Consecutive overall stability constants of metal complexes in solution from diffusion current data

Abstract

It is shown theoretically that the change in polarographic diffusion current of a metal ion in solution, produced by the presence of excess coordinating ligand, is directly proportional to the ligand number, n̄, of the system. The relationship is confirmed by the observed behaviour of the cadmium ion when complexed with four different heterocyclic ligand species for which stability constant data were determined by conventional means. The formation curve for a particular metal-ligand system may be constructed provided that a reliable assessment of the ligand number may be made at one or (preferably) two values of the free ligand concentration. Since a diffusion-controlled polarographic wave-height is given by the Ilkovič equation, regardless of the degree of reversibility of the electrode process, the method may be used to determine the stabilities of complexes of metal ions that behave irreversibly at the dropping mercury electrode.