well as computer program (written to calculate formation constants directly from the polarographic data) were verified by experiment. Three metal ions, viz. PbII, CdII, and BiIII, were studied with the ligand N,N′,N″-tris(hydroxyethyl)triazacyclononane and formation constants, expressed as log K1, were found to be 12.22, 10.61 and 16.36 from polarography, respectively. Results obtained for lead and cadmium are compared with those obtained from potentiometry. The validity of the proposed method and the application of DPP for polarographically inactive complexes are demonstrated.